[alsa-devel] USB device gives wrong data?

Clemens Ladisch cladisch at fastmail.net
Mon Oct 22 08:34:54 CEST 2007

william estrada wrote:
> I think I have found a problem with the USB sound driver?  It appears
> that the current drive returns a x'81' for silence but it should be a
> x'7f'.

The USB audio driver returns whatever data is sent by the device

> When I use my laptop's sound device and record without a mic, I get
> a file full of x'7f's.  If I do the same with my cheap-o USB device
> I get a file full of x'81's.

For unsigned 8-bit samples, silence is x'80'.

Apparently, both devices have some DC offset and are quite noisy.

> I have written a recording program that I have a 'silence' filter
> that filters out segments of dead air.  It works with the laptop's
> sound device but not the USB device.  The filter works by remove
> x'7f's after allowing a selectable amount.

Your filter should be able to detect silence even when noise is present.

> Sorry for the multiple posting, not sure where is should have gone.

The the linux-audio-dev list, of course.  ;-)


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