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Digital Video

Digital video systems work by taking an analogue signal, converting it to digital form with an analogue to digital (A/D) converter, compressing the signal by removing redundant information and finally transmitting it over a digital network where the signal is decompressed. This is the derivation of the word CODEC, CODer/DECoder.

The key parameter characterising digital video is the data rate which is measured in bits per second (bps). Transmission rates range from 64kbps (64 thousand bps) to 140Mbps (140 million bps). The quality of the received vide is dependent on the data rate.

Effects of compression

The BBC uses a data rate of 140Mbps on digital transmission links which gives results which are as good as and in some cases better than analogue links. Channel 4 uses moderate compression of the data stream to produce a bit rate of 35Mbps. This gives results which are visually as good as 140Mbps albeit with a slight delay between the transmitted and received signals due to the compression/decompression process. Below ~35Mbps the loss in picture quality due to the reduction of data rates start to become noticeable.

The quality of the received signal is characterised by its resolution and delay. Increasing bit rates from 128kbps to ~700kbps results in an increase in the definition of the received picture whilst above this rate the picture quality remains the same but the delay in reconstruction is reduced due to the smaller amount of processing needed.

SuperJanet, the successor to the Joint Academic Network (JANET), carries video at 2Mbps. Rapid changes of picture content such as occur when switching between video sources cause slight delays and loss of definition whilst the picture is being updated. Overall quality is roughly equivalent to a good domestic VHS video recorder. This has been found to be acceptable for most meetings, lectures and seminars.

Transmission at 384kbps (six 64kbps ISDN channels) provides a clear and stable picture for scenes with minimal changes in picture content. However when there are major changes in the transmitted picture the quality of the received picture degrades, there is loss of resolution and noticeable blurring.

At 128kbps (as used by the BT VC7000 video phone) picture quality is noticeably jerky and resolution bad with quite noticeable delays in movement. Due to the jerky nature of the video signal there is a noticeable loss in synchronisation between the received audio and movement of the speakers lips. During sessions run by LIVE-NET at 128kbps it was necessary to keep pictures from the overhead camera and slides still for an appreciable time in order to allow the picture to stabilise. If both sides attempted to speak at the same they would experience difficulty in the audio in the form of clipping.

The bandwidth of a digital video signal is split into two sections, one for the video and the other for the accompanying audio. The data rate for audio must be at least 48kbps to achieve an acceptable quality. For a 128kbps channel this results in only (128 - 48) = 80kbps being available for the video. If the data rate is doubled to 256kbs the bandwidth available for the video signal nearly quadruples resulting in a dramatic increase in picture quality.

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