On 10/15/2012 01:01 AM, Adriano Moura wrote:
2012/10/14 Tanu Kaskinen <ta...@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 <ta...@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.
https://launchpad.net/~diwic
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