Voice Compression

Two basic variations of 64 Kbps PCM are commonly used: ^-law and a-law. The methods are similar in that they both use logarithmic compression to achieve 12 to 13 bits of linear PCM quality in 8 bits, but they are different in relatively minor compression details (^-law has a slight advantage in low-level, signal-to-noise ratio performance). Usage is historically along country and regional boundaries, with North America using ^-law and Europe using a-law modulation. It is important to note that when making a long-distance call, any required law to a-law conversion is the responsibility of the ^-law country.

Another compression method used often is adaptive differential pulse code modulation (ADPCM). A commonly used instance of ADPCM is ITU-T G.726, which encodes using 4-bit samples, giving a transmission rate of 32 Kbps. Unlike PCM, the 4 bits do not directly encode the amplitude of speech, but they do encode the differences in amplitude, as well as the rate of change of that amplitude, employing some rudimentary linear prediction.

PCM and ADPCM are examples of waveform codecs—compression techniques that exploit redundant characteristics of the waveform itself. New compression techniques were developed over the past 10 to 15 years that further exploit knowledge of the source characteristics of speech generation. These techniques employ signal processing procedures that compress speech by sending only simplified parametric information about the original speech excitation and vocal tract shaping, requiring less bandwidth to transmit that information.

These techniques can be grouped together generally as source codecs and include variations such as linear predictive coding (LPC), code excited linear prediction compression (CELP), and multipulse, multilevel quantization (MP-MLQ).

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