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

[Alexander E. Patrakov]

вс, 7 окт. 2018 г. в 16:42, Karl Ove Hufthammer <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.

-- 
Alexander E. Patrakov
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