ZyXEL

ZyXEL voice modems

There are two slightly different sets of compression algorithms implemented in the program for ZyXEL modems.

The first set includes ZyXEL ADPCM 3 and ZyXEL ADPCM 2 speech coding and proprietary algorithms. Their implementation is taken directly from ZyXEL programming library sources. Default compression method is ZyXEL ADPCM 3.

You can choose the compression method with best quality/performance for your modem. If one does not work, try another or an alternative method with different baud rate.

Another set is ZyXEL ADPCM [alternative] - an alternative implementation of the same methods. This set includes ZyXEL ADPCM 4 bps, which is otherwise not available and which gives the best compression quality. 4 bps ADPCM is not available in older ZyXEL modems.

Note that bits-per-sample regulation is only available in [alternative] set.

CELP coding is not currently implemented.

Compression methods found in modern ZyXEL modems
Method Bit Depth Sampling Frequencies Comment
ZyXEL ADPCM 2 2 7200, 8000, 9600, 11025 Poor sound quality
ZyXEL ADPCM 3 3 7200, 8000, 9600, 11025 Classic 3-bit ADPCM scheme
ZyXEL ADPCM 3 (new) 3 7200, 8000, 9600, 11025 3-bit ADPCM scheme with added sync bits in the voice data.
ZyXEL ADPCM 4 4 7200, 8000, 9600, 11025 Good sound quality
IMA ADPCM (Intel/DVI) 4 7200, 8000, 9600, 11025 Standard Intel/DVI (IMA ADPCM), good sound quality
A-law PCM 8 8 7200, 8000, 9600, 11025 Very good sound quality
u-law PCM 8 8 7200, 8000, 9600, 11025 Very good sound quality (if supported)

(*) Bold is a recommended setting. Default sample rate for ZyXEL modems if 9600 Hz.

Modem Notes Modem Notes