User`s guide

VoiceFinder AP2120 VoIP Gateway User’s Guide Version 1.00
AddPac Technology Co., Ltd. -
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decodes bit stream to analog voice signal. In special cases, it also indicated
the compression type (For example, G.723a CODEC).
Typcially, telephone network uses PCM Codec. PCM does sampling of analog
sound 8,000 times/ 1 sec. (Sampling gap: 125micro sec.) and converts each
sample to number code. On the telephone network, PCM code uses 8 bits, so
the standard transmission speed of digital telephone is 64Kbps.
The other commonly used compression algorithem is ADPCM(Adaptive
Differential Pluse Code Modulation). The typical ADPCM is ITU-T G.726 using 4
bits sample encoding with the transmission speed of 32Kbps. It encodes the
difference of amplitude and its rate.
PCM and ADPCM is Waveform Codec compression method. In addition to
waveform CODECs, there are source CODECs that compress speech by
sending only simplified parametric information about voice transmission; these
CODECs require less bandwidth. Source CODECs include linear predicative
coding (LPC), code-excited linear prediction (CELP), and multi-pulse,
multi-level quantization (MP-MLQ). Coding techniques are standardized by the
ITU-T in its G-series recommendations. The most popular coding standards for
telephony and voice packet are:
z G.711 : Describes the 64-kbps PCM voice coding technique. In G.711,
encoded voice is already in the correct format for digital voice delivery in
the PSTN or through PBXs.
z G.726 : Describes 40, 32, 24, 16Kbps ADPCM voice coding technique.
Used for Packet Voice, ordinary telephone network and PBX. In that case,
public telephone network or PBX needs to support ADPCM.
z G.729 : Describes CELP compression where voice is coded into 8-kbps
streams. There are two variations of this standard (G.729 and G.729 Annex
A) that differ mainly in computational complexity; both provide speech
quality similar to 32-kbps ADPCM.
z G.723.1 : Describes a compression technique that can be used for
compressing speech or audio signal components at a very low bit rate as
part of the H.324 family of standards. This CODEC has two bit rates
associated with it: 5.3 and 6.3 kbps. The higher bit rate is based on ML-MLQ
technology and provides a somewhat higher quality of sound. The lower bit
rate is based on CELP and provides system designers with additional