GNOME Bugzilla – Bug 564019
Downmixing 5.1 -> stereo does not anymore include LFE channel information
Last modified: 2008-12-11 19:59:20 UTC
As per http://lwn.net/Articles/310346/, filing a bug about a regression I have noticed, ie. that GStreamer does not anymore (at least automatically) include LFE channel when downmixing from 5.1. Easiest way to see this to fetch a file called Test AC3 v2.0.avi from Internet (unfortunately probably non-free file as such), ie. http://www.google.com/search?q="test+ac3+v2.0" Try it out, and observe that during LFE playback you don't here anything using stereo output. For that matter, those of you with true 5.1 equipments would probably want to test it out anyway - with a quick test I was seemingly not really able to get the LFE information to my amplifier even in multi-channel output when using totem-gstreamer (5.1 setting, and there is another bug with totem filed that the pass-through cannot be selected when using gstreamer backend). Using GStreamer packages from ubuntu's development version (9.04). I think it worked during 8.04 LTS times (gstreamer 0.10.18), and at least during 7.10 (gstreamer 0.10.14). I'm not really sure if this problem is in the AC3 decoder or somewhere higher in the channel mapping code. If anyone has information about a similar libre test clip that could be included in the test suite, please tell.
Could you play this file with GST_DEBUG=a52dec:5,audioconvert:5 GST_DEBUG_NO_COLOR=1 totem /path/to/file &> log and then attach the created file "log" (probably needs to be compressed).
And also tell us which audio configuration you selected in Totem's preferences (default is Stereo).
Created attachment 124412 [details] Log using analog stereo output from the motherboard, Totem set to Stereo.
Created attachment 124413 [details] Log from digital output, Totem (unfortunately) set to Stereo.
The analog output is what used to work before. Now it doesn't give me the LFE in the downmix anymore. I'm using GStreamer set to ALSA in gstreamer-properties, with no pulseaudio installed. So I frustrated with the diminishing options to get proper 5.1 -> 2 downmix. I have an asound.conf with proper pcm.51to20 device, but also that cannot be specified for GStreamer (at least not in gstreamer-properties), only to Xine or MPlayer. Since I really like GStreamer (and totem) otherwise, I purchased an SPDIF card to try out, but unfortunately it's even worse with GStreamer - I hear nothing using GStreamer output. I included a log file using also that digital output, but unfortunately Totem set to Stereo because pass-through cannot be really selected (I now found the bug - it's bug #331875). I tried also 5.1 and other selections. I don't get anything through the speakers using Totem (GStreamer) or even Rhythmbox. The only place I do get sound out from the digital output using GStreamer is gstreamer-properties test signal, where I've explicitly selected the Digital device. I have no idea why the actual applications produce nothing. Xine, MPlayer etc. play fine, even totem-xine using any speaker selection though pass-through is the recommended one. I'm currently using totem-xine since it gives me LFE via the digital route for the amplifier to down-mix when using Pass-Through, and also with plain xine and mplayer I'm able to use the mentioned 51to20 alsa output device. The only thing that now sucks is that I'd want my GStreamer back some day, and now that I have the digital output, preferably with that :O (btw, I mentioned testing 5.1 in the original text - in that case I meant only analog 5.1 output, but I really don't have even enough wires for it currently)
This a bug in liba52 then. The downstream elements claim to only support stereo (configuration problem on your side) and then the liba52 gstreamer element sets the output to stereo. liba52 then handles the downmixing. The GStreamer downmixing via audioconvert includes information from LFE when downmixing from 5.1 to stereo. Closing this bug as NOTGNOME
I'm still a bit curious since the last upstream release of liba52 is from several years ago, and there seemingly hasn't really been patches in the packaging either, yet this problem has only occurred for less than a year with GStreamer. I also did not understand the stated configuration problem on my side - like stated, the log using analog output is in a situation where GStreamer is configured to use Stereo, and only one RCA wire (stereo front channels) is leading to the amplifier. But probably if liba52 cannot handle it, Totem should use that GStreamer's downmixing feature for providing better experience for the users. A wishlist bug, then?