[pulseaudio-discuss] Mono Upmixing Only

Tanu Kaskinen tanuk at iki.fi
Thu Mar 21 00:58:16 PDT 2013

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


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