World is moving through various technological advancements and man is pacing with them all. An easiest example is the telecommunication field. From ancient days men used different methods for telecommunication and now we are in the era of mobile technology and internet. The mobile phone which came to catch moving man became all purpose devices in recent days.
Here I am going to explain about the new technology 3G or Third Generation Technology. Till now ,we are familiar with 2G technology which enables us voice communication over air, internet connectivity with EDGE or GPRS connectivity options that provide speed up to maximum of around 65 KB/s. 2G technology is not so much advanced because of its limited bandwidth and transfer rate. But 3G technology when compared with 2G have some advantages. In terms of speed 3G technology is offering a speed of 3MB/s.
3G technology represents a shift from voice oriented communication to multimedia oriented communication and services. Demand for voice services has become the requirement of past days and now it has changed to data or multimedia requirements.
COMPARISON OF 2G AND 3G TECHNOLOGIES
2G TECHNOLOGY
2G technology is based on low band data signaling. One of the popular 2G technology is the GSM-Global Systems for Mobile Communications. It's first implemented in 1991 and using in about 140 countries and around 248 million users around the globe are using it.
Now what's GSM? It's a combination of Frequency Division Multiple Access (FDMA) and Time Division Multiple Access (TDMA).GSM uses a frequency of 25 MHz of bandwidth with spectrum of 900 MHz FDMA is used to divide the available 25 MHz bandwidth into 124 carriers with 200 kHz frequency. It is then again divided using TDMA into 8 time slots. But later frequency band width of 1.5 GHz is implemented in GSM to increase data capability.
In addition to GSM another technology called Personal Digital Communication (PDC) emerged in Japan. Also the CDMA technology which emerged in North America and it implemented a system called spectrum technology which breaks the speech into small digitized segments and encode to recognize call. 2G technology has some data handling capacity such as fax and sort message service with a speed of 9.6 kB/s. But it's not suitable for data applications like web browsing and multimedia applications.
In order to overcome this drawback new methods like high speed circuit switched data (HSCSD),General packet radio service (GPRS), Enhanced Data Rates for Global Evolution (EDGE) technologies where evolved. It improved data speed up to 57.6 KB/s. This data speed is achieved by 14.4 kB/s data coding and aggregating of four radio channels with time slots of 14.4 kB/s. But all these developments become unsatisfactory because of low bandwidth and speed.HSCSD is the previous step towards the 3G technology.
3G TECHNOLOGY
The 3G technology can be defined as the convergence of existing technologies in 2G. Both terrestrial and satellite components are included in 3G.An important advantage of 3G is that we can integrate existing technologies of 2G such as CD MA,GSM,TDMA into 3G.
This technology uses a Core network and Radio Access Network (RAN).Core network is consisted of packet switching module which includes 3G, SGSNs and GGSNs which has same function as that of GPRS. The circuit switched domain contains 3G MSC for switching of voice calls. Also the access and charge of the services are done through the core network. RAN is entirely different from core network according to its function.
RAN consist of elements like Node B and Radio Network Controllers (RNCs).Node B is equivalent to the base transceiver station in 2G network and RNC is the Base Station Controller in 2G network. It functions as radio source management and helps in switching between circuit switched and packet switched domains. The connection between elements in RAN and between RAN is made possible with the help of lub, lur and lu interfaces. The lu interface is split into circuit switched d and packet switched. The lu interface is based on ATM. Also voice is embedded on virtual networks using AAL2 technology. These switching between 3G SGSNs for data and 3G MSC for voice are made according to the requirement.
The oscilloscope is an instrument that lets you view transient waveforms in electrical and electronic circuits. For example in the case of televisions, the waveforms found in the various points of the circuits are well defined, and through analysis we can easily diagnose what the problems of performance. Oscilloscopes are the most versatile instruments that exist and have been using television repair technicians to doctors. An oscilloscope can measure a large number of phenomena, provided the appropriate transducer (an element that converts a physical quantity into an electrical signal) will be able to give us the value of a pressure, heart rate, power, sound, vibration level in a car, etc. It is important that the oscilloscope used to display signals that allow for at least 4.5 cycles per second, allowing the verification stages of video, vertical and horizontal sweep to power supply. While the most common is the simple trace oscilloscope, is one stroke better double in more than one phenomenon or waveform can be displayed simultaneously. Oscilloscope operation is based on the possibility of deflecting an electron beam through the creation of electric and magnetic fields. In most oscilloscopes, electronic diversion, call deflection is achieved by electric fields. This is the electrostatic deflection. A minority of specialized equipment oscilloscope display response curves, using the electromagnetic deflection system, as the used in television. This latter type of oscilloscope has no control of exploration time.
The process of electron beam deflection occurs in the vacuum created within the cathode ray tube called (TRC). In this screen is where information is displayed implemented. The cathode ray tube electrostatic deflection is equipped with two pairs of deflection plates horizontal and vertical respectively, that allow properly controlled on-screen representation of the phenomena that you want to analyze.
This representation can be seen inscribed on Cartesian coordinates where the horizontal and vertical axes represent time and voltage respectively. The scale of each of the Cartesian axes recorded on the screen can be changed independently of one another, to give the signal representation is most suitable for measurement and analysis.
The dimensions of the CRT screen are now standard in most instruments, to 10 cm in the horizontal axis (X) by 8 cm in the vertical (Y). On the screen are recorded divisions of 1 cm square, directly on the CRT or on a part superimposed on it, which is printed a grid of 80 cm square. This grid is made where the representation of the signal applied to the oscilloscope. The oscilloscope, as a device that is widely used, is represented in the market for instruments under many forms, not only as to the purely physical aspect but in terms of its internal characteristics and therefore their performance and potential application of same. However, despite the possible differences, all scopes have common operating principles. The most widely used are those that could define as "basic oscilloscopes. With the oscilloscope, one can display signal waveforms alternating voltage by measuring the peak to peak, medium and rms. The basic circuits are:
* Vertical input attenuator
* Vertical Amplifier
* Step vertical deflection
* Amplifier trigger sample (trigger)
* Shooting Mode Selector (interior or exterior)
* Amplifier trigger pulse
* Time Base
* Erase pulse amplifier
* Step horizontal deflection
* Cathode ray tube
* Circuit power.
