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ResidualDecoding Help
post Sep 13 2012, 22:44
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I read the documention of here http://flac.sourceforge.net/format.html#residual about the residual. And I think I ve got a quite good overview but I still wasn t able to write a full decoder.
So I searched after that on the internet and I found two different codes. The first one is the official libflac libary.
That code is very very big if you take a look at the FLAC__bitreader_read_rice_signed_block function in bitreader.c.

The other libary was the flake libary of cuetools (also posted in this forum):
That libary does all that very very easy and in much less code. But I dont understand the code.
So may anyone of you explain me what that code does and how is it possible that the code is so small.
So I thought about that it may does not support highest compression level. And I got it right. It does not support all flac files export from audacity.
If I use compression level 8 it will fail.

So may someone of you tell me a bit about it and how the residual is stored ...

here is the code of cuetools:
        unsafe void decode_residual(BitReader bitreader, FlacFrame frame, int ch)
            // rice-encoded block
            // coding method
            frame.subframes[ch].best.rc.coding_method = (int)bitreader.readbits(2); // ????? == 0
            if (frame.subframes[ch].best.rc.coding_method != 0 && frame.subframes[ch].best.rc.coding_method != 1)
                throw new Exception("unsupported residual coding");
            // partition order
            frame.subframes[ch].best.rc.porder = (int)bitreader.readbits(4);
            if (frame.subframes[ch].best.rc.porder > 8)
                throw new Exception("invalid partition order");
            int porder = frame.blocksize >> frame.subframes[ch].best.rc.porder;
            int res_cnt = porder - frame.subframes[ch].best.order;

            int plen = 4 + frame.subframes[ch].best.rc.coding_method;
            // residual
            int order = frame.subframes[ch].best.order;
            int* r = frame.subframes[ch].best.residual + order;
            int partitions = 1 << frame.subframes[ch].best.rc.porder;
            for (int p = 0; p < partitions; p++)
                if (p == 1) res_cnt = porder;
                int n = Math.Min(res_cnt, frame.blocksize - order);

                int k = frame.subframes[ch].best.rc.rparams[p] = (int)bitreader.readbits(plen);
                if (k == (1 << plen) - 1)
                    k = frame.subframes[ch].best.rc.esc_bps[p] = (int)bitreader.readbits(5);
                    for (int i = n; i > 0; i--)
                        *(r++) = bitreader.readbits_signed((int)k);
                    bitreader.read_rice_block(n, (int)k, r);
                    r += n;
                order += n;

Please help me sad.gif
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post Sep 14 2012, 00:02
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This psuedocode-heavy explanation of FLAC decoding with residuals starting on page 28, which might help clarify how it operates from a programmer's perspective.

This post has been edited by tuffy: Sep 14 2012, 00:02
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