mirror of https://github.com/bsnes-emu/bsnes.git
62 lines
1.6 KiB
C++
62 lines
1.6 KiB
C++
auto DSP::brrDecode(Voice& v) -> void {
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//state.t_brr_byte = ram[v.brr_addr + v.brr_offset] cached from previous clock cycle
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int nybbles = (state._brrByte << 8) + apuram[(uint16)(v.brrAddress + v.brrOffset + 1)];
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const int filter = (state._brrHeader >> 2) & 3;
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const int scale = (state._brrHeader >> 4);
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//decode four samples
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for(auto n : range(4)) {
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//bits 12-15 = current nybble; sign extend, then shift right to 4-bit precision
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//result: s = 4-bit sign-extended sample value
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int s = (int16)nybbles >> 12;
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nybbles <<= 4; //slide nybble so that on next loop iteration, bits 12-15 = current nybble
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if(scale <= 12) {
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s <<= scale;
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s >>= 1;
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} else {
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s &= ~0x7ff;
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}
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//apply IIR filter (2 is the most commonly used)
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const int p1 = v.buffer[12 + v.bufferOffset - 1];
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const int p2 = v.buffer[12 + v.bufferOffset - 2] >> 1;
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switch(filter) {
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case 0:
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break;
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case 1:
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//s += p1 * 0.46875
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s += p1 >> 1;
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s += (-p1) >> 5;
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break;
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case 2:
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//s += p1 * 0.953125 - p2 * 0.46875
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s += p1;
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s -= p2;
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s += p2 >> 4;
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s += (p1 * -3) >> 6;
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break;
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case 3:
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//s += p1 * 0.8984375 - p2 * 0.40625
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s += p1;
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s -= p2;
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s += (p1 * -13) >> 7;
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s += (p2 * 3) >> 4;
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break;
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}
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//adjust and write sample (mirror the written sample for wrapping)
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s = sclamp<16>(s);
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s = (int16)(s << 1);
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v.buffer[v.bufferOffset + 0] = s;
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v.buffer[v.bufferOffset + 12] = s;
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v.buffer[v.bufferOffset + 24] = s;
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if(++v.bufferOffset >= 12) v.bufferOffset = 0;
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}
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}
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