An alternating current is constantly changing that value and reverses its direction at regular intervals. In the case of an alternator, these changes result from the rotation of the armature or induced, as each turn of the winding wire cuts the lines of magnetic field in one direction and then in the opposite direction, causing the electrons to move alternately in one direction and then in the opposite direction. Accordingly, an alternation of intensity change is suffering an alternating current as it moves in one direction, growing in intensity from zero to its maximum and back to zero. Two alternatives, one in one direction and the other in the opposite direction or not, form a cycle. In a two-pole alternator, when the armature has made a complete revolution will have traveled 360 electric and a cycle will have occurred. The number of cycles that occur during a second is the frequency of the alternating current, which is symbolized by the letter f. Another important parameter is the alternating current period, which is symbolized by the letter T, the period and frequency are reciprocals of each other, fulfilling the following equation: The frequency is usually measured in cycles per second or Hertz (Hz). In the next figure we can get a clearer idea of the period and frequency of a wave:
Types of Oscilloscopes
Electronic equipment is divided into two types: analog and digital. Early work with continuous variables while the latter do so with discrete variables. The first work directly with the applied signal is amplified once deflects an electron beam vertically in proportion to its value. In contrast digital oscilloscopes used a prior analog-digital converter (A / D) for digitally storing the input signal, then reconstructing this information on the screen. Both types have their advantages and disadvantages. The analog is preferable when priority display rapid changes in the input signal in real time. Digital scopes are used when one wants to study events and non-repetitive (voltage spikes that occur at random).
Analog oscilloscopes
When the probe is connected to a circuit, the signal passes through the latter and goes to the vertical section. Depending on where we place the control of the vertical amplifier attenuate or amplify the signal. At the start of this block already has sufficient signal to attack vertical deflection plates and are responsible for diverting the electron beam which emerges from the cathode and hits the fluorescent coating inside the screen, meaning vertical. Up if the voltage is positive with respect to the reference point (GND) or downwards if negative.
The signal also passes the section thus shot to start the horizontal sweep (this is responsible for moving the electron beam from the left of the screen to the right in a given time). The layout (move from left to right) is achieved by applying the upslope of a sawtooth to the horizontal deflection plates, and may be controlled in time acting on the TIME-BASE command. The path (path from right to left) is done much faster with the falling part of the sawtooth.
Thus the combined action of horizontal layout and vertical deflection traces the graph of the signal on the screen. The trigger section is necessary to stabilize the repetitive signals (ensures that the route starts at the same point of the signal-repeating). As a conclusion to properly use an analog oscilloscope we need to make three basic settings: The attenuation or signal amplification needed. Using the AMPL command to adjust the amplitude of the signal before it is applied to vertical deflection plates. The signal should occupy an important part of the screen without going to push the limits. The time base. Use TIME-BASE command to set time which is a division of the screen horizontally. For repetitive signals is desirable that the screen can be observed around a couple of cycles. Triggering of the signal. Using TRIGGER LEVEL control (trigger level) and TRIGGER SELECTOR (type of shot) to stabilize the best possible repetitive signals.
Of course, too, must adjust the controls that affect the display: FOCUS (focus), intensity (intensity) never excessive, Y-POS (vertical beam) and X-POS (horizontal beam). Digital oscilloscopes as also explained in earlier sections are an additional set of data processing which can store and display the signal. When you connect a digital oscilloscope probe to a circuit, the vertical section adjusts the signal amplitude in the same way as into the analog oscilloscope. The analog-digital converter data acquisition system sampling makes the signal to set time intervals and converts the DC voltage signal into a series of digital values called sample. In the horizontal section of a clock signal determines when the A / D converter takes a sample. The speed of this clock is called the sampling rate and is measured in samples per second.
The sampled digital values are stored in a memory as cue points. The number of signal points used to reconstruct the signal on the screen is called registration. The trigger section determines the start and end points of signal in the register. The display section receives these record points, once stored in memory to the signal present on screen. Depending on the capabilities of the oscilloscope, one can have additional processes on the sampled points, you can even have a pre-trigger to observe processes that take place before the shooting. Fundamentally, a digital oscilloscope is operated in a manner similar to an analog one, in order to take the steps needed to adjust the AMPL command, TIME-BASE command and control involved in the shooting.
Terminology
There is a general term to describe a pattern that repeats in time wave. There are sound waves, ocean waves, brain waves and of course, stress waves. An oscilloscope measures the latter. A cycle is the smallest part of the wave that repeats over time. A waveform is a graphical representation of a wave. A voltage waveform is always present over time on the horizontal axis (X) and the amplitude on the vertical axis (Y). The waveform gives us valuable information about the signal. At any moment we can visualize the height reached and, therefore, whether the voltage has changed over time (if we observe, for example, a horizontal line we can conclude that in this time interval the signal is constant). With the slope of the diagonal lines in both rising edge and falling edge in, we know the speed in moving from one level to another could be also seen sudden changes in the signal (acute angles) usually due to transient processes.
Types of waves
Waves can be classified into four types:
* Sine waves
* Square and rectangular waves
* Triangular waves and sawtooth.
* Pulses and edges or steps.
Sine waves: These are the fundamental waves and that for several reasons: They have some very interesting mathematical properties (such as combinations of sinusoidal signals with different amplitude and frequency can be reconstructed any waveform), the signal obtained from the sockets of any home have this form, the test signals produced by a generator oscillator circuits are also sinusoidal signal, most of the sources of power in AC (alternating current) produce sinusoidal signals. The damped sinusoidal signal is a special case of this type occur in waves and oscillation phenomena, but are not sustained over time.
Square and rectangular waves: Square waves are basically waves that pass from one state to another voltage at regular intervals, in a very short time. They are usually used to test amplifiers (this is because these signals contain in themselves all frequencies). Television, radio and computers use a lot of these signals, mainly as clocks and timers. The rectangular waves are different from the square to have no equal intervals in which the voltage remains at high and low. They are particularly important for analyzing digital circuits.
