games/opl3: use correct sampling rate

games/dmid uses the same sample rate as the chip for music, but other
applications do not.  opl3 and its older version opl2 (not in 9front)
read an input stream of commands in basically IMF format, something
used in other id Software games and some others, which assumes a
given input sampling rate:  700 Hz for Wolfenstein 3D music, 560 Hz
for Commander Keen, 60 Hz for Ultima 6, etc.

The opl3 emulation on the other hand is not really intended to run at
a sampling rate different that the chip's 49.716 kHz sampling rate.
Previously, we assumed it runs at 44.1 kHz and just used the input
rate as a divisor to get the number of samples per delay tic.

From what I understand, the correct way to use it for accurate
emulation is to run the opl chip emulator at its intended sampling
frequency, then downsample to 44.1 kHz.  This means better output
but more code.  The alternative is to basically do the same as
before rev 8433, except with no buffering, but at accuracy/quality
loss.  This change implements the former and just forks pcmconv to
deal with resampling.

2 files changed, 43 insertions, 9 deletions

diff --git a/sys/src/games/dmid.c b/sys/src/games/dmid.c
index 254d42df9..9361605a4 100644
--- a/sys/src/games/dmid.c
+++ b/sys/src/games/dmid.c
@@ -8,7 +8,7 @@ typedef struct Opl Opl;
 typedef struct Chan Chan;
 typedef struct Trk Trk;
 enum{
-	Rate = 44100,
+	Rate = 49716,		/* opl3 sampling rate */
 	Ninst = 128 + 81-35+1,
 
 	Rwse = 0x01,
@@ -236,7 +236,7 @@ setoct(Opl *o)
 	e = freq[n] + (d % 0x1000) * (freq[n+1] - freq[n]) / 0x1000;
 	if(o->c->i->fixed)
 		e = (double)(int)e;
-	f = (e * (1 << 20)) / 49716;
+	f = (e * (1 << 20)) / Rate;
 	for(b=1; b<8; b++, f>>=1)
 		if(f < 1024)
 			break;
diff --git a/sys/src/games/opl3/opl3m.c b/sys/src/games/opl3/opl3m.c
index 3d74e9e65..17dbae1e5 100644
--- a/sys/src/games/opl3/opl3m.c
+++ b/sys/src/games/opl3/opl3m.c
@@ -6,6 +6,10 @@ void	opl3out(uchar *, int);
 void	opl3wr(int, int);
 void	opl3init(int);
 
+enum{
+	OPLrate = 49716,	/* 14318180Hz master clock / 288 */
+};
+
 void
 usage(void)
 {
@@ -16,15 +20,18 @@ usage(void)
 void
 main(int argc, char **argv)
 {
-	int rate, r, v, dt, fd;
+	int rate, n, r, v, fd, pfd[2];
 	uchar sb[65536 * 4], u[5];
+	double f, dt;
 	Biobuf *bi;
 
 	fd = 0;
-	rate = 44100;
+	rate = OPLrate;
 	ARGBEGIN{
 	case 'r':
 		rate = atoi(EARGF(usage()));
+		if(rate <= 0 || rate > OPLrate)
+			usage();
 		break;
 	default:
 		usage();
@@ -35,14 +42,41 @@ main(int argc, char **argv)
 	bi = Bfdopen(fd, OREAD);
 	if(bi == nil)
 		sysfatal("Bfdopen: %r");
-	opl3init(rate);
-	while(Bread(bi, u, sizeof u) > 0){
+	opl3init(OPLrate);
+	if(pipe(pfd) < 0)
+		sysfatal("pipe: %r");
+	switch(rfork(RFPROC|RFFDG)){
+	case -1:
+		sysfatal("rfork: %r");
+	case 0:
+		close(0);
+		close(pfd[1]);
+		dup(pfd[0], 0);
+		execl("/bin/audio/pcmconv", "pcmconv", "-i", "s16c2r49716", "-o", "s16c2r44100", nil);
+		sysfatal("execl: %r");
+	default:
+		close(1);
+		close(pfd[0]);
+	}
+	f = (double)OPLrate / rate;
+	dt = 0;
+	while((n = Bread(bi, u, sizeof u)) > 0){
 		r = u[1] << 8 | u[0];
 		v = u[2];
 		opl3wr(r, v);
-		if(dt = (u[4] << 8 | u[3]) * 4){	/* 16-bit stereo */
-			opl3out(sb, dt);
-			write(1, sb, dt);
+		dt += (u[4] << 8 | u[3]) * f;
+		while((n = dt) > 0){
+			if(n > sizeof sb / 4)
+				n = sizeof sb / 4;
+			dt -= n;
+			n *= 4;
+			opl3out(sb, n);
+			write(pfd[1], sb, n);
 		}
 	}
+	if(n < 0)
+		sysfatal("read: %r");
+	close(pfd[1]);
+	waitpid();
+	exits(nil);
 }