[Asterisk-Users] Correction: Asterisk sound files, audio bandwidth, and sound quality

[steve at daviesfam.org]

On Wed, 28 Sep 2005, Stephen Bosch wrote:

> steve at daviesfam.org wrote:
> >>When I listen to the GSM compressed prompts, I can hear subtle noise
> >>when the person is speaking -- this is irrespective of whether I listen
> >>to the prompts through the TDM-400 on an analogue phone or whether I do
> >>so directly on a workstation. It has to be possible to do better than that.
> >>
> >>It doesn't have to be CD quality sound, but it should be clear. How can
> >>the sound quality be improved within the limitations of an 8 kHz sample
> >>rate? 
> > 
> > I presume you know the answer: augment the .gsm files with 8k sample rate 
> > .wave files?
> 
> If I knew the answer, I wouldn't be asking. There are other ways to
> improve sound quality, like changing the resolution.

I'm sorry, I wasn't trying to be funny.

The recorded prompts supplied with Asterisk are encoded with the .gsm 
codec.  That makes them sound like audio sounds on your GSM cellphone.  
Which is noticably worse than true PCM audio.

Now in the telephone world "best quality" still isn't very good - its ulaw 
or alaw encoded 8kHz audio.  That's frequency response up to 3.5kHz and 
about 12 or 13 bits of dynamic range.

But the fuzzyness you hear on the standard Asterisk prompts is due, I'm 
sure, to the use of gsm compression.

Its not necessary that they are stored in gsm - Asterisk has got an 
elaborate logic for finding the "best" version of the prompt, so you can 
store the same prompt in lots of formats and Asterisk will choose the 
"best".

You can see the formats with "show file formats"

So you need them stored in a non-compressed format - the simplest is 
probably wav format, another possibility is to use sox to put them into 
ulaw directly

Now Digium hasn't made the standard prompts available in a format other 
than gsm.  I don't know why.

For us we recorded the prompts in South African voice and so we have 
those.  You need to either extract the original non-compressed prompts 
from Digium (if they have them), or take it as an opportunity and record 
your own set in Canadian accent.

Hope that helps.

Steve