Triangular waves and sawtooth: They occur in circuits designed to control voltages linearly, such as, for example, the horizontal sweep of an analog oscilloscope or the horizontal and vertical scanning of a television. The transitions between the minimum and maximum signal change at a constant rate. These transitions are called ramps. The sawtooth wave is a special case of triangular signal with a ramp down of much steeper than the ascending ramp.
Pulses and flanks or steps: Signals, such as the flanks and pulses that only occur once are called transients. One side or step indicates a sudden change in voltage, for example when connecting a switch. The pulse would indicate, in this instance, the switch is connected in a certain time and was disconnected. Usually the pulse represents one bit of information through a circuit of a digital computer or also a small flaw in a circuit (eg a false momentary contact). It is common to find signs of this type on computers, X-ray equipment and communications.
Voltage: Voltage is the electrical potential difference between two points in a circuit. Normally one of these points is usually ground (GND, 0V), but not always, for example to measure the peak to peak voltage signal (Vpp) as the difference between the maximum and minimum value of this. The word usually means the difference amplitude between the maximum value of a signal and ground. In the range of values experienced by an alternating current or a sinusoidal electromotive force in the course of a cycle, the highest possible is when the inductor cut the maximum number of lines of force. This is called "Maximum Value" and is positive to negative 90 degrees and 270 degrees electrical. Instantaneous value is called the value of current or voltage at any one time. The maximum value is an instantaneous value, as well as the value of zero and any other between these two. From a practical standpoint, is of great importance to the "cash value" or rms, which is the value recorded measurement instruments for alternating current.
The rms value is that which produces the same thermal effect (heat) that the of a direct current. For example, if a direct current of 5 amps heated water from a vessel at a temperature of 90 º C, an alternating current that produces the same temperature rise will have an rms value of 5 amps. The average value of a pure sine wave AC is zero, since the positive half-wave is equal and opposite to the negative half-wave. Hence, when speaking of average value always refers to the average of a half-wave. The average value of a symmetrical sine wave is defined as algebraic average of the instantaneous values during a half period. We can also say that the average value is an ordered such that the area of the resulting rectangle equals the area of half period. It is represented by adding the subscript med to the letter of the magnitude of which concerned, Eave, Imed, Pmed, etcetera. Its mathematical expression: Relations between peak to peak values, maximum and effective.
The maximum value is half the peak to peak value and the rms value is obtained by dividing the peak to peak value by, for example if we get on the measured value of peak to peak voltage of 18 volts and wish to obtain the maximum value and rms value, proceed as follows: Then the maximum voltage in this example is 9 volts, the rms voltage is 6.364 volts and the average voltage is 5.730 volts.
Phase: The phase can be explained much better if we consider the sine waveform. The sine wave can be extracted from the movement of a point on a circle of 360 degrees. A sinusoidal signal cycle covers 360 degrees. When comparing two sinusoidal signals of the same frequency may be that both are not in phase, ie, that do not coincide in time steps equivalent points of both signals. In this case it says that both signals are outdated, being able to measure the gap with a simple rule of three: Where T is the time delay between a signal and another.
Parameters that affect the quality of an oscilloscope
Bandwidth: Specifies the frequency range in which the oscilloscope can measure accurately. By convention the bandwidth is calculated from 0Hz (continuing) to the frequency at which a sinusoidal type signal displayed a 70.7% of the value applied to the input (which corresponds to an attenuation of 3dB).
Rise time: It is one of the parameters that will give us, along with its predecessor, the highest frequency of use of the oscilloscope. It is an important parameter if desired pulses reliably measure and flanks (remember that this kind of signals have transitions between voltage levels very fast). An oscilloscope can not display pulses with rise times faster than your own.
Vertical Sensitivity: Indicates the ease of the oscilloscope to amplify weak signals. Data is provided in mV per vertical division, usually in the order of 5 mV / div (reaching up to 2 mV / div).
Speed: For analog oscilloscopes, this specification indicates the horizontal sweep speed, allowing us to observe events faster. It is usually the order of nanoseconds per division horizontal.
Gain Accuracy: Indicates the precision with which the oscilloscope vertical system attenuates or amplifies the signal. It usually provides maximum percentage of error.
Accuracy of time base: Indicates the precision of the system time base oscilloscope horizontal display time. It is also often given as a percentage of maximum error.
Vertical resolution: It is measured in bits and is a parameter that gives us the resolution of A/D converter digital oscilloscope. It tells us that precision signals are converted into digital values stored in memory. Calculation techniques can increase the effective resolution of the oscilloscope.
Operation of the Oscilloscope
The following are the steps for the proper management of the oscilloscope:
Grounding: A good ground is very important for measurements with an oscilloscope. For safety ground is required to place the oscilloscope. If contact occurs between a high voltage and the housing of a scope not grounded, any part of the case, including managers, can give you a dangerous shock. While a well-placed ground oscilloscope, the current in the previous case the user would go through, is diverted to the ground connection. To ground an oscilloscope is needed to unite the chassis of the oscilloscope with the neutral reference point voltage (usually called ground). This is achieved using power cables with three conductors (two for food and one for ground). The oscilloscope needs, moreover, share the same mass with all circuits under test to which it connects. Some oscilloscopes can operate at different voltages and it is very important to ensure it is set to the same at our disposal on the taking of blood.
Be grounded self: Working in integrated circuits (ICs), especially CMOS, you must register yourself grounded. This is because certain parts of these integrated circuits are susceptible to break down the static tension that holds our own body. To resolve this problem you can use a conductive strap that is properly connected to ground, discharging the static electricity that holds your body.
Initial setting of controls: After connecting the oscilloscope to the outlet and feed by clicking on the switch: You should be familiar with the scope's front panel. All scopes have three basic sections that call: Vertical, Horizontal, and Trigger. Depending on the particular employee oscilloscope, we have other sections. There are some BNC connectors, which are placed the measuring probes. Most of today's oscilloscopes have two channels usually labeled as I and II (or A and B). Having two channels allows us to compare signals from very comfortable. Some advanced oscilloscopes have a switch labeled AUTOSET or PRESET to adjust the controls in a single step to adjust the signal perfectly to the screen. If the oscilloscope does not have this characteristic, it is important to adjust the various controls of the device to its standard position prior to measurement.
