changelog

从FFMPEG转换而来的H264解码程序,VC下编译..

dialog normalisation
--------------------

dialnorm results in an nomalized output of -31 dbFS in Line Mode (see [1] 3.61.)
For 2-channel output (A52_STEREO and A52_DOLBY) this is lowered by additional -3 db
(see [1] 3.6.1)

For more information about Line Mode see [1]


downmixing adjustment
---------------------

the downmixing adjustment prevents clippings of the output samples. The value is based on
worst-case calculations. This has been implemented in liba52 some time ago.

Now this adjusment is compared to the attenuation provided by dialnorm. Since the dialnorm
value attenuates the samples, it's possible that no additional adjustment is neeed.

If not downmixing the signal, dynamic range compression can be applied to the signal.
Bothe 'sides' of compression are allowed, meaning high-level cut compression and low-level
boost compression.

If the signal is downmixed to a 2 channel mode (A52_STEREO or A52_MONO) or to mono
the situation changes.
High-level cut compression isn't allowed, only low-level boost can be applied. (see [1] page 3.6.1
to 3.6.3)

Note: In [1] downmixing is only mentioned when downmixing to 2 channels or to mono. And therefor the
current implementation only disables high-level cut in 2 channel mode or in mono-mode.

This is the current source code portion of that feature (parse.c)

if ((state->output == A52_MONO) || (state->output == A52_STEREO) || (state->output == A52_DOLBY))
{
    if (range > 1.0)
        state->dynrng = state->level * range;
}
else
    state->dynrng = state->level * range;


/* some old notes .... ( by Simone)
* If dynamic range compression is applied to the signal, there's no need to use the downmixing
* adjustment, because the encdoding step takes this into account and calculates the dynanmic range compression
* according to that and stores the resulting value in the ac3 stream.
* ( see [1] downmixing and overload protection, page 4-9)

* If no dynamic range compression is applied the downmixing adjustment has to be used. For more inforamtion
* out this adjustment see [2] Downmixing, page 92-97.

* Planned feature:

* If a user specified fractional dynamcic range is used (as stated in [3]) a frctional adjustment
*  used as well.

* e.g.  0 < dyn_fract < 1    ->   adj_fract = 1 - dyn_fract

*/












Reference documentation used:

[1] Dolby Digital Professional Encoding Guidelines
[2] ATSC A52/A
[3] ../doc/liba52.txt