near schemes. This is because humans are more sensitive to small changes at low volume than changes of the same magnitude at higher volumes. ITU-T Range of Description of Encoding Used with ITU-T Number Compression Technique (Comments) Codec Recommendations G.711 56-64 Kbps Pulse Code Modulation (PCM) of H.320 voice frequencies G.722 64 Kbps 7 kHz audio-coding within 64 H.320, H.322, H.323 Kbps (PCM) G.722.1 24 or 32 Kbps Modulated Lapped Transform H.320, H.323 (MLT) G.723 5.3 and 6.3 Multipulse-Maximum H.322, H.323, H.324 Likelihood Quantization (MP- G.728 16 Kbps Low-Delay Code Excited Linear H.320, H.322, H.323 Prediction (CELP) G.729 8 Kbps Conjugate-Structure Algebraic- H.323 Code-Excited Linear-Prediction (CS-ACELP) G.729A 8 Kbps CS-ACELP H.323 Figure 6-5. CCITT/ITU-T Audio Encoding Recommendations. The CCITT (now known as the ITU-T) has standardized PCM as G.711 audio. Using G.711 coding, speech and sounds with bandwidths of approximately 3.4 kHz are converted to 56 or 64 Kbps bit streams. A slightly more recent ITU-T standard, G.728, encodes 3.4 kHz audio frequencies Coding and Compression into 16 Kbps bit streams. It encodes speech to fit a simple, analytical model of the vocal tract. It generates synthetic speech that is very similar to the original waveform. The code-excited version LPC goes a step further to compute the errors between the original speech and the LPC model. It transmits both the model parameters and highly-compressed representation of the errors. CELP results in compression that is remarkably faithful to the original signal. Indeed, G.728 audio virtually replicates G.711 quality at one-quarter the bandwidth. There are many different audio-encoding standards. The ones that relate to video communication are listed in the chart below, along with an indication of with which ITU-T video compression standard they are associated (for a thorough discussion of video compression standards, refer to Chapter 7). DIGITAL SIGNAL PROCESSORS Compression methods can be categorized in one additional way