These are the recommended steps:
- Set the oscilloscope to display the channel I. (At the same time be positioned as the trigger channel I).
- Set to an intermediate scale volts / division of channel I (ie 1v/cm).
- Send command calibrated position variable volts / division (central pot).
- Disable any vertical multipliers.
- Set switch input for channel I in DC coupling.
- Place the automatic shooting mode.
- Turn off the shot off or delayed to a minimum.
- Place the control level to the minimum necessary for assessing the stroke on the screen, and the line of focus adjusted for the cleanest possible display (usually stay with signaling controls close to the vertical).
Probes measure: With the steps above, we are able to connect the probe to measure the channel input connector I. It is very important to use the probes designed to work specifically with the oscilloscope. A probe is not far from a cable with a clamp, but a connector specifically designed to prevent noise that might disturb the measurement. In addition, the probes are constructed to have minimal effect on the measuring circuit. This ability of the probes is called the loading effect, to minimize use a passive attenuator, usually x10. This type of probe is usually provided with the oscilloscope and is an excellent general use probe. For other types of measures are used special probes, such as current probes or activities.
Passive Probes: Most passive probes are marked with an attenuation factor, usually 10X or 100X. By convention attenuation factors appear with the "X factor behind the split. In contrast amplification factor "X" appear in front (X10 or X100). The probe is probably the most used 10X, reducing the signal amplitude by a factor of 10. Its use extends from frequencies above 5 KHz and signal levels greater than 10 mV. The 1X probe is similar to above but introduces more load on the test circuit, but can measure signals with lower levels. For convenience of using special probes have been introduced with a switch that allows use 1X or 10X. When using such probes make sure the position of this switch before making a move.
Probe Compensation: Before you use a 10X attenuator probe is necessary to adjust the frequency on the oscilloscope in particular to be working. This adjustment is called the compensation of the probe and comprises the following steps.
- Connect the probe channel input I.
- Connect the probe tip to the point of compensation signal (Most scopes have a power to adjust the probes, otherwise it is necessary to use a square wave generator).
- Connect the alligator clip of the probe to ground.
- Note the reference square signal on the screen.
- With the screwdriver adjustment, acting on the trimmer to see a perfect square wave.
Active Probes: They provide an amplification before applying the signal to the oscilloscope. May be necessary in circuits with a very low output power. This type of sensors needed to operate a power source.
Current Probes: Allow direct measurement of currents in a circuit. They come to measure AC and DC. They have a clip that covers the cable through which you want to measure current. When not placed in series with the circuit cause very little interference in it.
What can we do with an oscilloscope?
- Measure directly the tension (voltage) of a signal.
- To measure directly the period of a signal.
- To determine indirectly the frequency of a signal.
- Measure the phase difference between two signals.
- Determine which part of the signal which is DC and AC.
- Determine which part of the signal is noise and how this varies over time.
Measurement of tension with the oscilloscope
The screens on the oscilloscope are calibrated with a grid so that in terms of gains selected for the internal circuitry, we may use them as reference points to measure tension. So if the key gain selector was in the position of 1V/div, which corresponds to 1 volt per division, to focus the signal enough to get different readings on its intensity from the waveform. Figure for example, is an example of signal voltage of 3 volts or 6 volts peak-peak voltage, if the key is in position selector 1V/div. This procedure applies only to alternating signals.
Also, the voltages can be measured with the oscilloscope. Once the line centered on the screen, we applied in the vertical input we want to measure voltage. The remoteness of the line in the vertical (up or down) will depend on the input voltage. If the signal is analyzed waveform known-sine, triangular, rectangular-plus peak values easy to obtain other values such as average, the rms value. Likewise if it is an audio signal is known, we can calculate power. In each of the vertical positions of the attenuator can be read directly the voltage required to deflect the stroke an inch, vertically.
This allows us to make measurements of voltage on the screen, whether continuous or alternating. In both cases, the switch coupling placed in the proper position. The measure of an alternating voltage is done by counting the inches or tables in the grid it occupies on the screen signal, multiplying by the conversion factor selected with the switch from portrait, bearing in mind that the larger the space occupied by the signal on the screen, the more reliable the measurement made. When performing a voltage measurement, or its component in a waveform, which we will measure will be the vertical displacement experienced by the deflection from a given reference. This shift also will indicate the polarity of the DC voltage measurement, as towards the top of the grid (positive stress) or to the bottom (negative voltage).
Measuring Times with the oscilloscope
The distance over time, between two given points can be calculated from the physical distance in centimeters between those points and multiplying by the factor indicated on the switch time base. In the above example if the key selector was in the time interval 01 seconds, the cycle time would be drawn 1 seconds, i.e., this would be a wave of period equal to 1 seconds.
Frequency measurement
The natural frequency of a given signal can be measured on an oscilloscope under two different methods:
- Since the measure of a period of that signal by applying the above method and using the formula:
- By comparing a frequency of known value and we want to know.
In this case the oscilloscope is operated under X / Y (external deflection).
Applying each of the signals to the inputs "X" and "Y" of the oscilloscope and in case there is a complete harmonious relationship between them, the screen enters a so-called to the view which you can find the number of times a frequency contains the other and therefore deduct the value of the unknown frequency.
Measure Phase: The previous system of measurement of frequency by using the "curves, may also be used to determine the lag in degrees between two different signals at the same frequency. We work the oscilloscope with external horizontal deflection by applying to its horizontal and vertical inputs (X/Y) the two signals that you want to compare. Through this connection will be formed on the screen a "curve" that we will properly interpreted the phase difference between the two waveforms being compared. In the previous drawings, are examples of the application system.
