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.

--HG--
branch : SDL-1.2
extra : convert_revision : svn%3Ac70aab31-4412-0410-b14c-859654838e24/branches/SDL-1.2%402118

src/audio/SDL_audiocvt.c

@@ -44,11 +44,7 @@ void SDLCALL SDL_ConvertMono(SDL_AudioCVT *cvt, Uint16 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;
 			}
@@ -62,14 +58,7 @@ void SDLCALL SDL_ConvertMono(SDL_AudioCVT *cvt, Uint16 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;
 			}
@@ -85,14 +74,10 @@ void SDLCALL SDL_ConvertMono(SDL_AudioCVT *cvt, Uint16 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;
 				}
@@ -100,14 +85,10 @@ void SDLCALL SDL_ConvertMono(SDL_AudioCVT *cvt, Uint16 format)
 				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;
 				}
@@ -124,18 +105,10 @@ void SDLCALL SDL_ConvertMono(SDL_AudioCVT *cvt, Uint16 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;
 				}
@@ -143,18 +116,10 @@ void SDLCALL SDL_ConvertMono(SDL_AudioCVT *cvt, Uint16 format)
 				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;
 				}