[pulseaudio-discuss] Messing around with surround, also found a patch

David Henningsson david.henningsson at canonical.com
Tue Oct 16 05:55:57 PDT 2012

On 10/15/2012 01:01 AM, Adriano Moura wrote:
> 2012/10/14 Tanu Kaskinen <tanuk at iki.fi>
>> I didn't get why 5.1 REAR should end up in 7.1 SIDE. If the content
>> creator decided to put audio in a REAR channel, why shouldn't it be
>> played to a REAR speaker? Is 5.1 content actually created with the
>> assumption that in practice the so-called "rear" channels end up in
>> "side" speakers?
> If you do a search for "5.1 speakers setup" you will notice that most
> setups place the surround speakers at -90º/90º and nobody calls then
> Rear or Back speakers, but Side or Surround speakers.

If you mean Google image search, I find both 90, 110 and 135 angles. 
Wikipedia suggests 110, and the manual to my receiver suggests 120. I 
can't agree that 90 is the standard. And I also find several images 
referring to a "Rear" channel rather than "Surround" channel.

> I don't know why
> we call then REAR in a 5.1 arrange... maybe it's an Alsa thing that we
> borrowed? They also list then as REAR. But anyway, 5.1 arrangement is
> definitely meant to be played trough surround/side speakers.

Most onboard sound cards that support 5.1 have 3 jacks: Green = front, 
Orange = center+lfe, Black = Rear/Back/surround.
Those who support 7.1 have 4 jacks: Green = front, Orange = center+lfe, 
Black = Rear/Back/surround and Gray = Side.

That looks to me as if the Side jack is the one extra, that should be 
silent (or extrapolated), when playing back 5.1 material on a 7.1 system.

OTOH, referring to my receiver's manual, it defaults to upmixing the 5.1 
surround channels to be played on both rear and side in a 7.1 speaker 
setup, but has the option of playing it only on the sides, rather than 
only on the rear.

Maybe this is a case where there are conflicting de-facto standards?

> 2012/10/14 Tanu Kaskinen <tanuk at iki.fi>
>> I don't understand why openal would use PA_CHANNEL_MAP_WAVEEX to define
>> its channel map, though. When openal creates a stream in pulseaudio, I
>> would expect it to know exactly the channel map, and if it doesn't match
>> with the WAVEEX definition, then it should use some other definition (or
>> initialize the pa_channel_map struct manually).
>> That said, if the mapping that you suggest for WAVEEX isn't any less
>> conformant than the current mapping, this sounds like a useful change to
>> make. Potentially it can break other applications that assume that the
>> mapping is what it is now, but I find the existence of such applications
>> quite unlikely...
> Again, I think they either didn't want to mess with
> pa_channel_map_init or didn't realize WAVEEX was broken for 6.1 and
> up... Well, few people have surround setups, even less so 6.1/7.1. (I
> also don't, But I'm using binauralization with some nifty jack and
> jconvolver DSP)

Nevertheless, it sounds like this is something to be fixed in OpenAL 
rather than PulseAudio? Better specify the channel map explicitly.

David Henningsson, Canonical Ltd.

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