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

[Andy Furniss]

Moritz Barsnick wrote:

> And is LFE really ignored when downmixing?)

In Dolby and AAC specs it is excluded and there isn't any way for the
metadata to include it. Of course players/ffmpeg may do whatever they
want. IIRC the ITU 5.1 spec also shows how to include it (10db analogue
boost for movie but not music) if you have full range speakers.

DTS meta data can include it and I have a sample that does use it. The
sample in this post doesn't have any downmix meta so if request_channels
2 was used it would get the default codec matrix which doesn't have LFE.

DTS codec matrixes are not fully normalised. This is default -

C L R Ls Rs LFE

[dca @ 0x2a1e620] Stereo downmix coeffs:
[dca @ 0x2a1e620] L, input channel 0 = 0.501187
[dca @ 0x2a1e620] R, input channel 0 = 0.501187
[dca @ 0x2a1e620] L, input channel 1 = 0.707107
[dca @ 0x2a1e620] R, input channel 1 = 0.000000
[dca @ 0x2a1e620] L, input channel 2 = 0.000000
[dca @ 0x2a1e620] R, input channel 2 = 0.707107
[dca @ 0x2a1e620] L, input channel 3 = 0.501187
[dca @ 0x2a1e620] R, input channel 3 = 0.000000
[dca @ 0x2a1e620] L, input channel 4 = 0.000000
[dca @ 0x2a1e620] R, input channel 4 = 0.501187
[dca @ 0x2a1e620] L, input channel 5 = 0.000000
[dca @ 0x2a1e620] R, input channel 5 = 0.000000


This is my sample that does have metadata and includes LFE

[dca @ 0x24f02a0] L, input channel 0 = 0.501190
[dca @ 0x24f02a0] R, input channel 0 = 0.501190
[dca @ 0x24f02a0] L, input channel 1 = 0.707092
[dca @ 0x24f02a0] R, input channel 1 = 0.000000
[dca @ 0x24f02a0] L, input channel 2 = 0.000000
[dca @ 0x24f02a0] R, input channel 2 = 0.707092
[dca @ 0x24f02a0] L, input channel 3 = 0.415680
[dca @ 0x24f02a0] R, input channel 3 = 0.000000
[dca @ 0x24f02a0] L, input channel 4 = 0.000000
[dca @ 0x24f02a0] R, input channel 4 = 0.415680
[dca @ 0x24f02a0] L, input channel 5 = 0.446686
[dca @ 0x24f02a0] R, input channel 5 = 0.446686