ZyXEL Elite 2864 User Guide - Page 177

Chapter 16 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 playback the recorded digitized voice either off-line for local message listening or online for a message announcement. For interactive voice applications, DTMF tone detection capability is important so that a computer can react according to the remote 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 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 still maintaining good voice quality requires a sophisticated and complicated signal processing algorithm. It also requires a lot of digital signal processing computation power. The ZyXEL 2864 series modems support four voice digitization schemes. A summary of these four schemes is listed below: Digitization Scheme 2-ADPCM 3-ADPCM New 3-ADPCM 4-ADPCM Speech Compression Algorithem ADPCM, 2bits/sample ADPCM, 3bits/sample ADPCM, 3bits/sample ADPCM, 4bits/sample Data Rate 19200 bps 28800 bps 30720 bps 38400 bps 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 16-1