[FFmpeg-user] Input 5.1 DTS, output 2.0 MP3: Atrocious Quality

Bazza lamia at jeack.com.au
Sun May 17 03:07:33 CEST 2015

On Sat, 16 May 2015 10:25:09 +0000 (UTC), Carl Eugen Hoyos
<cehoyos at ag.or.at> wrote:

>Since your answer makes no sense (is ac3 doubly bad?), 
>maybe you could map 1, 2, 3, 4 to out.mp2, out.ac3 and 
>the two wav files?

They say a picture is worth 1000 words etc so I'll do it this way.

This is John's problem. Bad sound when 'downing' 5.1 to stereo.
In his case, an attempt to use libmp3lame. Ignoring bitrates, the
problem seems to boil down to whether the '-ac 2' option is being
fully honoured (in channel numbers AND levels). 
The simple command 'ffmpeg -i inter.dts -ac 2 -ab 320k out.mp3',
for an MP3 file, will produce this result. Plainly distorted. -)
The links are captured PNG image files from Adobe's Audition.

You asked for 4 tests to be carried out. 
Here they are. In order.

> $ ffmpeg -i inter.dts -ac 2 out16.wav
    Conversions to WAV types are OK and will downmix. 
    This one did. Sounds are fine

> $ ffmpeg -i inter.dts -ac 2 -acodec pcm_s32le out32.wav
    Likewise for this other (32) codec. Performed as expected.

> $ ffmpeg -i inter.dts -ac 2 -ab 640k out.ac3
    The process FAILS in this case(s). For AC3 and AAC.
    For these types of outputs, Audition does not recognize
    the file types so, to get around the problem of display,
    the AC3 and AAC types were converted back into a WAV type.
    The channels are there. The output level is just overloaded.
    The resultant output is virtually identical to John's initial
    problem wherein (I guess) the input channels are 'unweighted'

> $ ffmpeg -i inter.dts -ac 2 -ab 320k out.mp2
    I initially thought that the MP2 was similarly irregular but
    now, I think I got myself confused with all the test files.
    This MP2 conversion appears to be OK. It too, is not importable
    into Audition so like (3) above, it had to be rendered to WAV.
    However, it seems to behave properly as expected. 

This is as far as I can go. 
I'd like to believe that '-ac 2' was universal -)
It is, in the sense that all channels are mixed.
The volume option is a workaround. 

More information about the ffmpeg-user mailing list