[linux-dvb] Re: Video Disk Recorder version 0.90

[Matjaz Thaler <matjaz.thaler@guest.arnes.si>]

> > And a question:
> > If a dvd has DD5.1, what is the sound output? DolbyPrologic or a
> > 2-channel-downmix? My problem is that the different audiotracks
>
> 2-channel Dobly Surround Downmix, look at ac3dec/downmix.c. I don't now
> anything about ac3, any hint and help there is welcome. Newer versions
> of ac3dec do use a slightly different approach there, but there library
> interface is not suited for cdr needs.

I patched downmix.c for dolby surround sound for dvdplayer.
Here are my observations:

If you want to downmix 5 channels to dolby surround with correct volume
level you can use the following algorithm:

RIGHT = right + 0.7 centre + 0.7*(left_sur + right_sur)
LEFT = left + 0.7 centre - 0.7*(left_sur + right_sur)

Sound calculated by above algorithm is distorted.
To downmix 5 channels to dolby surround without overflow
you must divide both channels by 3.1:

RIGHT = (right + 0.7 centre + 0.7*(left_sur + right_sur))/3.1
LEFT = (left + 0.7 centre - 0.7*(left_sur + right_sur))/3.1

The problem in this case is very low volume level.

I did a lot of testing and the compromise was division by 1.7.
With this divider you get distorted sound for example at very
loud explosions, the volume level is acceptable but lover than
DD 2.0 sound as you observed.

We can balance DD 5.1 and DD 2.0 volume level by decreasing
DD 2.0 volume level but this doesn't solve different volume level
for LPCM sound.


Matjaz




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