VCRs Working
Helical Recording: This is the system universally used in video recorders. This format has been chosen with a clear objective, to obtain the highest density of information and record as much time in the film shorter. The recording-playback heads are placed or mounted on a carrier cylinder head that is tilted with respect to the longitudinal movement of the tape. The cylinder comprises two concentric drums, one fixed and one rotating mobile, on which are the heads. The time for each head start and end in contact with the tape determines the information content. In the VCR (video recorders), this time coincides with a field of video information. Example: on a track is recorded odd field information, this information is recorded by the head "A", while on the runway nearby, the head "B" prints the odd field, so it records all information of a schedule. This means that in a full rotation of cylinder, two tracks recorded or read and whose contents will be the 365 lines on a chart according to our rules.
The applicability of this method lies in the switching of heads that occurs once per field. To make this switch produces minimal interference; the change is made on recent field lines before vertical sync pulses i.e. each track begins with the start of vertical sync pulse. The cylinder heads are mounted at the same height but 180 degrees opposite. While the head "A" begins to make contact with the tape on the bottom, the head "B" leaves the contact at the top. In the helical scan, you get a high speed tape-head as the tape progresses slowly. The film moves with a speed of about 2 [cm / s], however the front of the tape head is moved to the tangential velocity of cylinders. The cylinder rotates at 1800 rpm in NTSC standard "M" and 1500 rpm in our system Standard PAL "N", this equates to 30 and 25 turns per second respectively. The cylinder has a diameter of 60 [mm] perimeter accordingly be: P = ∏. D = 3.1416 x 60 = 188.49 [mm] = 0.1885 [m]. The head gives a full turn in 1 / 25 [s] in the PAL N, hence the tangential velocity is: Vt = 0.1885 x 25 = 4.71 [m / s] Applying the equation which is discussed, we can determine the frequency response or frequency limit recording. Here: Note: This example has used a distance d = 0.5 [μm] for the gap. In conclusion, the helical recording method for obtaining a high frequency response, which combined with FM recording, produce the necessary response to the luminance signal.
Safety band (zero guard band) for High-density Recording
To avoid interference between adjacent tracks recorded was a clear space between them, known as a band of space security. Obviously the protection band is an advantage from the standpoint of image quality but has the disadvantage of tape waste by the lack of utilization of total surface of the magnetic material deposited. In the non-professional video recorders, it tends to eliminate the said protection band, eventually the system known as high-density recording. The problem with this system lies in the probability that the head to move on the next track, and reading some information from another field, ie unwanted information. This is further complemented with a mechanical problem of great demand for follow leads. To minimize this drawback, to the extent of abandoning the need for a band without recording protection was devised with the use of air-gap heads tilted. Head "A" is constructed with a gap whose angle to the vertical is + , while the head "B" is constructed with an angle - for the same. The result is that consecutive tracks are recorded with a definite and opposite azimuth. The angle "" is an approximate value of 6 º. During playback head "A" act with maximum efficiency to cover the tracks recorded for the same, just as the head "B" will to walk the tracks recorded by him. Now suppose that one of the heads by mistake advances in monitoring and runs in part by the wrong runway. At various points the gap will be different and even opposing values of magnetic field, so that the average field induced by the cross will be minimal (adopting inclinations are such that the output voltage is almost zero). The head "A" crosses the track itself and part of the track adjacent "B" information "A" will be reproduced correctly, while the information provided by the track "B" will be ignored.
Put another way, the inclinations of the air gaps are equivalent to the effect of a "gap" larger than the actual width. For reference, you can compare with the tilt of the head in an audio recorder and its influence on the reproduction of high frequencies. This technique for eliminating interference from adjacent tracks read by tilting air gap is valid for the luminance signal that modulates a carrier frequency. Treatment of the chrome signal is different and presents problems not solved by this method.
Elementary scheme for recording video
The luminance signal and color are obtained after a separate circuit equivalent to those used in televisions. Being well-defined spectra, the filter does not require special features. If we note that the separation is sacrificed in some frequency response in the luminance signal, as already mentioned. This restriction is part of the permissible limitations for non-professional recording, otherwise they should adopt more complex methods. The color information is limited in its frequency response to 500 kHz. The color signal is applied to a converter or mixer which in turn receives its other input the voltage of a constant frequency oscillator (crystal oscillator) in order to effect the shift of spectrum required, the frequency of 629 KHz. At the output of the converter, we find the color signal with all its properties, i.e. an AM signal suppressed carrier, double sideband, but now centered on the frequency mentioned above. Importantly, we run the entire spectrum and that the sidebands remain far apart and position on the new carrier. As the chrome signal is recorded directly and that their frequencies are low compared to the FM signal containing luminance, both heads opposite azimuth, is not sufficient to eliminate cross-field interference, i.e. between adjacent tracks. This problem and its solution is discussed later as phase shift.
For information on FM luminance records, never mind the linearity or distortions that may occur this signal. For chrome, the situation is different, this message is recorded directly, therefore, be kept good linearity between recording and playback, otherwise they would suffer abnormalities in terms of color hue and saturation. As with audio, for recording color requires a high frequency signal (RF) to be added to the color signal and act by way of pre polarization. In recording this video signal is the same RF carrier frequency modulated with the luminance information. The recording and transcript of the color signal requires lower tolerances than those used for black and white information. For example, small deviations from the color sub carrier, the TV can not recognize the signal and only see black and white image.
Color Reproduction
The recovery of the original signal color conversion is done through a reverse way as in the recording. The imprecision of the mechanical system and the elasticity of the tape, resulting in reproduction as frequency variations. If we call the signal fsc printing, in reproduction we have (± f'sc f) where f error of reproduction (according to the values given above is (629 KHz ± f)). By means of a converter that is applied to the same RF as that used in the recording, it reconstructs the color information in its original value of sub carrier. For the error disappears, a comparator circuit is used, this device compares the output of the mixer with a standard oscillator, the comparator circuit produces a control voltage which changes the oscillator frequency used for conversion, so that the signal has no error output. (4.211 MHz ± f) - (0.629 MHz ± f) = 3.582 MHz
f value has been introduced by adding to the oscillator frequency of 4.211 MHz is the effect of the control voltage. The simplified process used in reproduction.
