asterisk users - Sep 2005 - Correction: Asterisk sound files, audio bandwidth, and sound quality

Sorry -- I goofed on the sample rates! Apologies!

Hello, everybody:

I'm developing an application using Asterisk and a TDM-400 card.

I understand the concept of the difference between GSM and WAV files
when using Asterisk, but I'm not happy with the sound quality with the
GSM compression. It's merely *acceptable* for a telephone call, but for
anything else, it leaves something to be desired.

Case in point -- if you compare the sound quality of the prompts with,
say, an Octel voicemail system, there's no contest -- and this is
through a standard telephone.

1. Can I *record* audio from a TDM-400 channel at a sample rate above
8 kHz? Can I record at 16 bits? (This is most important.)

2. Can I use a prompt with a sample rate above 8 kHz? Will the
TDM-400 card support it? How can I do this?

The voip-info wiki hints that Asterisk can support whatever codecs
listed after a "show codecs" command:
> *CLI> show codecs
> Disclaimer: this command is for informational purposes only.
>         It does not indicate anything about your configuration.
>         INT    BINARY        HEX   TYPE    NAME   DESC
>
--------------------------------------------------------------------------------
>           1 (1 <<  0)      (0x1)  audio    g723   (G.723.1)
>           2 (1 <<  1)      (0x2)  audio     gsm   (GSM)
>           4 (1 <<  2)      (0x4)  audio    ulaw   (G.711 u-law)
>           8 (1 <<  3)      (0x8)  audio    alaw   (G.711 A-law)
>          16 (1 <<  4)     (0x10)  audio    g726   (G.726)
>          32 (1 <<  5)     (0x20)  audio   adpcm   (ADPCM)
>          64 (1 <<  6)     (0x40)  audio    slin   (16 bit Signed
Linear PCM)
>         128 (1 <<  7)     (0x80)  audio   lpc10   (LPC10)
>         256 (1 <<  8)    (0x100)  audio    g729   (G.729A)
>         512 (1 <<  9)    (0x200)  audio   speex   (SpeeX)
>        1024 (1 << 10)    (0x400)  audio    ilbc   (iLBC)
>       65536 (1 << 16)  (0x10000)  image    jpeg   (JPEG image)
>      131072 (1 << 17)  (0x20000)  image     png   (PNG image)
>      262144 (1 << 18)  (0x40000)  video    h261   (H.261 Video)
>      524288 (1 << 19)  (0x80000)  video    h263   (H.263 Video)
>     1048576 (1 << 20) (0x100000)  video   h263p   (H.263+ Video)
What is the 16 bit signed Linear PCM format? How do I get Asterisk (1.2)
to use such a sound file instead of a *.gsm file?

Your feedback is very much appreciated.

Cheers,

Stephen Bosch

Stephen Bosch wrote:
> 1. Can I *record* audio from a TDM-400 channel at a sample rate above
> 8 kHz? Can I record at 16 bits? (This is most important.)
No TDM hardware supports sample rates above 8KHz, since that is what the 
PSTN is designed for. Zaptel hardware can possibly be convinced to 
supply 16-bit signed linear audio, but I don't know of anyone who has 
tried it. Normally the hardware provides the audio stream already 
compressed to 8-bit ulaw or alaw.
> 2. Can I use a prompt with a sample rate above 8 kHz? Will the
> TDM-400 card support it? How can I do this?
See above: no.
> What is the 16 bit signed Linear PCM format? How do I get Asterisk (1.2)
> to use such a sound file instead of a *.gsm file?
Provide a .raw file with that data in it, and Asterisk will play it. If 
it's going out a TDM channel, it will be converted to ulaw or alaw as 
required before it gets there.