Specifications

VoiceFinder VoIP Gateway Configuration Guide (APOS 2.0) Release Version 3.1
AddPac Technology Co., LTD
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3.1.2. Codec and Mean Opinion Score
Codec (Coder-Decoder) is a device which converts the voice analog signal to the digital beat
stream and the digital beat stream to the analog voice signal.
In general, PSTN uses PCM Codec. PCM samples the analog signal 800 times per seconds (the
sampling interval is 125 micro-seconds) and converts the analog sound to the digital signal by
changing each sample to the numeric code. For this PSTN network, PCM uses 8 bit for the
code, therefore, the standard bandwidth requirement is 64 Kbps.
Sometimes the other compression format, Adaptive Differential Pulse Code Modulation
(ADPCM), is used. A typical example of ADPCM is to encode by using 4-bit in ITU-T G.726
standard, the bandwidth is 32kbps. This 4-bit compression format does not encode the voice
amplitude directly, but it encodes the amplitude difference rate as a very elementary prediction
method.
PCM and ADPCM utilize the characteristics of repetition with Waveform and they are the
example of compression technique. The new compression techniques for utilizing the
characteristics of voice generating source has been developed during the last 10 or 15 years.
These techniques were used for the signaling process for compressing voice by sending the
cyclic information which represents the original voice vibration and the lingual shape. So this
information requires a bandwidth for transmission. These techniques can bind ‘source’ codec
together which includes the variance formats such as LPC (Linear Predictive Coding), CELP
(Code Excited Linear Prediction) and MP-MLQ (Multi-Pulse, Multi-Level Quantization).
CELP, MP-MLQ, PCM and ADPCM coding method has been standardized in the
recommendation of the international standardization institution, G. series of ITU-T.
The followings are the formats of voice coding which are used most often in a call shop and
packet voice.
z G.711 : This is the format that stipulated 64Kbps PCM voice coding, which has been
explained earlier, and delivers voice through PSTN or PBX.
z G.726 : This format stipulates 40, 32, 24, 16Kbps of ADPCM voice coding and
commonly used in PSTN, PBX. However, PSTN and PBX network must have ADPCM
processing system.
z G.729 : This format stipulates CELP compression, which can code voice to 8Kbps stream.
In this standard, there are 2 variance formats (G.729 and G.729a). Complexity involved in
calculation has a big difference in these 2 variance formats, but the both provide 32 Kbps