- Added dynamic recentering for the OPL synth. The chip has four basic

waveforms, and three of them are non-negative. This can cause a tendency
  for the resulting output waveform to go into very high ranges depending on
  the timbres used, and Heretic's exemplify this problem.
- Reduced the OPL volume level slightly.
- Fixed: The waveform view from snd_drawoutput was upside-down.


SVN r949 (trunk)

docs/rh-log.txt

@@ -1,4 +1,10 @@
 April 28, 2008
+- Added dynamic recentering for the OPL synth. The chip has four basic
+  waveforms, and three of them are non-negative. This can cause a tendency
+  for the resulting output waveform to go into very high ranges depending on
+  the timbres used, and Heretic's exemplify this problem.
+- Reduced the OPL volume level slightly.
+- Fixed: The waveform view from snd_drawoutput was upside-down.
 - Various fixes for compiling working 64-bit binaries with Visual C++. The
   number of changes was pleasantly small, and a cursory check seems to show
   everything working alright.

src/oplsynth/fmopl.cpp

@@ -1638,7 +1638,7 @@ static bool CalcVoice (FM_OPL *OPL, int voice, float *buffer, int length)
 				output[0] += tl_tab[p];
 			}
 			// [RH] Convert to floating point.
-			buffer[i] += float(output[0]) / 7168.f;
+			buffer[i] += float(output[0]) / 10240;
 		}
 
 		// advance

src/oplsynth/opl_mus_player.cpp

@@ -3,6 +3,7 @@
 #endif
 #include <string.h>
 #include <assert.h>
+#include <math.h>
 
 #include "opl_mus_player.h"
 #include "doomtype.h"
@@ -21,6 +22,7 @@ OPLmusicBlock::OPLmusicBlock()
 {
 	scoredata = NULL;
 	NextTickIn = 0;
+	LastOffset = 0;
 	TwoChips = !opl_onechip;
 	Looping = false;
 	io = NULL;
@@ -47,6 +49,7 @@ void OPLmusicBlock::Restart()
 	OPLplayMusic (127);
 	MLtime = 0;
 	playingcount = 0;
+	LastOffset = 0;
 }
 
 OPLmusicFile::OPLmusicFile (FILE *file, char *musiccache, int len)
@@ -209,6 +212,7 @@ bool OPLmusicBlock::ServiceStream (void *buff, int numbytes)
 			{
 				YM3812UpdateOne (1, samples1, samplesleft);
 			}
+			OffsetSamples(samples1, samplesleft);
 			assert(NextTickIn == ticky);
 			NextTickIn -= samplesleft;
 			assert (NextTickIn >= 0);
@@ -231,6 +235,7 @@ bool OPLmusicBlock::ServiceStream (void *buff, int numbytes)
 						{
 							YM3812UpdateOne (1, samples1, numsamples);
 						}
+						OffsetSamples(samples1, numsamples);
 					}
 					res = false;
 					break;
@@ -256,6 +261,72 @@ bool OPLmusicBlock::ServiceStream (void *buff, int numbytes)
 	return res;
 }
 
+void OPLmusicBlock::OffsetSamples(float *buff, int count)
+{
+	// Three out of four of the OPL waveforms are non-negative. Depending on
+	// timbre selection, this can cause the output waveform to tend toward
+	// very large positive values. Heretic's music is particularly bad for
+	// this. This function attempts to compensate by offseting the sample
+	// data back to around the [-1.0, 1.0] range.
+
+	double max = -1e10, min = 1e10, offset, step;
+	int i, ramp;
+
+	// Find max and min values for this segment of the waveform.
+	for (i = 0; i < count; ++i)
+	{
+		if (buff[i] > max)
+		{
+			max = buff[i];
+		}
+		if (buff[i] < min)
+		{
+			min = buff[i];
+		}
+	}
+	// Don't slide if we don't have to, because doing so introduces noise.
+	// However, if the amplitude is low, we do want to slide so that when
+	// the song ends, the wave will be around 0 and not click when the song
+	// starts over.
+	if (min - LastOffset > -0.5 && max - LastOffset < 0.5 && max - min > 0.5)
+	{
+		offset = LastOffset;
+	}
+	else
+	{
+		offset = (max + min) / 2;
+		// If the new offset is close to 0, make it 0 to avoid making another
+		// full loop through the sample data.
+		if (fabs(offset) < 1/256.0)
+		{
+			offset = 0;
+		}
+	}
+	// Ramp the offset change so there aren't any abrupt clicks in the output.
+	// If the ramp is too short, it can sound scratchy. cblood2.mid is
+	// particularly unforgiving of short ramps.
+	ramp = MIN(512, count);
+	step = (offset - LastOffset) / 512;
+	offset = LastOffset;
+	i = 0;
+	if (step != 0)
+	{
+		for (; i < ramp; ++i)
+		{
+			buff[i] = float(buff[i] - offset);
+			offset += step;
+		}
+	}
+	if (offset != 0)
+	{
+		for (; i < count; ++i)
+		{
+			buff[i] = float(buff[i] - offset);
+		}
+	}
+	LastOffset = float(offset);
+}
+
 int OPLmusicFile::PlayTick ()
 {
 	BYTE reg, data;

src/oplsynth/opl_mus_player.h

@@ -15,11 +15,13 @@ public:
 
 protected:
 	virtual int PlayTick() = 0;
+	void OffsetSamples(float *buff, int count);
 
 	double NextTickIn;
 	double SamplesPerTick;
 	bool TwoChips;
 	bool Looping;
+	double LastOffset;
 
 	FCriticalSection ChipAccess;
 };

src/sound/fmodsound.cpp

@@ -1970,11 +1970,11 @@ void FMODSoundRenderer::DrawWave(float *wavearray, int x, int y, int width, int
 	if (screen->Accel2D)
 	{ // Drawing this with lines is super-slow without hardware acceleration, at least with
 	  // the debug build.
-		float lasty = wavearray[0] * scale + mid;
+		float lasty = mid - wavearray[0] * scale;
 		float newy;
 		for (i = 1; i < width; ++i)
 		{
-			newy = wavearray[i] * scale + mid;
+			newy = mid - wavearray[i] * scale;
 			screen->DrawLine(x + i - 1, int(lasty), x + i, int(newy), -1, MAKEARGB(255,255,248,248));
 			lasty = newy;
 		}