Vox Audio File Conversion

The programs vox and devox are C programs developed in 1997 when I worked at Sprint to convert audio files between OKI ADPCM (Dialogic vox) file format and linear audio files, which work with PC audio hardware. Conversions to/from other formats can be accomplished with sox.

No current development work is going on with the vox program. It is believed to do its intended job, and I am longer work in the voice processing industry.

Vox works on all known versions of Unix, including Linux.

The Standard

I originally wrote the program based on a description of the algorithm found in the book PC Telephony - The complete guide to designing, building and programming systems using Dialogic and Related Hardware by Bob Edgar, pg 272-276, third edition, 1995, Flatiron Publishing, Inc., New York. Relavent pages are shown here (scanned). I think the book is out of print, so hopefully listing a few pages from the book is okay.

Edgar-pg272.jpg, Edgar-pg273.jpg, Edgar-pg274.jpg, Edgar-pg275.jpg, Edgar-pg276.jpg.

Since developing the program, I found that Dialogic published a standard for the ADPCM algorithm. Here is the standard, as once posted on Dialogic’s web page. I don’t know if they still post the standard.

Disclaimer: (dated 6/26/2003) A few people have pointed out to me that there is a minor difference between my program and the Dialogic standard. The difference relates to how the step size is calculated. The difference is also reflected in Edgar’s text, which my code is based on. I don’t how or why the difference arouse. I have a few comments about said difference:

1. I wrote the program to a published algorithm available to me at the time.
2. I might be able work on a new version of the program which will match the Dialogic standard; however, since I am not in the voice processing industry, I don’t have any hardware to use to test the new code. If you have some hardware and are willing to help with testing, let me know. I don’t get the impression that there is much demand for a new version of it.
3. The differences seem to be fairly minor. If you just want to be able to do simple conversions for listening purposes, my code, as it is now, will work for you. If you require exact conversion, then my code may not be what you want.

Download vox as source code

• Compressed tar file
• Zip file

Copyright 2003, Tim Bower tim(at)ksu.edu

Web page up-dated July, 2009.