[FFmpeg-devel] [GSoC] [PATCH 2/2] mlpenc: Working MLP/TrueHD encoder

[Andy Furniss]

Jai Luthra wrote:
> On Sat, Sep 17, 2016 at 05:07:28PM +0100, Andy Furniss wrote:
>> Nice work, this is just a sort of related question really from a
>> user who hasn't taken any notice of TrueHD for a few years.
>>
>> Last I looked I couldn't find much in the way of specs for TrueHD
>> and noticed that the Decoder didn't have anything related to
>> dynamic range control.
>>
>> Is there more info about now?
>>
>> IIRC testing decoder - (depending on content) for downmix or 2
>> channel sub stream the absence of DRC was a bit of a show stopper
>> in that full range can be way too much. The AC3 version of course
>> did have DRC meta and ffmpeg would correctly fully apply it for
>> downmix, so for a stereo listener like me AC3 = good, TrueHD (or
>> DTS) = bad.
>
> The proprietary suite for TrueHD does support dynamic range
> compression, but I'm unsure if (and where) the metadata for DRC is
> present in the bitstream.
>
> The bitstream isn't fully reversed yet, just enough to get things
> working. I think this is a very useful feature for downmixed
> substream to sound good, so I'll try to figure it out in my spare
> time.

Thanks, it would be really good if the meta could be found even just for
the the decoder. I guess it would be a lot more complex for encode anyway.

I notice we both use the term downmix, maybe that's not quite the right
thing to say WRT substreams. I mean that in the sense that THD
substreams are AIUI the base and the higher channel counts are derived.
Which means the stereo mix can be different from what a downmix would
make. I've got a 7.1 speaker check sample somewhere and the stereo
substream has all the speakers at the same level, which is a nice
feature.