ZyXEL ADPCM codec

Julian849

I have a ZyXEL USB Omni56K Duo modem and want to send and receive voice streams on it, but to reach adequate quality I probably need to implement some "ZyXEL ADPCM" encoding because plain PCM provides too small sampling rate to transmit even medium quality voice, and it doesn't work through USB either (probably because even this bitrate is too high for USB-Serial converter in it mybkexperience).

This mysterious codec figures in all Microsoft WAV-related libraries as one of many codecs theoretically supported by it, but I found no implementations.

Can someone offer an implementation in any language or maybe some documentation? Writing a custom mu-law decoding algorithm won't be a problem for me.

Thanks.

#Others_Jan_2020