Re: normalize audio in mp4s
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.
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!
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?
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