On Thu 12 Mar 2020 at 02:38:28 (+0100), Emanuel Berg wrote: > Reco wrote: > > >>> Personally I just reencode audio in such cases. > >>> For instance (0:1 is the first audio track, use 0:2 for > >>> english): > >>> > >>> ffmpeg -i ford-v-ferrari-2019.mkv -map 0:0 -map 0:1 \ > >>> -c:v copy \ -c:a libopus -b:a 128k -ac 2 -clev 1.414 > >>> -slev .5 \ ford-v-ferrari-2019.good_audio.mkv > >> > >> That's a useful little recipe, and pleased the OP, but can > >> you just clarify the arguments -clev 1.414 -slev .5 > > > > Sure. First things first - [1]. clev stands for > > center_mix_level, slev for surround_mix_level. Both values > > are dB. > > Thanks for the details! > > This command, it solves the 5.1/surround -> stereo issue, > right? That's the -map, -c:a, -b:a, and -ac 2 things.
My understanding is that -map 0:0 -map 0:1 asks for the video and the portuguese respectively, -c:v copy asks to copy the video (v), -c:a libopus asks for the opus audio encoder, -b:a 128k is 128k bits/sec audio, -ac 2 is two channels of audio (a). > But later on ... > > > What ffmpeg does by default then downmixing is: > > > > FL = FL + 0.707 * FC + 0.707 * BL > > > > where FL stands for Front Left channel, FC is Center, BL is > > Back Left channel. And it's the same for right channel. > > > > Your typical movie has dialogue on center channel, gunfire > > & cars' sounds come from the front channels, ambient comes > > from the back ones. > > ... namely _here_, it "sounds" like you are addressing the > other issue? > > With the -clev and -slev? > > 1.414 because of 2*0.707 ? > > Neither -clev nor slev are in the ffmpeg(1) man page BTW - > and that page is 2419 lines! You need to install ffmpeg-doc and then point your browser (Ctrl-O typically) at file:///usr/share/doc/ffmpeg/manual/ffmpeg-all.html which is a little less than 30000 lines! These arguments still worry me. I *think* the resampler is being used, §28 on that page, and I would expect to see -clev 3 and -slev -3 in order to increase the centre and decrease the surround by 3 dB each (which is doubling/halving the power IIRC). ¹ You could test this by using some more extreme values (up to ±32, which should cut one out altogether) to make sure you hear the effect. > But OK, I can use 'ffprobe -hide_banner 2>&1 $1 | grep -i > audio' and then it should say stereo and not 5.1, gotcha... > > The other problem I can detect myself by (not) hearing... > > Are there anything else you routinely look for and correct? On Thu 12 Mar 2020 at 02:52:31 (+0100), Emanuel Berg wrote: > Reco wrote: > > > Your typical movie has dialogue on center channel, gunfire > > & cars' sounds come from the front channels, ambient comes > > from the back ones. > > > > So, what's needed is to gain volume on dialogues > > OK, so that's > > -clev 1.414 > > since dialog comes from the center. > > > while downplaying ambient somewhat. > > Is surround (-slev 0.5) front _and_ back? Or just back, as > ambient comes from the back? From your description above, it > sounds like one should decrease the front level? But perhaps > this is done as well? AIUI the surround channels are at the back. Just having clev and slev makes it a little unclear. I suppose it's possible that the front L/R channels stay at 0dB while you increase clev and decrease slev, and if you want the front L/R channels decreased, you've just got to do it by increasing clev some more. I'm guessing here. On Thu 12 Mar 2020 at 03:07:23 (+0100), Emanuel Berg wrote: > Reco wrote: > > > FL = FL + 0.707 * FC + 0.707 * BL > > With mediainfo(1) as recommended by Mr. Smedegaard, I got > this > > $ mediainfo ford-v-ferrari-2019.mkv | grep -i channel > Channel(s) : 6 channels > Channel layout : L R C LFE Ls Rs > Channel(s) : 6 channels > Channel layout : L R C LFE Ls Rs > > L = left, R = right, C = center? Yes, all at or near the front. > LEF = low-frequency effects? [1] > > Ls, Rs = left and right subwoofers? > > > [1] https://en.wikipedia.org/wiki/Low-frequency_effects Ls, Rs are the surround channels, LFE is the sub-woofer. You only need one of the latter because you can't locate it with your ears at this frequency. https://en.wikipedia.org/wiki/5.1_surround_sound ¹ Confusingly, the downmix levels at §15.2.1.2 are specified as levels rather than dB. Perhaps this is because they want to be able to have 0 as a possible option, which would be -∞dB. Cheers, David.
