Voice Technology Basics
Voice Technologies Compression
What makes voice compression possible is the power of Digital
Signal Processors. DSPs have continued to increase in performance and decrease
in price over time, and as they have, it has made it possible to use new compression
schemes that offer better quality and use less bandwidth. The power of the DSP
makes it possible to combine this traffic onto a line that formerly supported
perhaps only a LAN connection, but now can support voice, data, and LAN integration.
Looking at this chart, quality and bandwidth tend to trade off. PCM is the standard
64Kbps scheme for coding voice; it is the standard for toll quality. The other
compression schemes - ADPCM at 32Kbps, 24Kbps and 16Kbps - offer less quality
but more bandwidth efficiency. The newer compression schemes -LDCELP at 16Kbps
and CS-ACELP at 8Kbps - offer even higher efficiency but with very high quality
very acceptable in a business environment.
ADPCM—Adaptive Differential Pulse Code Modulation:
consumes only 32 Kbps compared to the 64 Kbps of a traditional
voice call; often used on long-distance connections.
LPC—Linear predictive code: a second group
of standards that provide better voice compression and, at
the same time, better quality. In these standards, the voice
coding uses a special algorithm, called linear predictive
code (LPC), that models the way human speech actually works.
Because LPC can take advantage of an understanding of the
speech process, it can be much more efficient without sacrificing
voice quality.
CELP—Code-Excited Linear Predictive voice
compression: uses additional knowledge of speech
to improve quality.
CS ACELP: Further improvements known
as conjugate structure algebraic compression enable voice
to be coded into 8-kbps streams. There are two forms of this
standard, both providing speech quality as good as that of
32-kbps ADPCM.
Voice Quality Guidelines
Silence Suppression by Voice Activity Detection
Voice activity detection (VAD) provides for
additional savings beyond that achieved by voice compression.
Telephone conversations are half duplex by nature , because
we listen and pause between sentences. Sixty percent of a
64-kbps voice channel typically contains silence. VAD enables
traffic from other voice channels or data circuits to make
use of this silence.
The benefits of VAD increase with the addition of more channels,
because the statistical probability of silence increases with
the number of voice conversations being combined.
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