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mp3dec: fix volume for mp3dec

the libmad samples need to be scaled properly. the
previous assumption was that it is 32 bits, but it
really is MAD_F_FRACTBITS + sign bit and rest used
for overflow.

thanks eekee!

--- a/sys/src/cmd/audio/mp3dec/main.c

+++ b/sys/src/cmd/audio/mp3dec/main.c

@@ -46,7 +46,7 @@

 		rate = pcm->samplerate;

 		chans = pcm->channels;

-		snprint(fmt, sizeof(fmt), "s32r%dc%d", rate, chans);

+		snprint(fmt, sizeof(fmt), "s%dr%dc%d", MAD_F_FRACBITS+1, rate, chans);

 		if(ifd >= 0){

 			close(ifd);

@@ -82,6 +82,13 @@

 		p = buf + j*4;

 		for(i=0; i < pcm->length; i++){

 			v = *s++;

+

+			/* clipping */

+			if(v >= MAD_F_ONE)

+				v = MAD_F_ONE-1;

+			else if(v < -MAD_F_ONE)

+				v = -MAD_F_ONE;

+

 			p[0] = v, v>>=8;

 			p[1] = v, v>>=8;

 			p[2] = v, v>>=8;