hout introducing significant distortion. Figure 3.16 shows a block diagram of a hybrid video coder. It relies on interframe prediction, followed Structure of the Coded Information Spatial Spatial Inverse Frame Buffer Fig. 3.16 Generic DCT based hybrid codec. Temporal Feedback Camera Coder Channel Buffer Prediction Fig. 3.17 Simplified codec Frame with buffer feedback to control quantization settings. Memory by coding of the prediction error, typically using a DCT. Therefore, when there is a lot of unpredictable movement in the scene the coder generates a high bitrate, but when there is little or no movement it generates a low bitrate. The output of the coder is thus a variable bitrate (VBR) and is suitable for transmission over most packet-based networks. If the codec is required to be used over a constant bitrate (CBR) channel, then the addition of a smoothing buffer on the output is required; this is connected to the quantizer as shown in Figure 3.17. As the buffer overflows, the quantizer step size is increased SO the coding process pro- duces fewer bits. This feedback introduces a time-varying impairment into the video. Thus, these types of compression schemes, when operated at substantial compression ratios, introduce either bitrate or quality variations. Structure of the Coded Information The previous sections have discussed the use of compression techniques to reduce the overall bitrate required to code video. Having undergone heavy compression by exploiting spatial, temporal, and statistical redundancy, each remaining bit is worth N bits from the original uncompressed bitstream, where N is the compression ratio. The result is a stream of bits with an increased value. If this stream were transmitted in this basic form, a single Chapter 3 Digital Video Transmission error within the stream would make all subsequent information difficult or impossible to decode. This can be likened to reading a book where all of the letters have shifted by an unknown amount (e.g., a C becomes an e and a d becomes an f); even 