[vdr] DVD with AC3 audio question

Dr. Werner Fink werner at suse.de
Tue Mar 7 11:52:13 CET 2006


On Tue, Mar 07, 2006 at 09:04:59AM +0200, Kartsa wrote:
> 
> Now I tried without the -a option and got the PCM light on in the receiver
> but there were no sound. Not even noise. I got VDR somehow freesed while I
> was testing when I was trying to select the audio (for some reason there
> seemed to be two audio tracks on the DVD even though I had put there only
> one :/). There was something like "tried to open OSD although OSD allready
> open" in the log file (I can not verify that right now). So I had to reboot
> in order to get normal working state again. Then it began to record and I
> could not test with a commercial DVD (wife factor, you know). But I have to
> say that the DVD I made works fine in my set top DVD player.
> 
> Anyway the reason can then be my receiver if it tests the bit and ignores
> PCM structure but somehow stops the noise.

For small AC3 frames (each with a delay of 32ms at 48kHz) which
are filled into a PCM frames of 6144 bytes (32ms at 48kHz) with
a special start sequence and padded with zeros you get for the
most time silence if the PCM frames are handled as a linear PCM
stream.

> So how is it that this setting seems to work for some and not for some
> (unles it is all about the receiver).

IMHO the receiver will work if and only if the nonlinear data bitw
is set in the S/P-DIF data transfer stream.  The problem could be
that not only the DVB-S FF cards can not set this bit but also
some older soundcards or soundchips can not set this bit.

On the other hand, without the nonlinear data bit some receivers
may not like gaps in the data stream caused by underruns (e.g.
high load or high data rates which can not handled by the dvd
plugin and/or VDR) and switch back into linear PCM mode.


       Werner

-- 
AC3 loop through sound card http://bitstreamout.sourceforge.net/
Howto http://www.vdr-portal.de/board/thread.php?threadid=1958
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