linux-kernel - Re: [PATCH] ALSA: hda/realtek - Keep analog bypass muted on ALC892

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Date:	Thu, 09 Aug 2012 09:33:43 +0200
From:	Takashi Iwai <tiwai@...e.de>
To:	Thierry Reding <thierry.reding@...onic-design.de>
Cc:	Jaroslav Kysela <perex@...ex.cz>,
	David Henningsson <david.henningsson@...onical.com>,
	alsa-devel@...a-project.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] ALSA: hda/realtek - Keep analog bypass muted on ALC892

At Thu, 9 Aug 2012 09:18:48 +0200,
Thierry Reding wrote:
> 
> On Thu, Aug 09, 2012 at 09:02:10AM +0200, Takashi Iwai wrote:
> > At Thu,  9 Aug 2012 08:47:19 +0200,
> > Thierry Reding wrote:
> > > 
> > > Unmuting the analog bypass causes any input to be directly bypassed to
> > > the outputs, which is usually not expected by the user.
> > > 
> > > Signed-off-by: Thierry Reding <thierry.reding@...onic-design.de>
> > > ---
> > > Note that this patch could be generalized to include all codec variants
> > > that have this second channel. As I don't have access to all the data
> > > sheets I'll leave that decision up to somebody with more hardware
> > > available.
> > 
> > Hmm, are you really sure that this is exclusive switch between two?
> > All Realtek codecs with this mixer widget mixes two sources up.
> > Otherwise it must not be a "mixer" widget at all but it must be a
> > "selector" widget.
> 
> It's not an exclusive switch. But the second input is the analog bypass
> that basically takes the signal directly from the input path and routes
> it to the output.

Ah so you mean it as the analog loopback path.
Normal users even want this often.

> > And I haven't received a bug report regarding this although ALC892 is
> > no new chip and the auto-parser has been enabled for so long time.
> 
> I've seen this happen when feeding a signal into the CD-IN (NID 0x1c),
> which is then directly routed to the output, so I actually hear that
> signal at the output, even if there is no software loopback to capture
> the input and play it back via the DAC.
> 
> This is probably not a very common use-case, so maybe nobody else has
> come across it. Looking at the block diagram in the ALC892 datasheet
> (page 5) clearly shows where the input of this second switch comes from.
> I don't think this is intended to be unmuted by default, but only in
> very special cases where you actually want to pass the analog input
> signal directly to the output without going through the ADC and back out
> the DAC.

Each input has the analog loopback mixer switch (and off as default).
Why it doesn't suffice?


thanks,

Takashi
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