[alsa-devel] [patch][saa7134] do not change mute state for capturing audio

[Stas Sergeev]

19.07.2011 17:00, Mauro Carvalho Chehab wrote:
> Several video boards have the option of plugging a loop cable between
> the device output pin and the motherboard line in pin. So, if you start
> capturing, you'll also enabling the output of such pin, as the kernel
> driver has no way to know if the user decided to use a wire cable,
> instead
> of the ALSA PCM stream.
> So, if users with such cables are lucky, it will play something, but,
> on most cases, it will just tune into a non-existing station, and it will
> produce a white noise.
This needs to be clarified a bit (for Lennart).
Initially, before the board is tuned to some station,
the sound is wisely muted. It is muted for both the
capturing and the pass-through cable.
As far as I can tell, if you want to probe the card by
capturing, you can capture the silence, you don't need
any real sound to record.
The problem here is that the particular driver has a
"nice code" (or a hack) that unmutes both the capturing
and the pass-through cable when you capture anything.