VHS format - Treatment of the signals
We started from a composite video signal of appropriate amplitude in our case 1 Vpp coming from a tuner or camera. In the case of the VCR (VCRs) actually provides a set consisting of a tuner similar to those used in color televisions, plus an amplifier and its associated detector. The TV signal is transmitted, as we have a carrier frequency, which changes according to the selected channel. But it is always recorded in VHS format. The same signal may be from another VCR in the case of a cassette copy as mentioned before from a camera. We also understand that the audio signal as sound Interporto (IS) has previously been separated, to be derived by another route and recorded on tape in the traditional form of audio recorders in an auxiliary runway.
Recording process - The luminance signal
The composite signal entered, is separated in the same way that a television in three parts namely: luminance signal or sync signal or color sub carrier signal. These signals are separated in the same way that a TV through a system of filters and traps, each is then entered in various circuits for processing. The luminance signal is input separately after a controlled gain amplifier, as you know these amps are used to control and maintain the level of the signal. Then passed to other amplifiers, from this point takes the signal to a sync separator, sync that is used to servo circuits, recording the pulses of control and processing circuits of the color signal. The standard signal (standard) 1 Vpp video has, however, for various reasons, this signal can vary in amplitude, therefore we must ensure a constant level for the proper functioning of the later stages. That is, the signal must be maintained within well defined limits, regardless of variations in input voltage. But no one should act to normal variations of average image brightness.
A system that simply reacts to the signal average or peak to peak amplitudes, reduce the gain when the average brightness and amplify it were high during dark scenes. The result would be a uniform brightness, gray, regardless of the content of the scene. What the system should correct a total amplitude error and not dependent variations of the scene. Consequently the above, it can not be taken as reference material program, the only thing that can be taken as reference is the horizontal sync pulse. For which it has a sync separator. The AGC detector operates as the type triggered the TV. The AGC cocked black compares the value of the signal with the sync. Then in some VCR is used or trap adds another color to remove all traces of the chrome signal. The filter used has a cutoff frequency of 3.38 MHz, the effect of this filter on the luminance signal is to reduce the definition to some 240 horizontal lines. In other words increasing the pixel size reduces the definition or picture quality. The signal continues, being entered into a circuit called emphasis, its function is to improve the signal to noise ratio. As the luminance signal is frequency modulated, and in all cases of frequency modulation, it tends to degrade the response to higher frequencies, degrade signal to noise ratio. The signal to noise ratio of a frequency modulated signal depends on the relationship between carrier frequency and signal frequency, and consequently when the frequency information increases the signal to noise ratio gets worse.
This high-pass filter increases the gain at high frequencies producing an effect of over-drive, commonly known as "Overshot". After the playback will be made the reverse process to restore the relationship to information correct amplitudes. Here are two circuits called latch and trimmer respectively, whose objectives are: Reset the zero reference level, which is lost in pre-emphasis circuit (coupling capacitors); The level of continuous change with average brightness of the scene and should be transferred during the processes-modulation and frequency modulation. On some machines interlocking precedes the pre-emphasis circuits, so that the peaks are generated at this stage are not determinants of interlocking dc level. Peak cut black and white. This allows us to have a maximum level of black and white, not allowing the signal "Y" on module to the VCO (voltage controlled oscillator) from which we obtain the FM signal. The signal thus obtained is entered as mentioned at the VCO, obtaining the corresponding output FM signal or carrier whose center frequency is 3.8 MHz, in all cases, with a sweep of 1 MHz this signal, via band pass filter whose center frequency is 3.8 MHz is applied to the head amplifiers, to be recorded on the tape.
Reproduction of the luminance signal
The signal coming from the head amplifier is composed of two frequencies:
• The color signal with a center frequency of 629 KHz or, 627 MHz, according to the standard and
• FM signal containing the luminance information with a frequency sweep from 3.4 to 4.4 MHz, between the extremes of synchronism and white of the scene.
For the different processes, it needs the information previously separate, voltage output from the head amplifiers, resulting two paths, one a ceramic filter to achieve high-pass luminance signal (FM) and the second band pass filter for color tension. The signal Y separated enters a controlled gain amplifier to adjust levels. The output of the amplifier enters a circuit called "DOC" or "DROP OUT", whose function is to eliminate noises that appear in reproduction. These sounds may be due to: mechanical interference, electronic interference sources or oscillators switched servo control stage, the wear and tear of the tape, all these noises appear as small white dots on the screen in the form of drops. As we know, the sounds are very short duration transients.
To achieve effective implementation of the function uses a level detector, a delay line of 64 microseconds and an electronic key switch. The operation is as follows: the signal at the terminals of the key switch are on one hand the direct signal and the other the signal stored in the delay line for the previous stroke, lack of information detected by the corresponding circuit, during that time the key is switched reproducing information from the previous line ended noise becomes the normal condition. This process is feasible because of the redundancy of information, i.e. the great similarity between two consecutive lines. The corrected signal is amplified again and proceeds to detect it with a circuit similar to those used in FM radio receivers. Then he proceeds to de-stress and to interlock signal to regain the level of zero, and finally amplified again and sent the signal to a mixer, to add color signal. The information thus obtained can be used directly by entering the TVC video input, or is applied to a modulator for transmission to the TCF, to enter by the RF input.
Process of recording color signal
Once separated color signal through a band pass filter whose center frequency is 3.58 MHz and 1.2 MHz bandwidth, the signal is attached to an amplifier gain control (AGC), this amplifier operates in a manner that allows information to exceed certain limits, causing congestion and deterioration of image quality. Then the signal is again filtered, which removes any trace of undesirable such as radio frequency of 4.5 MHz for sound (IS). This signal is inserted into a first converter or converter MAIN, which causes the frequency shift required.
The mixer requires another signal for operation of 4.211 MHz, which in this case stems from a second converter whose behavior explained shortly, allowing the displacement of the spectrum to 629 kHz. For these values of frequency, as clarification is important to mention that change according to the rule, therefore generically continue using the values of 629 KHz, 4.21 MHz and 3.58 MHz, having made that reservation. Like all the output converter circuit yields the sum and difference frequencies, a resonant circuit through which we want to adopt. The second converter receives at its inputs the following voltages: The first, coming from a crystal oscillator at 3.582 MHz and phase control. The phase control is effected by a phase comparator circuit which teem BURST pulses from the channel or camera.
