[vdr] pre-emphasis flag on S/PDIF out etc. (mp3 plugin) ?

[Lutz Pressler <Lutz.Pressler@SerNet.DE>]

Hi,

I haven't searched extensively where it would have to be
implemented, probably someone here can tell off the cuff:

CD-DA (and also the MP3 format, even if obviously most
decoders don't care) defines a "pre-emphasis" flag for
tracks (or even via subchannel flag for parts of it?).
If it is set, the higher frequency part of the signal
is amplified in the PCM data. CD players correct that
by analogue circuitry after D/A conversion.
The reason is to reduce D/A noise.

Even if today there are better anti noise measures
and the pre-emphasis mechanism is not used for current
productions, there are quite a lot of CDs (especially
classics) which use it.

Now for S/P-DIF/VDR/MP3 plugin:

1. The MP3 plugin is able to play back CDs via Digital
   Audio Extraction. At the moment, the pre-emphasis
   flag is not taken care of.

2. If that would be changed, it can be used in two ways:
a) deemphasize on the fly (cf. sox "deemph" effect).
b) send the unaltered PCM data through the S/P-DIF interface
   but also set the pre-emphasis flag, which is defined in
   the bitstream. Digital receivers should do the correction
   on their own then.

   Is this feasable for output via the DVB card respectively
   an sound card? API change necessary?

Probably recognition of the pre-emphasis flag is more difficult
than the rest (cdfs for CDs, what is libmad doing for MP3s?).

Worth the effort? What do you think? Unfortunately
pre-emphasized data cannot be played back correctly as it is
(apart from converting manually).


Greetings,
  Lutz


-- 
Lutz Preßler, Göttingen, Germany  -- phone: +49-700-77377537




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