8.5 Voice Compression

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Even though 64 Kbps is the industry standard for encoding and transmitting voice using PCM, this is still a lot of bandwidth to allocate to voice traffic on a private network. Therefore, alternative voice-carrying capabilities can make more efficient use of the available bandwidth, which also would result in substantial cost savings. The voice compression algorithm ADPCM offers toll-quality voice at 32 Kbps, cutting the bandwidth requirement in half without sacrificing voice quality. Other implementations of ADPCM permit reliable voice transmission at 24 and 16 Kbps, which allows the user to further reduce the bandwidth required for voice.

The conversion from PCM voice to ADPCM voice can even be implemented “on the fly” without interrupting calls in progress. The conversion may be implemented through time-of-day reconfiguration, event-driven reconfiguration, or manually through keyboard command at the multiplexer’s system controller.

Other compression techniques can bring voice down to 10 Kbps or lower. These very low bit rate voice (VLBRV) techniques are typically employed in situations where the bandwidth is close to full utilization, such as during peak hours of the day, but the organization still has an additional requirement for voice and does not want to pay for more bandwidth. Because of the trade-off between quality and maximum bit compression, VLBRV schemes are more suited for internal communication than customer service lines, for example, where the detectable loss of quality may be a critical factor in providing support.

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