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Topic: What is the deal with 5.1(back)? (Read 4277 times) previous topic - next topic
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What is the deal with 5.1(back)?

As far as I know, 5.1 in movies and music is always 5.1(side). Where is 5.1(back) used? And why is this Bizarro 5.1 the only 5.1 supported by Opus?

Re: What is the deal with 5.1(back)?

Reply #1 –
That doesn't make a lot of sense.  What are you playing and what are you playing it on?  And what convinces you that the channel mapping is wrong?

Re: What is the deal with 5.1(back)?

Reply #2 –
What got me wondering about this is that ffmpeg won't allow 5.1(side) to be encoded by libopus. Maybe that's just a bug in ffmpeg, but still - why does Opus insist on calling the 5.1 channels "back" channels, when the corresponding channels in 7.1 are called "side" channels? It's confusing.

Re: What is the deal with 5.1(back)?

Reply #3 –
what do you mean with "side"?  5.1 does not have "side" speakers, only 6.1 and 7.1 do.

(front left, front right, center, surround, back left , back right)

I am guessing you are talking about the order where center and surround are inside of the stream of 6 channels, but I've only used opus in stereo, so I can't comment exactly on what it does.

Re: What is the deal with 5.1(back)?

Reply #4 –
Quote from: [JAZ] on
what do you mean with "side"?  5.1 does not have "side" speakers, only 6.1 and 7.1 do.

Some audio formats distinguish between 5.1(side) and 5.1(back). And in those formats, 5.1(side) is normal 5.1.

Re: What is the deal with 5.1(back)?

Reply #5 –
I don't think I'd agree that 5.1 (side) is the "normal" 5.1.  Even ffmpeg has "5.1" and  "5.1 (side)" although neither of them match the Opus map.  I don't think it will map the channels the way you want for opus automatically, but can't you do it by specifying the channelmap explicitly to ffmpeg?  Or perhaps you could encode via FLAC or WAV with an explicit channel map?

Or do you know how to encode it, but just wondering why Opus isn't the same as everything else?  That one is easy, almost every codec has a different standard mapping for 5.1.  The Opus map is the same as Vorbis (ie. Ogg) and is FL, FC, FR, RL, RR, LFE (using the Ogg names for the channels).  AC3 5.1 is FL, FC, FR, SL, SR, LFE, which is almost the same as Opus, while DTS is FC, FL, FR, SL, SR, LFE.  WAV is different again, and don't even ask about AIFF, they seem to have deliberately done something dumb just to be different.

Whether you call the two sides or rears is somewhat arbitrary since they are neither.  They are "surrounds" and are not intended to be in the same position as either the side or rear speakers in, for example, a 7.1 layout.  DD originally tended to call them rears, while many newer surround systems call them sides.  It may matter if your system understands the difference (or doesn't understand, depending on your point of view) and doesn't send a particular channel to the speaker you have connected.

Re: What is the deal with 5.1(back)?

Reply #6 –
Indeed 5.1 side is normal 5.1. See "Note 3" in header of column of table - https://en.wikipedia.org/wiki/Surround_sound#Standard_speaker_channels
Opus doesn't save waveformatextensible channel mask.  It only uses standard channels layouts as in vorbis specification - https://xiph.org/vorbis/doc/Vorbis_I_spec.html#x1-810004.3.9 As you can see there is no 5.1 side. Only 5.1 rear (which is the same as 5.1 back). So opus handle all 5.1 as 5.1 back.
 

Re: What is the deal with 5.1(back)?

Reply #8 –
Quote from: lothario15 on
To OP: I posed a similar question a little while ago.  See here: https://hydrogenaud.io/index.php/topic,110670.0.html

It seems the terms 'back' and 'side' may be arbitrary terms for the same placement position.

Exactly.  I don't get the problem.  Whether a codec refers to a channel as side or rear in a 5.1 setup, they go to the same speaker.

So is this just a semantics complaint, or is there an actual problem?  I understand that ffmpeg has some issues encoding Opus 5.1, but that's ffmpeg's problem.  Are there are other problems, perhaps upmixing to 7.1 setups?