Hitolis wrote:
> When downmixing from 7.1 or 5.1 to 2.0 stereo, matrix normalisation does not
> work with the DTS MA format. Downmixing currently works for TrueHD and AC3;
> E-AC3 always works.
I reproduced the level difference on 5.1. Note the limit of my test: ffmpeg
cannot encode DTS-HD MA, so I used ffmpeg's DTS core encoder (dca) and the same
decoder you are hitting. ffmpeg n8.1.3 here.
Method: one channel excited at a time (1 kHz sine, -20 dB, 48 kHz), 5.1(side),
single channel reading -41.09 dBFS RMS. Decode to 2 channels, measure on f32le.
excited channel dts -ac 2 ac3 -ac 2
FC (center) -44.10 dB -45.62 dB -> gain 0.707 vs 0.593
SL/SR (surround) -44.10 dB -47.13 dB -> gain 0.707 vs 0.500
LFE dropped dropped (~-114 dB)
So in the DTS path the center arrives 1.5 dB hotter and the surrounds 3.0 dB
hotter than the same signal through AC3. With five or six channels summing that
is exactly the "too loud, clips" you hear.
The cause is not the dca decoder misreading the layout. It is that the Dolby
decoders downmix inside the decoder and dca never does. In libavcodec/ac3dec.c,
ac3_downmix() rewrites avctx->ch_layout to stereo when a stereo downmix is
requested, and the sample loops then call ff_ac3dsp_downmix() with
s->downmix_coeffs, which come from the bitstream (cmixlev, surmixlev, dmixmod).
dca_core.c only handles the XXCH case where the decoder was told the encoder
already downmixed; for core and XLL it hands out the native layout, and
libswresample/rematrix.c applies its generic matrix: center 0.707, surround
0.707, by default. The DTS downmix metadata is ignored. Hence no attenuation
where AC3 has it.
Workaround I ran here, on a stream that decodes to 5.1(side):
ffmpeg -i input.mkv -filter_complex \
"[0:a]pan=stereo|c0=0.707*FL+0.707*FC+0.5*SL|c1=0.707*FR+0.707*FC+0.5*SR" \
-c:a pcm_f32le out.wav
With the surround-only stimulus that lands at -47.11 dB, matching the AC3 path
(-47.13). If the decoded layout is 7.1, add +0.5*BL and +0.5*BR. Mind the
names: pan fails with 'Channel "SL=c0" does not exist in the chosen layout' if
the stream is 5.1 rather than 5.1(side).
Two smaller things. "Invalid downmix layout" is not an ffmpeg string: I grepped
libavcodec/ac3dec.c, libavcodec/dca_core.c and libswresample/rematrix.c in
master and there is no such message, so that line is most likely your player's
own downmix setting, not the decoder's. And 'ffplay -downmix 2' is not a current
option; -ac 2 is the one that takes effect.
Iris
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