Hello. [Note: the whole e-mail is purely theoretical. I still don't have any surround-sound system in my room. A salesman told me that I should buy 5.1 speakers, not 7.1, because the TV is centered on the long side of the room, and thus not enough space exists behind the listening area. I agree with this argument - so will not be able to test what I am talking about anyway.] I tried to play surround-sound files using various players and look how PulseAudio remixes things, and found some strange results. First, let's start with a simple third-party 5.1 AC3 file: http://distribution.bbb3d.renderfarming.net/audio/bbb3d_sunflower_soundtrack_surround.ac3 If you play it using ffplay, vlc, or with anything gstreamer-based, this will yield the following channel map: front-left,front-right,front-center,lfe,rear-left,rear-right (Surround 5.1). If you use "mpv --channels 6", then it will yield this instead: front-left,front-right,front-center,lfe,side-left,side-right Now let's see what happens in PulseAudio when a user has 5.1 speakers. With ffplay, vlc, and anything gstreamer-based, each channel's sound comes to the correct speaker. With mpv, rear-left and rear-right channels get remixed from other channels (if remixing is enabled), which is bad. If remixing is disabled, then rear speakers stay silent, and the sound in the corresponding channels gets lost. Obviously, the ffplay result is correct here. And now let's see what happens if a user has 7.1 speakers. With ffplay, vlc, and anything gstreamer-based, PulseAudio will send the original signals to the channels mentioned in the channel map, and will remix side-left and side-right outputs from the remaining channels. With mpv, it will send the last two channels of the input stream to side-left and side-right speakers, and remix rear-left and rear-right. That's unless the user disables remixing. If the user disables remixing, then, with ffplay, side channels will remain silent, and with mpv, rear channels will be silent. However, I'd guess that the mpv result is correct here. See http://www.thx.com/consumer/home-entertainment/home-theater/surround-sound-speaker-set-up/ - the "rear" channels have moved between 5.1 and 7.1 layouts. What 5.1 calls "rear" is actually "side" in 7.1 terminology. See also http://www.razersupport.com/gaming-audio/razer-tiamat-71/ - in particular, the answer to the "What is the difference between 5.1 and 7.1 surround sound?" question. How should PulseAudio deal with this nomenclature mess? -- Alexander E. Patrakov
