[pulseaudio-discuss] Mono Upmixing Only

Carl Gridley crgridley at gmail.com
Thu Mar 21 02:45:38 PDT 2013

Hi Tanu,

Thanks for replying :-)

When upmixing is disabled, mono playback is silent but does not generate
errors that I can see from pulseaudio -vvv or any notifications.

Here's a bit of output:

I: [pulseaudio] source-output.c: Trying to change sample rate
I: [pulseaudio] source-output.c: Resampling enabled to 44100 Hz
D: [pulseaudio] module-suspend-on-idle.c: Sink
alsa_output.pci-0000_01_00.1.hdmi-surround-extra1 becomes busy.
D: [pulseaudio] resampler.c: Channel matrix:
D: [pulseaudio] resampler.c:        I00   I01   I02   I03   I04   I05
D: [pulseaudio] resampler.c:     +------------------------------------
D: [pulseaudio] resampler.c: O00 | 0.000 0.000 0.000 0.000 0.000 0.000
I: [pulseaudio] remap.c: Using generic matrix remapping
I: [pulseaudio] resampler.c: Using resampler 'peaks'
I: [pulseaudio] resampler.c: Using float32le as working format.
D: [pulseaudio] memblockq.c: memblockq requested: maxlength=33554432,
tlength=0, base=12, prebuf=0, minreq=1 maxrewind=0
D: [pulseaudio] memblockq.c: memblockq sanitized: maxlength=33554436,
tlength=33554436, base=12, prebuf=0, minreq=12 maxrewind=0
I: [pulseaudio] source-output.c: Created output 14 "Peak detect" on
alsa_output.pci-0000_01_00.1.hdmi-surround-extra1.monitor with sample spec
float32le 1ch 25Hz and channel map mono

This particular one is from a mono youtube video played in Chrome, the
behaviour is the same in other applications too.

For the plans comment, this is just a quote referring to fine-grained
control over upmixing from this thread:

That was several years ago though.

I think your option would be good, to make sure I understand it it would be:

- no upmixing of stereo sources (or any multichannel source) with the upmix
module disabled
- mono would still be upmixed (probably to all channels) regardless of the
upmix module being active or not

Or something functionally equivalent to the above.

If I can help with testing I'm happy to try any patch on my system.


On 21 March 2013 07:58, Tanu Kaskinen <tanuk at iki.fi> wrote:

> On Wed, 2013-03-20 at 10:34 +0000, Carl Gridley wrote:
> > Hi folks,
> >
> > I have found some old messages on this, but wondered if anything had
> > changed or anyone had any bright ideas on it now :-)
> >
> > I have PA 2.1 running on Kubutu 12.10.
> >
> > It is connected via HDMI to my AV receiver and so I have 5.1 PCM output.
> >
> > I've got two music speakers, and the surround speakers are much smaller,
> so
> > I turn 5.1 upmixing off. I also turn off LFE mixing as the receiver does
> a
> > good job and can set a crossover per channel.
> >
> > Everything sounds great, but mono sounds don't work as they can't get
> > upmixed to stereo. If I re-enable upmixing, mono works, but then all
> stereo
> > sources get upmixed which I'd prefer not to.
> I haven't bothered to try this myself: how do mono streams behave when
> upmixing is disabled? Does the stream fail to connect to the device, or
> does it connect but is silent, or something else?
> > I know from the previous notes there were no plans to allow more control
> > over upmixing, has this changed at all?
> No plans in what sense? Patches for enabling sensible use cases will be
> accepted, and this use case definitely is sensible. If you mean "no
> plans" as in "nobody has promised to do the work", then you're right,
> and nothing has changed.
> > An option just to enable mono
> > upmixing only would be fantastic :-)
> >
> > Or even map mono to the centre channel if one is reported.
> I'm not sure there is need to have any new options. If mono streams
> don't work at all in case upmixing is disabled, I don't see why anyone
> would want to keep the current behavior. We could have a special case
> for mono-only streams so that such streams would always be upmixed to
> all channels (or some smart selection of a subset of the available
> channels, but all channels is easier to implement).
> I looked at the remixing code to determine what it would take to
> implement this, and it seems simple enough that I'll write a patch
> myself.
> --
> Tanu
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