the signal. Thus, it is important to look for efficient coding schemes. Video Communications: The Whole Picture Coding Standard Coding Method Bit Rate G.711 64 Kbps G.721 ADPCM 32 Kbps G.722 ADPCM 64 Kbps G.723 ADPCM 20, 40 Kbps G.723.1 LD-CELP 5.3, 6.4 Kbps G.726 ADPCM 16, 24, 32, 40 Kbps G.727 ADPCM 16, 24, 32, 40 Kbps G.728 LD-CELP 16 Kbps G.729 8 Kbps Figure 13-4. ITU digital voice standards for speech codecs Almost all audio codecs included in video communication systems will comply with the ITU-T specifications described therein. The important ones for group-oriented systems are G.711, G.722, G.728, and G.729. As said earlier, G.728 audio compresses voice to 16 Kbps. G.711 and G.722 audio compress audio to 64 Kbps and, in an ISDN BRI connection, use half the network bandwidth for voice. G.711/G.722 compression makes sense at higher-end transmission speeds (712 Kbps). The difference between the two is that G.722 samples at twice the frequency of G.711 (7 kHz and 3.5 kHz, respectively) and thereby produces a more faithful representation of sounds other than simple speech. There is also G.723 encoding, developed by the DSP Group of Santa Clara, California (standardized by the ITU-T). G.723 uses Multipurpose-Maximum Likelihood Quantization (MP-MLQ) to diminish voice to about 6 Kbps. CELP predicts speech through a process of vocal tract modeling. It sends only the error (difference between the model and real events). CELP is useful in low-bandwidth conferences, because it leaves more bandwidth for video transmission. When inquiring about proprietary audio algorithms, it is important to ask the supplier to provide information on the range of frequencies encoded with each type and the respective data rates produced, as well as how bandwidth is assigned to audio. The more bits available for audio, the better the quality. Some systems allow the end-user to specify the audio bandwidth, and others offer flexible bit-rate audio that changes with the amount of video information transmitted. Some use rigid schemes that a