[pulseaudio-discuss] Weak bass in stereo mode – possibility of virtual 2.1 sound profiles

[Tanu Kaskinen]

On Mon, 2018-11-26 at 00:17 +0500, Alexander E. Patrakov wrote:
> On 11/16/18 11:13 AM, Tanu Kaskinen wrote:
> > On Thu, 2018-11-15 at 23:50 +0500, Alexander E. Patrakov wrote:
> > > Alexander E. Patrakov <patrakov at gmail.com>:
> > > > Tanu Kaskinen <tanuk at iki.fi>:
> > > > > On Sat, 2018-10-13 at 22:29 +0500, Alexander E. Patrakov wrote:
> > > > > > Note that I am unhappy with it, because it fixes only a particular
> > > > > > common problem case, while other related issues stay unfixed. E.g. if
> > > > > > one tries to play 5.1 audio on 7.1 system, with or without this patch:
> > > > > > 
> > > > > > - Front, Center and LFE channels are mapped 1:1, which is correct
> > > > > > - Rear (or what mpv calls "Side") source channels are mapped to Side,
> > > > > > which is also correct, because it's the Side speakers in the ITU-T 7.1
> > > > > > layout, not Rear, which have the nearest position to what Dolby
> > > > > > specifies for Surround AC3 speakers.
> > > > > > - The true rear channels get a mix between Front and Side, which is
> > > > > > clearly incorrect. ITU R-REC-BS.775-3-201208 says that both sets of
> > > > > > surround channels should be fed the same signal.
> > > > > 
> > > > > Regarding that last point, do I understand correctly that the
> > > > > recommendation is to duplicate the 5.1 rear(/side) channels to both the
> > > > > side and rear channels in 7.1? Doesn't that cause imbalance by
> > > > > amplifying the rear channels? Or is it compensated by attenuating the
> > > > > signal?
> > > > 
> > > > You understood correctly. In my big patch, it is compensated by
> > > > attenuating the signal. Anyway, I am going to resubmit it later today
> > > > without the half-baked change about the LFE volume.
> > > 
> > > Sorry, I have to do it later. I have too much non-PulseAudio work to do :(
> > 
> > No problem, I'll postpone reviewing the patch until you've submitted
> > the new version. From what I already saw, the general idea behind the
> > patch seems good.
> > 
> 
> Hello Tanu,
> 
> thanks for your patience. I have tried to undo the half-baked LFE change 
> today, but found two more bugs in my patch that I would rather fix 
> before resubmitting it:
> 
> 1. AUX input channels get sometimes routed to non-AUX output channels 
> rather than being dropped. Fixing this right how.
> 
> 2. Some inconsistencies if either input or output channel map contains 
> duplicates. Cannot really fix, because I don't know what's expected.
> 
> Could you please help with (2)? In particular, what would be the 
> expected channel matrix in the following two cases?
> 
> Case A:
> 
> front-left, front-right, front-center ->
> front-left, front-right, front-center, front-center
> 
> I would guess that we should generate two identical front-center output 
> channels, both duplicating the front-center input channel with the 
> coefficient being 1.0.
> 
> Case B:
> 
> front-left, front-right, front-left, front-right -> front-left, front-right
> 
> Would it be OK if I drop the redundant input channels, i.e. use only the 
> first copy of each channel name? Or should I average the redundant 
> inputs, or take a sum?
> 
> Is there any spec?

I'm not aware of any valid use cases for duplicated channels (except
mono, which can be interpreted as a channel without defined position),
so there's no spec and at least I don't have expectations of any
particular behaviour. Your suggestion for case A seems reasonable, and
for case B I think we should do whatever is easiest to implement.

-- 
Tanu

https://www.patreon.com/tanuk
https://liberapay.com/tanuk