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Fixed bug #292

I might be on crack here.

It looks like SDL_ConvertMono() in src/audio/SDL_audiocvt.c adds the left and
right channels of a stereo stream together, and clamps the new mono channel if
it would overflow.

Shouldn't it be dividing by 2 to average the two sample points instead of
clamping? Otherwise the mono sample point's volume doubles in the conversion.
This would also make the conversion faster, as it replaces two branches per
sample frame with a bitwise shift.

--ryan.
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slouken committed Sep 24, 2006
1 parent 208b75c commit 19cf0b67c78eb6574a15091c99c39f60c8500f31
Showing with 18 additions and 50 deletions.
  1. +18 −50 src/audio/SDL_audiocvt.c
@@ -45,11 +45,7 @@ SDL_ConvertMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
dst = cvt->buf;
for (i = cvt->len_cvt / 2; i; --i) {
sample = src[0] + src[1];
if (sample > 255) {
*dst = 255;
} else {
*dst = (Uint8) sample;
}
*dst = (Uint8) (sample / 2);
src += 2;
dst += 1;
}
@@ -64,13 +60,7 @@ SDL_ConvertMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
dst = (Sint8 *) cvt->buf;
for (i = cvt->len_cvt / 2; i; --i) {
sample = src[0] + src[1];
if (sample > 127) {
*dst = 127;
} else if (sample < -128) {
*dst = -128;
} else {
*dst = (Sint8) sample;
}
*dst = (Sint8) (sample / 2);
src += 2;
dst += 1;
}
@@ -87,29 +77,21 @@ SDL_ConvertMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
for (i = cvt->len_cvt / 4; i; --i) {
sample = (Uint16) ((src[0] << 8) | src[1]) +
(Uint16) ((src[2] << 8) | src[3]);
if (sample > 65535) {
dst[0] = 0xFF;
dst[1] = 0xFF;
} else {
dst[1] = (sample & 0xFF);
sample >>= 8;
dst[0] = (sample & 0xFF);
}
sample /= 2;
dst[1] = (sample & 0xFF);
sample >>= 8;
dst[0] = (sample & 0xFF);
src += 4;
dst += 2;
}
} else {
for (i = cvt->len_cvt / 4; i; --i) {
sample = (Uint16) ((src[1] << 8) | src[0]) +
(Uint16) ((src[3] << 8) | src[2]);
if (sample > 65535) {
dst[0] = 0xFF;
dst[1] = 0xFF;
} else {
dst[0] = (sample & 0xFF);
sample >>= 8;
dst[1] = (sample & 0xFF);
}
sample /= 2;
dst[0] = (sample & 0xFF);
sample >>= 8;
dst[1] = (sample & 0xFF);
src += 4;
dst += 2;
}
@@ -127,35 +109,21 @@ SDL_ConvertMono(SDL_AudioCVT * cvt, SDL_AudioFormat format)
for (i = cvt->len_cvt / 4; i; --i) {
sample = (Sint16) ((src[0] << 8) | src[1]) +
(Sint16) ((src[2] << 8) | src[3]);
if (sample > 32767) {
dst[0] = 0x7F;
dst[1] = 0xFF;
} else if (sample < -32768) {
dst[0] = 0x80;
dst[1] = 0x00;
} else {
dst[1] = (sample & 0xFF);
sample >>= 8;
dst[0] = (sample & 0xFF);
}
sample /= 2;
dst[1] = (sample & 0xFF);
sample >>= 8;
dst[0] = (sample & 0xFF);
src += 4;
dst += 2;
}
} else {
for (i = cvt->len_cvt / 4; i; --i) {
sample = (Sint16) ((src[1] << 8) | src[0]) +
(Sint16) ((src[3] << 8) | src[2]);
if (sample > 32767) {
dst[1] = 0x7F;
dst[0] = 0xFF;
} else if (sample < -32768) {
dst[1] = 0x80;
dst[0] = 0x00;
} else {
dst[0] = (sample & 0xFF);
sample >>= 8;
dst[1] = (sample & 0xFF);
}
sample /= 2;
dst[0] = (sample & 0xFF);
sample >>= 8;
dst[1] = (sample & 0xFF);
src += 4;
dst += 2;
}

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