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

[Andreas Schultz <aschultz@warp10.net>]

Hi Matjaz,

You have apparently a great deal of experience with the AC3 stuff :-),
what do you think about the downmix implementation in newer ac3dec 
version (like http://www.linuxvideo.org/devel/data/ac3dec.tar.gz).

Would it yield better results?

Andreas

Matjaz Thaler wrote:
>>>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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