What is the difference between 5.1 and 5.1 (side) audio?

[orion44]

FFmpeg uses the following 5.1 channel layouts:

5.1 (FL+FR+FC+LFE+BL+BR) and

5.1 (side) (FL+FR+FC+LFE+SL+SR)

When downmixing a 5.1 audio (DTS or AC3) to stereo, which channel layout should I use?

I made two batch scripts:

ffmpeg.exe -i "D:\Project\DTS.dts" -af "pan=stereo|FL=FL+0.7071*FC+0.7071*BL|FR=FR+0.7071*FC+0.7071*BR" "D:\Project\WAV.wav"
ffmpeg.exe -i "D:\Project\DTS.dts" -af "pan=stereo|FL=FL+0.7071*FC+0.7071*SL|FR=FR+0.7071*FC+0.7071*SR" "D:\Project\WAV.wav"

Which of the two should I use?

[tebasuna51]

The Side channels are at +-90º from the Center channel.
The Back channels are at +-140º

But the Surround channels in AC3 or DTS are at +-120º. None of them are equivalent.
In first times the Surround channels was assigned to Back channels but recently the Side are the preferred.
Both must be equivalents in standard 5.1.

ffmpeg suppose, without any reason, than DTS 5.1 is:
Stream #0:0: Audio: dts (DTS), 48000 Hz, 5.1(side)
Then you need use the Side channels, because the Back are ignored.
The correct behaviour must be admit both syntax.

The same for AC3:
Stream #0:0: Audio: ac3, 48000 Hz, 5.1(side)

[orion44]

FFmpeg reports:

Input #0, dts, from 'DTS.dts':
Duration: 02:13:48.56, start: 0.000000, bitrate: 1536 kb/s
Stream #0:0: Audio: dts (DTS), 48000 Hz, 5.1(side), fltp, 1536 kb/s

So I'll use the 5.1(side) channel layout.

Thanks!