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Date: Sat, 12 Feb 2011 13:44:21 -0800 (PST)
From: AW <arne_woerner@yahoo.com>
Subject: Re: PCTV USB2 PAL / adds loud hum to correct audio
To: linux-media@vger.kernel.org
Cc: dheitmueller@kernellabs.com
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Devin Heitmueller <dheitmueller@kernellabs.com> wrote:
> > When I use this command  simultaneously:
> > arecord -D front:CARD=PAL,DEV=0 -f S16_LE -c 2 -r 8000  /aux/tmp/bla.wav
> > I get correct audio with strong noise:
> > http://www.wgboome.de./bla.wav
> > (it is from input=1 for copyright  reasons... so there is silence plus 
noise)
> 
> The "-r" argument should  almost certainly be 48000, not 8000.
> 
Maybe...
That device is rather old...
And i didnt tell pactl anything about sample rate...

With the filter from the appendix the noise is gone...
But it feels like a dirty hack, because it would cut out (overly?) loud noise, 
2...

My wild guess is, that the usbaudio driver injects some bad samples 
(0x8000..0x9000) every appr. 256 bytes...

-Arne

appendix:
#include <stdint.h>
#include <unistd.h>

int main() {
uint16_t buf[1000000];
const int r = read(0,buf,sizeof(buf));
for (int i=0; i*sizeof(*buf)<r; i++) {
if (buf[i]/256>=0x80 && buf[i]/256<0x90) continue;
if (write(1,buf+i,2) != 2) break;
}
return 0;
}