Advanced Voice Capability; Voice Data Compression - ZyXEL Communications 2864 Series User Manual

ADVANCED VOICE CAPABILITY

Voice capability stands for the modem's ability to digitize incoming voice messages,
which the computer stores and forwards. It also means that the modem can play-
back the recorded digitized voice either off-line for local message listening or on-
line for a message announcement. For interactive voice applications, DTMF tone
detection capability is important so that a computer can react according to the re-
mote caller's touch tone input.

Voice Data Compression

The main issue in the digitized voice mode is the amount of storage required. A
good phone quality voice digitization will produce about 64 Kbits of data for each
second of voice recording. The hard disk will be filled up quickly by digitized voice
data at this digitization rate. Speech compression is needed to reduce the rate of
digitized data. A relatively simple ADPCM (Adaptive Differential Pulse Code
Modulation) algorithm can reduce the speech data rate to half the rate and main-
tain about the same voice quality. This algorithm can also be used to reduce the
speech data rate to 1/3 or 1/4 of the original rate, but with voice quality degrada-
tion. Reducing the speech data rate further and still maintaining good voice quality
requires a sophisticated and complicated signal processing algorithm. It also re-
quires a lot of digital signal processing computation power.
The ZyXEL 2864 series modems support four voice digitization schemes. A sum-
mary of these four schemes is listed below:
Digitization Scheme
2-ADPCM
3-ADPCM
New 3-ADPCM
4-ADPCM
whereby the sampling rate used is 9600 samples per second.
The New 3-ADPCM scheme is a 3-bit ADPCM scheme with addded sync bits in
the voice data to avoid loss of sample data synchronization due to data errors or loss
Chapter 16
Speech Compression Algorithem
ADPCM, 2bits/sample
ADPCM, 3bits/sample
ADPCM, 3bits/sample
ADPCM, 4bits/sample
16–1
Data Rate
19200 bps
28800 bps
30720 bps
38400 bps