Re: [alsa-devel] Bugs on aspire one A150

[Takashi Iwai <tiwai@suse.de>]

At Mon, 16 Mar 2009 19:06:38 +0100,
Andreas Mohr wrote:
> 
> On Mon, Mar 16, 2009 at 06:34:35PM +0100, Takashi Iwai wrote:
> > At Mon, 16 Mar 2009 18:30:00 +0100,
> > Andreas Mohr wrote:
> > > 
> > > Hi,
> > > 
> > > On Mon, Mar 16, 2009 at 06:15:39PM +0100, Takashi Iwai wrote:
> > > > At Mon, 16 Mar 2009 18:00:15 +0100,
> > > > Andreas Mohr wrote:
> > > > > > > And, is the behavior consistent regardless of the value high, i.e.
> > > > > > > the key is only whether the values for both channels are identical?
> > > > > > 
> > > > > > BTW, what if you record with the following definition?
> > > > > > Put the below to ~/.asoundrc
> > > > > > 
> > > > > > pcm.imix {
> > > > > > 	type plug
> > > > > > 	slave.pcm "hw"
> > > > > > 	ttable.0.0 0.5
> > > > > > 	ttable.0.1 -0.5
> > > > > > }
> > > > > > 
> > > > > > and record like
> > > > > > 
> > > > > > 	% aplay -Dimix -c1 foo.wav
> > > > > 
> > > > > Does NOT exhibit the "equal sliders == no sound" bug (apart from this sliders
> > > > > are acting normally, i.e. slider low == no sound), despite being a
> > > > > "plug" type definition (this is what you wanted to discern, right? ;).
> > > > 
> > > > Interesting.  This implies that one channel is inverted indeed.
> > > 
> > > Oh, you mean "inverted" as in "_hardware_ channel which provides opposite
> > > sample values as compared to the other channel"?
> > 
> > Right.  My guess is that the hardware provides left (or right) channel
> > is multiplied by -1  (yep "inverted" is no correct word choice).
> > 
> > > > As default the alsa-lib plugin downmixes a stereo stream to a mono
> > > > stream simply  by left/2 + right/2.  The above changes the routing
> > > > policy as left/2 - right/2.
> > > 
> > > That exactly matches my current stream of thought (while reading
> > > "one channel is inverted" above).
> > > 
> > > > So we need to pass some information to change this kind of thing...
> > > 
> > > That's something specific to ALC268 codec setup, right?
> > > ("ALC268 digital mic == left plus right channel, but inverted setup"?)
> > 
> > No.  It's specific to the digital-mic side.
> 
> I just tried connecting a headset and switching to E-Mic.
> What I can say is:
> - opposite levels does NOT happen there (E-Mic is "analog micro"-based, right?)
> - leaving E-Mic unplugged will actually record from i-Mic (due to properly
>   working EAPD mechanism, right?)

The record from mic-jack is via analog path.  The phase-inversion
appears only for digital-mic, AFAIK.

> BTW, the output (of arecord -Dplughw:0 -c2 -traw -fdat foo.wav) _does_ have
> inverted channels:
> 
> 00000130   2A 02 D2 FD  65 02 A5 FD  10 00 E7 FF  99 FF 72 00 *...e.........r.
> 00000140   53 FE A7 01  ED FF 12 00  4F FF A6 00  86 00 81 FF S.......O.......
> ... and so on in the entire file ...
> 
> 
> One question still: is this a hardware defect (i.e. could this possibly
> be swapped cables of the microphone connector in this model or so?
> Not plausible but...), or is this an existing property
> of the HDA's dig-mic base? You indicated it's the latter I think...

My guess is that it's a hardware implementation.
Maybe for the noise suppression via mic array.

The question is whether the left / right channels recorded from
digital mic are really raw data, or they are for modified data
(for differential, etc)...  It's hard to guess without the actual
data.


Takashi