[mythtv] [mythtv-commits] Ticket #6569: ac3 level is low for 5.1 source

[Mark Buechler]

Hi

On Sun, May 24, 2009 at 6:15 PM, Mark Spieth <mark at digivation.com.au> wrote:

> Comment(by danielk):
>>
>> Mark, have you tried submitting this to the ffmpeg folks?
>>
>
> no. the matrix is incomplete anyhow.
>
> I was watching F1 last night on onehd and it worked well up to a point and
> then it reverted to 5.1 soft (-10dB) and 2.0 normal.
> not sure why yet as it shouldnt have happened.
>
>  A hardcoded gain increase can result in clipping and is not an appropriate
>> way to address the "loud commercials" problem.  This problem varies across
>> different broadcasters/affiliates and really must be fixed at the source.
>> This patch would cause problems on channels that do set proper levels, as
>> well as on other ac3-bearing content, such as DVDs.
>> Additional reading: http://www.tvtechnology.com/article/12644
>>
>
> yes I agree.
> however across all of our HD channels doing 5.1, there was no difference in
> level.
> also a solution is better than none however bad it may be.
>
> we need some more stats for this from more sources to see how this tracks,
> and whether there are any exceptions.
> perhaps there is special processing done for 5.1 in the decoder that does
> not get enabled in the 2.0 case which causes this.
>
> I have seen posts which provide a patch for a52 ac3 decoder, but I suspect
> doesnt fix this in this way either.
> So I know others are seeing this as well.
>
> there is also a 2nd problem which Im testing where, if you are in 6ch mode,
> and ac3 2.0 switches, the audio out gos to 2ch mode and never goes back to
> 6ch mode and you loose the true 6ch mode when 5.1 is restored. all audio is
> then via upmixed functionality. havent tested this patch yet.
>
> cheers
> mark
>

I know all my ac3 5.1 sources have this issue as well. The old liba52 was
better than ffmpeg, but not by much. From what I've been able to tell, this
seems to be an issue with open source implementations of the decoder. Closed
source decoders seem to have appropriate levels.

- Mark.
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