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

[Tanu Kaskinen]

On Sat, 2018-10-13 at 22:29 +0500, Alexander E. Patrakov wrote:
> On 10/7/18 6:27 PM, Alexander E. Patrakov wrote:
> > вс, 7 окт. 2018 г. в 16:42, Karl Ove Hufthammer <karl at huftis.org 
> > <mailto:karl at huftis.org>>:
> > 
> >     Alexander E. Patrakov skreiv 07.10.2018 09:44:
> >      >
> >      >     I don’t understand why this is happening. Shouldn’t
> >      >     ‘remixing-use-all-sink-channels = no’ just affect *upmixing* of
> >      >     sound,
> >      >     and leave 5.1 material alone?
> >      >
> >      >
> >      > This is a known bug that appears because there are two 5.1
> >     standards:
> >      > proper 5.1 and 5.1 Side. The video player (I guess you use mpv)
> >     says:
> >      > the extra two channels have to come from the side.
> > 
> >     Looks like you’re right. If I run ‘ffprobe’ on my 5.1 test file, it
> >     returns:
> > 
> >         Stream #0:1[0x1100]: Audio: dts (DTS-HD MA) ([134][0][0][0] /
> >     0x0086), 48000 Hz, 5.1(side), s32p (24 bit)
> > 
> >     So the video file (and mpv) seems to  use the ‘5.1(side)’ standard.
> >     (Which is a bit strange, since this is supposed to be a video file to
> >     test a normal 5.1 setup, AFAICS.)
> > 
> > 
> > (speaking with my "DTS encoder author" hat on)
> > 
> > The issue is that it is a DTS file. DTS uses completely different 
> > channel names than what's found in PulseAudio source. The proper 
> > normative reference is:
> > 
> > https://www.etsi.org/deliver/etsi_ts/102100_102199/102114/01.03.01_60/ts_102114v010301p.pdf
> > page 19, table 5.4 (note that the presence of the LFE channel is 
> > transmitted separately).
> > 
> > Your file uses AMODE=0b001001=9, so the list of channels is: "Center", 
> > "Left", "Right", "Surround Left", "Surround Right". And there is also a 
> > 64x downsampled LFE channel. FFMmpeg-based decoders map "Surround" to 
> > "Side" because "Rear" also exists in other channel layouts and means 
> > something different. There is no way in DTS to express a layout with 
> > "Rear" but no "Surround" channels.
> > 
> > 
> > 
> >      > But your system does not have speakers there, it has them on the
> >     rear.
> >      > So PulseAudio attempts to remix. In fact, sound both with and
> >     without
> >      > remixing-use-all-sink-channels is wrong.
> > 
> >     What’s wrong about the remixing when one uses
> >     ‘remixing-use-all-sink-channels = yes’?
> > 
> > 
> > The rear channels will get not a copy of the side, but an average of 
> > front and side.
> 
> As promised, here is a patch.

Thanks a lot! Very nice fix for a common problem, I applied it now. I
expect to look into the big remixing rework patch soon.

> 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?

-- 
Tanu

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