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What are the differences between "side" channels and "back" channels in multichannel audio?

I'm converting .mlp files (from a DVD-A) to FLAC and I'm slightly confused by how the channels are positioned.

e.g. The conversion is:

avconv -i multichannel.mlp output.flac

Running mediainfo on the mlp and flac both give:

Channel(s)                               : 4 channels

If I convert the mlp to a wav, mediainfo gives me:

Channel(s)                               : 4 channels
Channel positions                        : Front: L R, Side: L R

If I convert the flac to a wav, mediainfo gives me:

Channel(s)                               : 4 channels
Channel positions                        : Front: L R, Back: L R

Everything else about the files is identical (including how they sound!)

What's the cause of this difference? Is there any real difference between "side" and "back"?

llogan
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Terence Eden
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2 Answers2

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There is long story about Side vs Rear in WAV, due to changes during time about how should be considered 5.1 (the change happenned when 71 was introduced).

After 7.1, "Surround" was split in "Side" and "Rear". Some tools map "Surround" (from 5.1, or 4.0) to "Side", some others to "Rear". So mapping depends mostly of the tool you use for conversion and the output format.

Jérôme, developer of MediaInfo.

Jérôme Martinez
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Off the top of my head (as I can't currently reach my DVD-A discs to check), I think that DVD-A DTS uses either 7.1 or DTS-HD audio format, both of which add 2 side speakers to the more familiar 5.1 setup. This might be what you're seeing in the .mlp file and your software could be picking the first 4 channels it sees (this last bit is pure speculation, sorry).

http://www.thx.com/consumer/home-entertainment/home-theater/surround-sound-speaker-set-up/

Whereas the flac file more likely has the 5.1 downmix which is giving you the 4 speakers you'd expect.

Wikipedia has details of channel numbers etc, as well as speaker names and set-ups further down the page.

https://en.wikipedia.org/wiki/Surround_sound#Channel_identification

lizconlan
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