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

Bazza lamia at jeack.com.au
Sat May 16 12:09:37 CEST 2015

On Sat, 16 May 2015 09:17:59 +0000 (UTC), Carl Eugen Hoyos
<cehoyos at ag.or.at> wrote:

>Bazza <lamia <at> jeack.com.au> writes:
>> >Please test the following:
>> >$ ffmpeg -i inter.dts -ac 2 out16.wav
>> >$ ffmpeg -i inter.dts -ac 2 -acodec pcm_s32le out32.wav
>> >$ ffmpeg -i inter.dts -ac 2 -ab 640k out.ac3
>> >$ ffmpeg -i inter.dts -ac 2 -ab 320k out.mp2
>> Carl, I did test some of this stuff.
>> - MP3 suffers overload
>> - WAVs are OK
>> - AC3 suffers overload
>> - AAC are OK
>> - MP2 are OK
>Sorry, I am apparently extremely dim-witted:
>Did you test the four lines above?
>Which of them sound ok, which of them do 
>not sound ok?
>Thank you, Carl Eugen

Tested them all Carl. It's not quite a case of "sounding OK"
it's that they are patently just "wrong". I'm using a Windows
/Zeranoe build. Observing results through Adobe's Audition.
Results are independant of bit rate (as you specify),I tried 
those (and some other rates). All fail.
Also, it appears to be independant of the codecs 16 Vs 32 etc.
The audio signals in John's sample have 6 mono. Most of those
are up to the clipping level when viewed separately.

When he "combines" them (let's say via declaring -ac 2) the
channels (all 6) do indeed mix but the 'numbers' are summed
greater than the streams capability - hence severe clipping and
overload. This appears to happen in the case of AC3 and MP3
(which is John's complaint).

In doing a bit of reading, it seems to be the case that L,R,
FL and FR are usually attenuated by 3dB per signal. Maybe that
'routine' is being bypassed (or not even called) in AC3 or MP3
situations. This is a pure guess but the numbers do "add up"
when we drop a volume by 10 dB. Luckily his levels nearly clip.

Now, I must add, Adobe's Audition does not like viewing AC3 
and AAC stuff directly so I do a re-convert from the AC3 and 
AAC back into WAV (just to view) but, no doubts about it, does 
not look good. Output in the non-behaving file format is just 
too high a level. FFMPEG generates no complaints so that's good.

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