Second, compilation is somewhat more complex. As a starting point, this signal is obtained by multiplying the horizontal frequency, by reference to the horizontal sync pulses. The frequency can be obtained 320 fH = 5 MHz or 160 fH = 2.5 MHz This tension is obtained from a VCO and delivered in the form of bursts or flashes when it is present the horizontal pulse. Then the tension enters a frequency divider circuit, this frequency divided by 8 if the incoming signal is 5 MHz or 4 if the input is 2.5 MHz in both cases the resulting frequency is 625 KHz . Part of this signal is fed back, to a phase comparator to effect control over the VCO frequency and keep as stable as possible.
One reason to perform an oscillator frequency of 4 or 8 times higher than is necessary, so you can get a more stable frequency drift frequency of 50 Hz in the oscillator will also appear divided on exit. After the division is performed the phase shift, this procedure is an encoding that is different for each system (PAL or NTSC). This point will be expanded later. This form of signal processing makes every head knows what the track that corresponds to read. Supplemented to the control pulses that are recorded in the bottom of the tape as an auxiliary signal, the control pulses are used not only for the operation of the circuit controlling the tape speed but also will position each head on the track right at the appropriate moment. It also reduces interference between adjacent tracks color.
Finally, after the phase shift is entered the second converter, also called sub-converter or converter side, where the output is obtained by the sum of the two frequencies [3.582 MHz + 625 kHz = 4.21 MHz] this tension resulting is usually separated by ceramic filters to finally enter the first converter (MAIN CONVERTER) as was mentioned above. At the start of the first converter we have full color information, ie a spectrum of double sideband and suppressed carrier centered on the new frequency of 629 KHz and a symmetrical bandwidth of ± 500 kHz. This output signal is achieved through a band pass filter, before being amplified and attached to a mixer circuit which will join the luminance signal (FM). The information emanating from the mixer output is applied to the heads through the respective rotary transformers.
Reproduction of the color signal
As discussed above the color signal undergoes heterodyne (Mixing) taken a carrier frequency of 629 KHz. The tension that comes from the head is a composite, made up of the FM signal containing luminance information, but (mixed-together), the color information, centered at 629 KHz. The latter signal is separated by a band pass filter with a bandwidth of ± 500 KHz and amplified by a controlled gain stage. This processor is the same used in the recording, unless reversed input filters in place of the 3.58 MHz filter (recording) is placed on 629 KHz (for playback) and added some stages. Once separated, the color information signal is applied to a controlled gain amplifier for the equalization of levels, then some more elaborate machines are usually placed another band pass filter to remove any trace of the FM signal. Then inserted to the first converter also called general converter or converter (MAIN CONVERTER). In this circuit the two signals entering the aforementioned of 629 KHz 4.21 MHz and a variety latter complex or elaborate, as in the recording process. From the controlled gain amplifier Burst Media is removed from a separate circuit for the phase control oscillator 3.58 MHz. The 3.58 MHz signal phasing is inserted to the second side converter or converter to be mixed with 625 KHz signal coming from the phase shift circuit. The 625 KHz signal from a controlled oscillator multiplies the horizontal frequency by reference horizontal pulses for regulation. The oscillator delivery [320 fH fH or 160] by the machine. This frequency obtained is divided by 8 (eight) or 4 (four) as appropriate. Obtained as a result of 625 KHz. This oscillator is a circuit that takes some of the output for refueling through a phase comparator for controlling the oscillator. This procedure is the same as made in the recording. The output of this circuit is 625 kHz phase enters the phase shift, which performs the reverse process in the recording made to achieve reconstruct the color signal with all its original properties.
Information gathered, as mentioned Entering secondary converter to beat it (mixing) with the RF of 3.58 MHz as a result of the mixture gives the sum and difference of those adopting the sum, 4, 21 MHz, using a band pass filter. After passing through the filter, the voltage is applied to the main converter where it multiplies the read voltage (629 KHz) of the output color obtained through a band pass filter, the frequency difference, 3.58 MHz, and is the signal color formed. With the respective amplitude modulation, suppressed carrier and bandwidth ± 500 kHz; characteristics of the chrome information. We now need amplifying and eliminate interference that occur. The killer is a circuit that disables the offset amplifier operation when it has lost the phase reference is the signal is lost identification, under these conditions is reproduced in black and white.
Head amplifiers and switching signals
The amplification system and switching heads: As is known should be considered two different situations in the explanation (RECORD) and reproduction (playback). Burning-in control terminal that switches electronic keys (REC) is a voltage of 12 V, this will close the keys grounded lower terminal heads, which in turn connects to mass input amplifiers reproduction, eliminating the source of internal noise in integrated circuits that can be induced at other stages. In the terminal control key (PLAY) the tension becomes zero, maintaining and ensuring that the key is open. The tension to be recorded is obtained at the output of MIXER (adder) FM signals modulated with luminance and color information with the respective phase shift centered at 629 KHz. This signal can not be incorporated in the head in this way, as explained in the introduction or the beginning of recording, the signal heads should be incorporated, must be a current or voltage proportional to the information recording. For all this, the step required is a voltage-current converter circuit between the mixer and the headers.
According to what is observed in the layout of the circuit, both heads are parallel, so there is no switching in recording and a head that is not supported on the belt despite being energized and no function either generates interference. The head is magnetizing the air. Reproduction: In this situation appears to excite the required voltage and close the electronic key (PLAY) and the potential vanishes (becomes zero) closing keys (REC). Under these conditions, the upper end of the headers is earthed and lower terminals connected to the amplifier head ready for playback. Having opened the keys (REC), the inputs of these amplifiers are open, allowing the entry of the information is read from the tape. Then, we must go through a stage of switching heads, an electronic key role in detecting mutation in which is supported on the tape head, ie the active head. The key switch heads (HEAD SW) receives the information directly from the carrier cylinder heads, through a strain called "PG". It is important to emphasize that in this way prevents the head off (the air is), do not inject the circuit noise or interference. There has been an express mention, but the heads are attached to the electrical circuit through a rotary transformer, where the static winding is connected to the switching circuit and connected to rotational head.
