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Chapter 10 - Voice Mode Operation
10-1
Chapter 10 - Voice Mode Operation
Voice mode refers to the Omni 288S’s ability to digitize incoming voice
messages, which the computer stores and forwards. It also means the modem can
playback 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 a computer can react according to the
remote caller's touch tone input. This Chapter summarizes the use of the Omni
288S’s numerous voice features.
Voice Data Compression
The main issue in 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. At this digitization rate, the hard disk will be filled up
quickly by digitized voice data. 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
maintain 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
degradation. Reducing the speech data rate further and maintaining good voice
quality requires a sophisticated and complicated signal processing algorithm. It
also requires a lot of digital signal processing computation power.
The Omni 288S supports seven voice digitization schemes:
Compression Method Bits/Sample Rates (samples/second)
ZyXEL 2 ADPCM 2 7200, 8000, 9600, 11025
ZyXEL 3 ADPCM 3 7200, 8000, 9600, 11025
ZyXEL 3 ADPCM (new) 3 7200, 8000, 9600, 11025
ZyXEL 4 ADPCM 4 7200, 8000, 9600, 11025
DVI ADPCM 4 7200, 8000, 9600, 11025
A-law PCM 8 7200, 8000, 9600, 11025
u-law PCM 8 7200, 8000, 9600, 11025
Note: The New 3-ADPCM scheme is a 3-bit ADPCM scheme with
added sync bits in the voice data. This helps to avoid loss of
data synchronization due to data errors or data loss. It is the
recommended 3-bit ADPCM scheme. It will automatically
recover from data errors when playing without producing noisy
voice signals.