odec for Audiovisual Services at pX64 kbps, which is described in the following. 16.10.2 The pX64 kbps Codec The pX64 codec has been designed for use with some of the common ISDN data rates, specifically the B channel (64 kbps), H channel (384 kbps), and H11/H12 channels (1.536/1.920 Mbps) for the equivalent DS1/E1 data rates. A functional block diagram of the codec (coder/decoder) is shown in Figure 16.9. However, the pX64 system uses a standard line rate (i.e., 525/625 lines) rather than the reduced line rate structure mentioned earlier. The reduced line rate techniques are employed in proprietary systems produced by such firms as Compression Labs Inc and Picture Tel. One of the most popular data rates for conference television is 384 kbps, which is six DSO or E0 channels. However, it is not unusual to find numerous systems operating at 64/56 kbps. 16.10.2.1 pX64 Compression Overview Sampling Frequency. Pictures are sampled at an integer multiple of the video line rate. A sampling clock and network clock are asynchronous. TELEVISION TRANSMISSION External control Coding control Source Video multiplex Transmission Transmission coder coder buffer coder (a) Video coder Video Coded signal stream Source Video multiplex Receiving Receiving decoder decoder buffer decoder (b) Video decoder Figure 16.9 Functional block diagram of the px64 video codec [From Figure 1/H.261, ITU-T Rec. H.261 (Ref. 20).] Source Coding Algorithm. Compression is based on interpicture prediction to utilize temporal redundancy, and transform coding of the remaining signal to reduce spatial redundancy. The decoder has motion compensation capability, allowing optional incorpo- ration of this technique in the coder. There is optional forward error correction 10 available based on the BCH (511,493) code. The codec can support multipoint operation. 16.10.2.2 Source Coder. The coder operates on non-interleaved pictures occurring 30,000/1001 (approximately 29.97) times per second. The tolerance on the picture fre- quency is +50 ppm. As in Section 16.9.