VOICE DATA COMPRESSION
The main issue in digitized voice is the amount of storage required. A good phone-quality voice digitization will produce about 64 Kbits of data for each second of voice. The hard disk will quickly be filled up by digitized voice at this digitization rate. Speech compression is needed to reduce the digitization data rate. A relatively simple ADPCM (Adaptive Delta Pulse Code Modulation) algorithm can reduce the speech data rate to half 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. We call this sophisticated speech data compression capability advanced voice capability.

The U-1496 series modems support five voice digitization schemes. Four schemes use the ADPCM algorithm and one uses the advanced CELP (Code Excited Linear Prediction) algorithm to achieve near-phone-quality voice at 9.6 kbps speech data rate. A summary of these four schemes is listed below: 

  Digitization       Speech Compression                    Data Rate
   Scheme                Algorithm

    CELP             Code Excited Linear Prediction         9600 bps
    2-ADPCM          ADPCM, 2 bits/sample                  19200 bps
    3-ADPCM          ADPCM, 3 bits/sample                  28800 bps
    3-ADPCM(NEW)     ADPCM. 3 bits/sample                  30720 bps
    4-ADPCM          ADPCM, 4 bits/sample                  38400 bps
where the sampling rate used is 9600 samples per second.

The ADPCM voice modes are supported on all U-1496 series modems. However, the advanced CELP scheme is supported only on the plus version and the U-1496 LCD model.

Silence detection is implemented to eliminate voice data coding in silence period.

The modem also supports simultaneous DTMF (Dual Tone Multiple Frequency) tone detection that allows a user to use the keypad of a tone-dialing, push-button telephone set to control, instruct, or answer the modem.

Automatic detection of voice, fax, and data.
ZyXEL Advanced Voice Capability.

Sam Leffler / sam@engr.sgi.com. Last updated $Date: 1995/02/20 01:24:15 $.