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

[Stephen Bosch]

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
8000 kHz? Can I record at 16 bits? (This is most important.)

2. Can I use a prompt with a sample rate above 8000 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