[Asterisk-Users] Low Volume WAV Files in Email Attachments

[Gregory Junker]

> Third, those complaining of low volume in emailed files are usually
> using a compressed format. In the uncompressed wav format, the volume is
> effectively doubled by shifting the audio data to the left one bit. This
> is done at the format level. Of course on playback via asterisk, it
> checks to see if it needs to shift the audio down and does so. So
> playback between asterisk recorded wav files should all sound the same
> on asterisk but isn't the same when played via a normal audio app.
> 

The complaints come mainly regarding the emailed attachements, which are 
WAV49 (MS-GSM) files, which (as far as I can tell) are just justified 
right and packed into 65 bytes per the IETF I-D.

These files are not played back within Asterisk, and honestly, most of 
what you said above here is rubbish. I just spent more time than I ever 
cared to spend (including studying the actual GSM codec spec from the 
ETSI), learning more than I ever cared to learn about GSM (which, btw, 
if you are concerned about patents, is just as subject to them if you 
believe Philips' claims), and the difference between uncompressed WAV 
files (which also suffer from attenuated signal levels) and the GSM 
and/or MS-GSM files is far far more than just "shifting the audio data 
to the left one bit".

There is an issue surrounding the recording of data through Asterisk. 
That is inarguable. The problem is that no one seems to agree on where 
to begin looking, so no one has, really. I don't know the origin of the 
GSM files that make up the Comedian VM system prompts, but they do not 
suffer from this problem. However, GSM files generated by the VM system, 
at the least, have a signal attenuation problem to the point that the 
emailed attachments are unusable, and by most accounts, the phoned-in VM 
retrieval is barely useful to boot.

Not only am I willing to try to track this down, I am furiously taken 
with the task, because it's a real issue that needs to be addressed, and 
I do understand that the actual devs have more important things to fix 
first. That's one of the nice things about open-source, eh? ;)