On Sun, 12 Jan 2025 20:16:12 +0100 Jan Stary <h...@stare.cz> wrote: > These are some example files from Dolby: > https://ott.dolby.com/OnDelKits/DDP/Dolby_Digital_Plus_Online_Delivery_Kit_v1.4.1/Test_Signals/muxed_streams/MPEG2TS/MPEG2TS_Muxed_Streams.html > > Thesr contain 5.1 audio. Playing it on my stereo gear > (I only have two ears anyway), I would expect the players > (mplayer, ffplay, mpv, vlc) to do the reasonable thing: > figure out the "right" position in the stereo panorama > and play it "there". > > But it seems only mplayer does that - for example, > all the other players play the "center" channel on the left. > > If anyone cares about this, can you plese confirm/report what > the various players do with this on your stereo hardware?
I had picked up Godzilla (2014) on DVD a couple months ago and -- while watching it in mpv under OpenBSD amd64/7.5-stable -- noticed that all the dialog audio is fully on the left channel. I investigated headphones, speakers, sndio, and my own ears, but wasn't invested enough to dig deeper once I had watched it in full. I never even got around to testing any other DVDs. (Lazy? Busy?) Your query inspired me. There are plenty of discussions of mixing down from 5.1 & 7.1 to stereo in ffmpeg, mpv, etc. The following is a fairly detailed summary & analysis of solutions: <https://superuser.com/a/1410620> In short, for ffmpeg, passing `-ac 2` will force it to mix down to stereo. Many of the solutions in the aforementioned link discuss alternative audio filters to mix the individual channels in different weights/volumes/mixes for enhanced center channel audio. My testing with mpv showed that using the `--audio-channels=stereo` configuration option correctly mixes down to stereo with dialog correctly mixed across both left & right channels. mpv(1) includes the following: > --audio-channels=auto-safe > > Use the system's preferred channel layout. If > there is none (such as when accessing a hardware device > instead of the system mixer), force stereo. Some audio > outputs might simply accept any layout and do downmixing > on their own. > > This is the default. Further review of sndiod(8): > -j flag > > Control whether program channels are joined or expanded if > the number of channels requested by a program is not equal to the > device number of channels. If the flag is off then client > channels are routed to the corresponding device channel, > possibly discarding channels not present in the device. If the > flag is on, then a single client channel may be sent on multiple > device channels, or multiple client channels may be sent to a > single device channel. For instance, this feature could be used > for mono to stereo conversions. The default is on I don't override the '-j' option for sndiod(8) on my workstation. If my understanding is correct, that means that sndiod(8) will allow a client client (mpv(1), in my case) to send more channels than my device (Intel 9 Series HD Audio; stereo) can process. Most likely, extra channels are probably being discarded. But, forcing to mix down to stereo (`mpv --audio-channels=stereo`, `ffmpeg -ac 2`, etc.) does the correct mixdown (at least according to the ATSC specification) before sending to sndiod(8) and the stereo audio device. I haven't yet tested adding `-j off` to my sndiod(8) flags, but have updated my mpv scripts. Thanks for your question! Morgan Aldridge
