156 The Technology of Video and Audio Streaming
156 The Technology of Video and Audio Streaming Table 8.2 Channel Data Parameters Uncompressed SD broadcast ADSL or Analog SD video source television cable modem modem Frame size 720 480 720 480 192 144 160 120 Frame rate 30 30 15 5 Color sampling 4:2:2 4:2:0 YUV12 YUV12 Video source rate 166 Mbit/s Uncompressed data rate 124 Mbit/s 5 Mbit/s 1.15 Mbit/s after scaling Target data rate 4 Mbit/s 500 kbit/s 35 kbit/s Total data reduction to 40:1 330:1 4700:1 meet target rate Scaled data rate 1:1.33 1:33 1:144 Compression from scaled 30:1 10:1 30:1 rate to target rate Note: SD is standard definition. The table shows how scaling down before encoding reduces the uncompressed data rate. A thumbnail video for a dial-up modem requires that the data rate is reduced to nearly 1/5000 of the original. This is accomplished by scaling down by a factor of 144:1, and then compressing by a further 30:1. Figure 8.1 shows the relative file sizes for encoded media. Data compression The raw data rate from a standard definition television camera is 166 Mbit/s. This has to be reduced to rates as low as 30 kbit/s for delivery to dial-up modems. How is a reduction in video data of almost 5000:1 achieved, while retaining a viewable picture? This reduction takes place in several steps. The scaling usually takes place on the video capture card. The compression is performed by the codec, a software application running on the main processors. The usual way to encode is first to capture the video with a dedicated PCI card installed in the encoding workstation. The card converts the video signal into an intermediate format that can be handled as a computer file, usually one of the Audio-Video Interleaved (AVI) formats. AVI has become the
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