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Avoid coefficient overflow in the lowpass filter (could easily happen with an output sample rate below 44100Hz)
Fixes #45

--- a/tsf.h

+++ b/tsf.h

@@ -995,8 +995,9 @@

 		if (dynamicLowpass)

 		{

 			float fres = tmpInitialFilterFc + v->modlfo.level * tmpModLfoToFilterFc + v->modenv.level * tmpModEnvToFilterFc;

-			tmpLowpass.active = (fres <= 13500.0f);

-			if (tmpLowpass.active) tsf_voice_lowpass_setup(&tmpLowpass, tsf_cents2Hertz(fres) / tmpSampleRate);

+			float lowpassFc = (fres <= 13500 ? tsf_cents2Hertz(fres) / f->outSampleRate : 1.0f);

+			tmpLowpass.active = (lowpassFc < 0.499f);

+			if (tmpLowpass.active) tsf_voice_lowpass_setup(&tmpLowpass, lowpassFc);

 		}

 		if (dynamicPitchRatio)

@@ -1241,7 +1242,7 @@

 	voicePlayIndex = f->voicePlayIndex++;

 	for (region = f->presets[preset_index].regions, regionEnd = region + f->presets[preset_index].regionNum; region != regionEnd; region++)

 	{

-		struct tsf_voice *voice, *v, *vEnd; TSF_BOOL doLoop; float filterQDB;

+		struct tsf_voice *voice, *v, *vEnd; TSF_BOOL doLoop; float lowpassFilterQDB, lowpassFc;

 		if (key < region->lokey || key > region->hikey || midiVelocity < region->lovel || midiVelocity > region->hivel) continue;

 		voice = TSF_NULL, v = f->voices, vEnd = v + f->voiceNum;

@@ -1292,11 +1293,12 @@

 		tsf_voice_envelope_setup(&voice->modenv, &region->modenv, key, midiVelocity, TSF_FALSE, f->outSampleRate);

 		// Setup lowpass filter.

-		filterQDB = region->initialFilterQ / 10.0f;

-		voice->lowpass.QInv = 1.0 / TSF_POW(10.0, (filterQDB / 20.0));

+		lowpassFc = (region->initialFilterFc <= 13500 ? tsf_cents2Hertz((float)region->initialFilterFc) / f->outSampleRate : 1.0f);

+		lowpassFilterQDB = region->initialFilterQ / 10.0f;

+		voice->lowpass.QInv = 1.0 / TSF_POW(10.0, (lowpassFilterQDB / 20.0));

 		voice->lowpass.z1 = voice->lowpass.z2 = 0;

-		voice->lowpass.active = (region->initialFilterFc <= 13500);

-		if (voice->lowpass.active) tsf_voice_lowpass_setup(&voice->lowpass, tsf_cents2Hertz((float)region->initialFilterFc) / f->outSampleRate);

+		voice->lowpass.active = (lowpassFc < 0.499f);

+		if (voice->lowpass.active) tsf_voice_lowpass_setup(&voice->lowpass, lowpassFc);

 		// Setup LFO filters.

 		tsf_voice_lfo_setup(&voice->modlfo, region->delayModLFO, region->freqModLFO, f->outSampleRate);