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VoIP Voice and Fax Signal Processing by Sivannarayana Nagireddi

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3.2 G.711 COMPRESSION

G.711 is an ITU-T-G.711 Recommendation (1988) pulse code modulation (PCM) logarithmic compression. The G.711 codec is an example of waveform-based compressing [URL (Cisco-coding)], which means that the decoder reconstructs the actual signal without making an assumption on any speech models. G.711 is supported in PSTN, hardware interfaces for voice samples, and VoIP deployments. PCMU is PCM using μ-law, which is popularly used in North America. PCMA is PCM compression using the A-law scheme used in Europe and some countries of Asia. PCMA and PCMU together are called as G.711-based compression scheme. G.711 takes a smaller code size of the order of 100 lines, and processing is insignificant compared with other voice processing operations. The basic difference between PCMU and PCMA [URL (PCM), URL (TI-PCM)] schemes are actual quantization, coding steps, dynamic range, and bit formats. Some more details on G.711 and power-level calculations are given in the subsequent sections.

3.2.1 μ-Law Compression of Analog Signal

In analog representation [Bellamy (1991), URL (Cisco-coding), URL (TI-PCM)] or for a continuous signal of μ-law compression, maximum input amplitude is mapped to normalized logarithmic output of ±1. For a given input x, the equation for μ-law encoding (compression) output y is given as follows:

image

In the equation, μ = 255 is the compression parameter, ...

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