[Bluez-devel] Bug in SBC

["Christian Hoene" <christian.hoene@gmx.net>]

Hello,

Let me forward some comments from Frans de Bont, the inventor of SBC. He
found a bug in the encoder but is not allowed to change the BlueZ
implementation. Thus, I forward you his error description because these
weeks I hardly have any time to look at the source code myself and - I have
to admit - I am getting older and coding lazy.

With best regards,
 
 Christian Hoene

PS:
Here Frans comments:

I also have taken a quick look into the filtering part of the encoder. It
seems that this code does not behave as the reference code. Unfortunately, I
cannot disclose this part of the reference source code. However, if someone
wants to correct this, the flow chart in the MPEG-1 Audio standard could be
taken as an example with slightly difference parameters, i.e. subbands=32
and filterlength=512.

You can find the referred flow chart in Figure C.4 on page 78 (in my version
of the ISO/IEC 11172-3 standard). Please note that the decoder flow charts
in A2DP have been derived from Figure A.2 on page 39.

You can find ... public version on:
http://www.mp3-tech.org/programmer/docs/iso11172-3.zip

For the encoder filterbank, you can find this in annex_c.doc, page 13,
FIGURE 3-C.2, "Layer I, II encoder flow chart".
The decoder filterbank is shown in annex_ab.doc, page 2, Figure 3-A.3,
"Layer III decoder flow chart".

BTW, I noticed that earlier versions of the Bluez SBC codec are indeed based
on the MPEG-1 flow charts. Probably the conversion from floating-point to
fixed-point has introduced the quality degradation.

Best regards,
Frans



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