Phase rotation
Recall that the fields consist of a signal read by the head "A" and another read by the head "B", or adjacent parallel tracks. If there is an error reading from any of the heads (CROSS TALK), there will be an interference, which translates to the screen, which are colored lines going from bottom to top, magnified depending on the colors red and blue. Sure, this happens if the correction is not made adequate or appropriate treatment of the color signal recording and playback. This process is already mentioned and called phase shift. The method starts from the principle that the color information between consecutive lines or close to any image, is very similar (the television picture has a high rate of redundancy). If we draw a perpendicular imaginary anywhere on the screen, is easy to see that between adjacent lines of a frame the color is almost the same. Is taken as the first example a machine working in NTSC, this system no phase change in the color information, therefore the process is simpler.
This reasoning is also to refer to the BURST phase, for which the color information takes the stage and to further simplify the explanation we consider a saturated color field, i.e. a full screen image or the same color other than white or black. For example red. By appropriate switching, the phase of the color information and BURST, will be changed, with an increase of 90 º line by line, differing both heads so that the increase for the head "A" is positive and increasing the torque Head "B" is negative. The statements shown in the table below where there are phases (rotational vectors) representing the color voltage on each line. While the phase signal is sent to the head "A" line by 90 degrees each, which is sent to the head "B" is late 90th. We understand progress, draw opposite to the waters of the clock. We know this is all we are doing to eliminate cross-field interference. So it is important to know which is the reading of the headers, the own and cross country.
The corrective effect is achieved by proceeding in reverse, making recover the original information, but now with the contents of interference. The result, each of the reports themselves have the same phase as in the recording, while the interferences have opposite phases between consecutive lines. So far, there is no evidence that this procedure is done to eliminate or minimize interference. But it's really efficient, because, assuming the interference of equal magnitude, but opposite, when the time of observation the box on the screen, the human eye resolved in an additive, making the cancellation of interference. In conclusion, the eye as a result of the persistence in the retina, which is canceled or minimizes interference. They say minimize, because in an image with movement (an actual scene), we can not ensure equality of interference between consecutive lines, but they are very similar (redundant information).
In the PAL system the phase shift process is somewhat different, due to the phase change of the system itself. This causes additional problems that we try to explain in simple terms. The turns are introduced into one of the heads, in this case the "B" and are the same as those made for the NTSC system, the former remains unchanged. So far everything seems easier, but the complication comes next. In the first section we discussed the signal phases as it originates, then the two lines is the signal applied to each head, the "A", not the additional rotation features. The next two rows are the phases representing the respective reading with crossed-field interference. Finally, in the last two rows, one has the reverse rotation and re-composition of the signal. This restores the original phase information, while interference again be out of phase, but with a difference, now is the opposite phase every two lines. The result is the same additive effect on the retina of the eye cancel interfering information. All these processes of phase shift, do not completely eliminate the interference of cross-field color should be complemented with the inclusion of a comb filter. The comb filter is not limited to processors implemented in color, but there is a much more general.
This type of filter can separate frequency spectra that reach the entry form intertwined. A basic circuit and consists of an adder and a delay line. In the NTSC system must take into account the phase shift introduced to the chrome signal, as a consequence of the adder at the entrances will have information from two consecutive lines where information of adjacent lines is in phase opposition. As a result, the output of the adder will have eliminated the vestiges of interference and duplicated head own information.
We should clarify that the combination of two adjacent lines is possible because the information content changes very little from one another online. In the PAL system the process of eliminating this is something more complex, given that we are interfering information that is out of phase every two lines, which means using for the cancellation of the delay line of a tH duration 2.
Introduction
To know more about a barcode scanner you should understand barcode. Each barcode is represented with specific symbols which are a series of bars. The strip of bar can consists of a number, character or alphanumeric character. In general a barcode consist of a start and end bar to indicate the starting and ending point of the barcode. There is an extra bar called the checksum bar whose purpose is to determine whether the barcode is correct after calculation and ensures the correctness of the barcode. And the barcode scanner’s photo sensors can read the barcodes and convert them to electrical pulses.
There are many varieties of barcode scanners models available and the usage depends on the type of industry. Some examples are desktop models, handheld and portable models. And the most popular type is the wireless barcode scanner. The names its self tell about it that it works wireless. They can connect to the base station or workstations wirelessly and can transmit data back to the terminal as long as they are within the wireless range radius. They allow you to read bar code labels from items without having to be attached to the base or computer by cord. They include a base from which they communicate much like a cordless phone and the base is responsible for charging the bar code scanner as well as communicating with it.
Where is it more useful?
Wireless barcode scanner is often used in the store industry where workers can easily handle the product and can scan a barcode. They are mainly used in industries where goods are required to move frequently. Using a wireless barcode scanner allows freedom of movement. Wireless barcode scanners are convenient than the handheld barcode scanners because they are not restricted by a cord. Benefits of using a wireless barcode scanner of any type can be very great and now they even come with Bluetooth technology. By using this technology the scanner can communicates with a range of products as long as they are also equipped with the Bluetooth technology.
Let us see some top wireless barcode scanner manufacturers. These are the lists of top manufacturing companies:
Symbol Wireless Barcode Scanner
PSC Wireless Barcode Scanner
Hand Held Wireless Barcode Scanner
Janam Wireless Barcode Scanner
And below are names of some other manufacturing companies that are not in the top list but are manufacturing Barcode Scanners of different types:
AML Wireless Barcode Scanner
HHP Wireless Barcode Scanner
Metrologic Wireless Barcode Scanner
Opticon Wireless Barcode Scanner
Percon Wireless Barcode Scanner
Unitech Wireless Barcode Scanner
Welch-Allyn Wireless Barcode Scanner
Though the wireless barcode scanner is a biggest investment, implementing a barcode system will always save your time and money. And when you want the best it is going to cost a little higher.