Flac - Sep 2004 - [Flac-users] Extending mp3/ogg application to play flac

Hello,

Flac is great and I have encoded all my "good" CDs into it!

Now I want to extend an existing application able to decode mp3 and ogg
vorbis to replay flac as well. It is a plugin to a set top box
application on Linux (VDR).

The existing code decodes into a mad_pcm structure using mad fixed point
format (mad_fixed_t).

I have read the API of libFlac++ and I understand what I have to do in
general.

However there are two issues I'm unsure about:

 - Do I need to convert the decoded audio data delivered to the
   FileDecoderWriteCallback to be compatible with mad_pcm/mad_fixed_t
data?

 - The mad_pcm structure expects 1152 samples per channel.
   How Do I obtain 1152 samples from the Flac decoder?

Any help is appreciated,
Lars

> not sure about the mad_fixed_t format, but probably.  all 
 > samples coming out of the FLAC decoder are linear PCM 32-bit 
 > signed ints. e.g. a sample from CD audio will be in the 
 > range -32768 (0xFFFF8000) to 32767 (0x00007FFF)
Mad_fixed_t is defined as a 4-byte signed int in the headers.
As the subsequent application seems to expect linear PCM, I'll simply
give it a try...

 > since the write callback will be called once for each frame, 
 > and the number of samples in a FLAC frame can vary, your 
 > write callback will probably have to write to an 
 > intermediate buffer.
Okay, I had this in mind but wasn't sure. Thanks for the reference.

Lars

--- Lars von Wedel <vonwedel@web.de> wrote:
> However there are two issues I'm unsure about:
> 
>  - Do I need to convert the decoded audio data delivered to the
>    FileDecoderWriteCallback to be compatible with mad_pcm/mad_fixed_t
> data?
not sure about the mad_fixed_t format, but probably.  all samples
coming out of the FLAC decoder are linear PCM 32-bit signed ints.
e.g. a sample from CD audio will be in the range -32768 (0xFFFF8000)
to 32767 (0x00007FFF)
>  - The mad_pcm structure expects 1152 samples per channel.
>    How Do I obtain 1152 samples from the Flac decoder?
since the write callback will be called once for each frame, and
the number of samples in a FLAC frame can vary, your write callback
will probably have to write to an intermediate buffer.  the same
thing has to happen in the winamp2 plugin, so you can look at that
code for an example (search for reservoir_ and SAMPLES_PER_WRITE).

http://cvs.sourceforge.net/viewcvs.py/flac/flac/src/plugin_winamp2/in_flac.c?view=markup

Josh



		
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