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audio/pcmconv: dithering

--- a/sys/src/cmd/audio/pcmconv/pcmconv.c

+++ b/sys/src/cmd/audio/pcmconv/pcmconv.c

@@ -141,7 +141,7 @@

 }

 int*

-resample(Chan *c, int *x, int *y, ulong count)

+resample(Chan *c, int *x, int *y, int count)

 {

 	ulong e;

 	int i, n;

@@ -197,6 +197,20 @@

 }

 void

+dither(int *y, int ibits, int obits, int count)

+{

+	static ulong prnd;

+

+	if(ibits >= 32 || obits >= ibits)

+		return;

+

+	while(count--){

+		prnd = (prnd*0x19660dL + 0x3c6ef35fL) & 0xffffffffL;

+		*y++ += ((int)prnd) >> ibits;

+	}

+}

+

+void

 siconv(int *dst, uchar *src, int bits, int skip, int count)

 {

 	int i, v, s, b;

@@ -497,6 +511,7 @@

 			l -= n;

 		n /= i.framesz;

 		(*iconv)(in, ibuf, i.bits, i.framesz, n);

+		dither(in, i.bits, o.bits, n);

 		m = resample(&ch[0], in, out, n) - out;

 		if(m < 1){

 			if(n == 0)

@@ -506,6 +521,7 @@

 		if(i.channels == o.channels){

 			for(k=1; k<i.channels; k++){

 				(*iconv)(in, ibuf + k*((i.bits+7)/8), i.bits, i.framesz, n);

+				dither(in, i.bits, o.bits, n);

 				resample(&ch[k], in, out, n);

 				if(m > 0)

 					(*oconv)(out, obuf + k*((o.bits+7)/8), o.bits, o.framesz, m);

-- 

⑨