Re: Soundmodem and sampling rate: force a rate?

David Rivenburg <driven@xxxxxxx> · Wed, 26 Oct 2005 00:46:48 -0500

J. Lance Cotton wrote:

I am trying to get soundmodem (usermode) working with a Soundblaster
Live card on SuSE 9.3 and the commercial OSS driver.

The problem is that apparently the SB card supports only 8k, 11.025k,
12k, ... sampling rates. At 1200bps AFSK, soundmodem demodulator asks
for (or gets) 9600Hz sampling rate.

Something, as far as I can tell, is opening the soundcard for recording 

at 9600Hz sample rate, but I've been told that that rate isn't supported 

by my SB Live card. I thought that in that instance, the "next closest" 

rate would be returned instead...

I do not know if this is a problem with the Commercial OSS driver, the 

SB Live card, or soundmodem. I've tried editing the soundcard/audioio.c 

file to "force" other sample rates. It does open the card at the forced 

rate, but the decoding still doe not work reliably. (The problem is 

exemplified in the samples I recorded and put at 

http://www.cmsworldwide.com/Files/soundsamples/index.html )

**Question: Is there any official way to have soundmodem request a 

user-custom sampling rate? Like a hidden config?

Thanks. 73 KJ5O

I came across a similar thing.  Looking at the soundmodemconfig scope 

function I saw horizontal lines in the waveform - this is clearly 

impossible for an AC coupled signal so I suspected that 9600 was not a 

valid sample rate for my audio codec.  Here is what I did to fix it:

in line 233 of soundcard/main.c change

unsigned int samplerate = 5000, mode;
to
unsigned int samplerate = 11025, mode;

then in line 710 of configapp/src/diag.c change

diagstate.samplerate = 5000;
to
diagstate.samplerate = 11025;

Even though the sample rate is set to 5000 there is a selection routine 

in audioio.c that steps it up to 9600.  I tried editing the valid sample 

rate list there but it still used 9600.

After making the above changes I have not had any trouble at all with 

soundmodem.  Not that there aren't any, but I don't know of another way 

to force a sample rate.

Hope this helps.

Dave
AD5OO
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