[pulseaudio-discuss] Automatically switch profiles depeding on the number of input-channels

Frederik Vogelsang frederik.vogelsang at googlemail.com
Tue Aug 2 02:16:21 PDT 2011

Thanks for your input, Colin.

2011/8/1 Colin Guthrie <gmane at colin.guthr.ie>:
> Dynamic profile switching is not currently supported, but it will be in
> the future.
OK, that is good to know.

> At present we're concentrating on passthough as that's generally what
> people have asked for more and it it semi related to this issue.
Yeah, passthrough is important to me as well. Probably more important
than profile switching :). Right now I have to use plain-ALSA for

> 2. Media Player requests DTS passthrough -> PA Rejects as the user has
> not said their receiver supports it.
>  Media Player requests AC3 passthrough -> Ditto
>  Media Player requests 6 channel PCM surround -> PA Switches to the AC3
> Profile and accepts 6ch PCM input.
That sounds good to me.

> As you can see the negotiation becomes far more complex... in step three
> above, why would PA prefer the 5.1 AC3 over the 5.1 Analog for example?
Well in my case I am using a HDMI connection and my graphics card only
supports either 2-channel PCM or DTS/AC3 passthrough. This is why I
have to use the PCM-to-AC3 conversion for multichannel PCM. Obviously
this would not make sense for 2-channel PCM sources.
If the device would be able to accept 6 or 8 channels directly this
should be preferred over AC3 encoding. But I guess users would not use
the a52-plugin in such a setup anyway.

> At the moment, I'm not massively in favour of doing automatic profile
> switching at all and rather just dealing more gracefully with the
> disabling of upmixing. The AC3 encoder output is a bit of a stick in the
> mud here tho', as it's got a kind special use case... I'm not really
> sure how best to deal with that one :s
I understand. In the long-term I would probably be fine with this, as
newer graphics cards support multichannel PCM directly.

I have also found a solution I can live with: for applications which
require multichannel PCM I simply use ALSA and redirect them to pulse
through the "pcm.!default"-section in my .asoundrc. But in "the
pcm.!surround51"-section I am still defining the a52-encoding
directly. This kinda works and since I do not use multichannel
material every day I am fine with it.

- Frederik

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