Compression Formats for Digital TVs
To see the need for compression formats, I will summarize the process of digitization of analog video. The standard defines the analog video lines per frame and frames per second (not all lines containing active video). To digitize analog video signal to sample all the active video lines. Each color sample is encoded into YUV signal (Y-luminance, U and V chrominance). An example of conversion from analog color television to a digital video signal is:
PAL: 576 active lines, 25 frames per second, for 720 pixels and 8 bit per sample to 13.5 Mhz:
- Luminance (Y): 720x576x25x8 = 82,944,000 bits per second
- Chrominance (U): 360x576x25x8 = 41,472,000 bits per second
- Chrominance (V): 360x576x25x8 = 41,472,000 bits per second
Total number of bits: 165,888,000 bits per second (approx. 166Mbits/sg). None of the common systems of video transmission provide sufficient transfer this wealth of information (the Video CD has a transfer rate of 1.4 Mbps and 6 Mbps cable)
Different Types of Compression
It is a standard defined specifically for video compression, used for transmission of digital video images. The algorithm used to compress still images also compares the present with the previous frames and future to save only the parts that change. The signal includes digital-quality sound. The disadvantage of this system is that due to its high complexity needs to rely on specific hardware.
There are different options depending on the application:
Standard MPEG-1 Video-CD chosen: VHS-quality digital sound.
MPEG-2 is used in the DVD (Digital Video Disk). Quality than MPEG-1.
High quality MPEG-3 video: 1920x1080x30 Hz with transfers between 20 and 40 Mbps
MPEG-4 is in development.
It basically consists of treating the video as a sequence of still images and independent compression and decompression using the JPEG algorithm, then, reassemble the video image. This can be done in real time and even with little investment in hardware. The disadvantage of this system is that it can be considered as a video standard for even includes the audio signal. Gold problem is that the dependence of transfers tends storage system, since the compression rate is not very large. In practice it is feasible to achieve the quality SVHS thus may work semi.
The conception of television more efficient (better quality) has brought the need to introduce digital television systems. The insertion of this technology we can see from how to work the TV signal from procurement, through the presentation of a television screen, so you could say that there are two general approaches to digital TV working from treatment Signal:
The analog signal is converted to digital TV to be broadcast and then be brought back to analog at the receiver, thereby increasing the efficiency of transmission and also has the ability to add new features.
A digital system itself, in which the signal is digitized before leaving the chamber for further processing and digital broadcast until presented to the receiver.
In evaluating the benefits of Digital TV among which we can cite: reliability, efficient multiplexing, miniaturization, data processing management, less calibration problems, increased complexity in managing capacity, versatility and have greater capacity channels we would realize that is the way best suited to the present and future changes taking place in humanity.
In addition, digital communications have benefited by the tendency of industry to use solid-state components for the construction of their systems. These components provide greater reliability than any other component used in analog systems. Thanks to these solid state components such as integrated circuits, the size of communications equipment has been tremendously reduced and consequently the data management is more efficient than analog techniques.
Modern trends in communication are aimed at the creation of increasingly complex systems that are managed with great ease due to digitization. This is due largely to the versatility of digital systems.Currently, digital TV is not widely used in the field of commercial television because of some problems of bandwidth, but, however, these problems are being overcome. Digital television in its infancy has been exploited in the field of aerospace research in studying the moon and other planets in the test which proved successful.
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