Crackling in CD audio

[GoogleCodeExporter]

CD audio in Sega CD games sounds very crackly sometimes. This is a bug that
appeared between Gens9f and Gens9j (listen to Track 02 or Track 06 of Sonic
CD in those two versions and the difference is incredibly obvious) so I'm
guessing it's a bug in the volume control feature.

Original issue reported on code.google.com by nitsuja-@hotmail.com on 27 Jul 2008 at 6:26

[GoogleCodeExporter]

I was totally wrong about the volume thing. Still not sure what it is, though. 
It's
weird: When I run the pre-built Gens9j executable (441kb) it has crackling, but 
when
I download the Gens9j source and built it, it does not have crackling (using 
the same
Gens.cfg file both times, of course). Maybe one of the Gens version numbers at
http://www.bluetoaster.net/emu/all.htm is incorrect.

Original comment by nitsuja-@hotmail.com on 27 Jul 2008 at 10:46

[GoogleCodeExporter]

The cause was a defective ASM -> C code port
        int i;
        for (length--, i = 0; length > 0; length--, i++)
        {
            Buf_L[i] += (CD_Audio_Buffer_L[CD_Audio_Buffer_Read_Pos] * CDDAVol) >> 8;
            Buf_R[i] += (CD_Audio_Buffer_R[CD_Audio_Buffer_Read_Pos] * CDDAVol) >> 8;
            CD_Audio_Buffer_Read_Pos++;
            CD_Audio_Buffer_Read_Pos &= 0xFFF;
        }

is appearantly not equivalent to
        int i = 0;
        for (; i <= _length; i++)
        {
            CD_Audio_Buffer_L[(CD_Audio_Buffer_Read_Pos + i) & 0xFFF] >>= 8;
            CD_Audio_Buffer_R[(CD_Audio_Buffer_Read_Pos + i) & 0xFFF] >>= 8;
            CD_Audio_Buffer_L[(CD_Audio_Buffer_Read_Pos + i) & 0xFFF] *= CDDAVol;
            CD_Audio_Buffer_R[(CD_Audio_Buffer_Read_Pos + i) & 0xFFF] *= CDDAVol;
        }
        __asm
        {
            mov ecx, _length
            mov esi, CD_Audio_Buffer_Read_Pos
            mov edi, Buf_L
            dec ecx

loop_L:
            mov eax, CD_Audio_Buffer_L[esi * 4]
            add [edi], eax
            inc esi
            add edi, 4
            and esi, 0xFFF
            dec ecx
            jns short loop_L

            mov ecx, _length
            mov esi, CD_Audio_Buffer_Read_Pos
            mov edi, Buf_R
            dec ecx

loop_R:
            mov eax, CD_Audio_Buffer_R[esi * 4]
            add [edi], eax
            inc esi
            add edi, 4
            and esi, 0xFFF
            dec ecx
            jns short loop_R

            mov CD_Audio_Buffer_Read_Pos, esi
        }

Original comment by Upth...@gmail.com on 27 Jul 2008 at 12:04

• Changed state: Fixed

[GoogleCodeExporter]

Your fix caused another problem, a fuzzy static sound in the background of 
almost
everything. The problem was, you lose information if you shift right before
multiplying by the volume. I don't think that shifting right after multiplying 
can
cause clipping issues in this case, unless the volume value is much higher than 
256
(which it isn't allowed to be), so I changed it back to multiply then shift.

Original comment by nitsuja-@hotmail.com on 27 Jul 2008 at 7:40