Implement fade in to prevent Triple Osc from clicking (#5199)

src/core/Instrument.cpp

@@ -23,8 +23,12 @@
  */
 
 #include "Instrument.h"
-#include "InstrumentTrack.h"
+
+#include <cmath>
+
 #include "DummyInstrument.h"
+#include "InstrumentTrack.h"
+#include "lmms_constants.h"
 
 
 Instrument::Instrument(InstrumentTrack * _instrument_track,
@@ -78,8 +82,96 @@ bool Instrument::isFromTrack( const Track * _track ) const
 	return( m_instrumentTrack == _track );
 }
 
+// helper function for Instrument::applyFadeIn
+static int countZeroCrossings(sampleFrame *buf, fpp_t start, fpp_t frames)
+{
+	// zero point crossing counts of all channels
+	int zeroCrossings[DEFAULT_CHANNELS] = {0};
+	// maximum zero point crossing of all channels
+	int maxZeroCrossings = 0;
+
+	// determine the zero point crossing counts
+	for (fpp_t f = start; f < frames; ++f)
+	{
+		for (ch_cnt_t ch = 0; ch < DEFAULT_CHANNELS; ++ch)
+		{
+			// we don't want to count [-1, 0, 1] as two crossings
+			if ((buf[f - 1][ch] <= 0.0 && buf[f][ch] > 0.0) ||
+					(buf[f - 1][ch] >= 0.0 && buf[f][ch] < 0.0))
+			{
+				++zeroCrossings[ch];
+				if (zeroCrossings[ch] > maxZeroCrossings)
+				{
+					maxZeroCrossings = zeroCrossings[ch];
+				}
+			}
+		}
+	}
+
+	return maxZeroCrossings;
+}
+
+// helper function for Instrument::applyFadeIn
+fpp_t getFadeInLength(float maxLength, fpp_t frames, int zeroCrossings)
+{
+	// calculate the length of the fade in
+	// Length is inversely proportional to the max of zeroCrossings,
+	// because for low frequencies, we need a longer fade in to
+	// prevent clicking.
+	return (fpp_t) (maxLength  / ((float) zeroCrossings / ((float) frames / 128.0f) + 1.0f));
+}
 
 
+void Instrument::applyFadeIn(sampleFrame * buf, NotePlayHandle * n)
+{
+	const static float MAX_FADE_IN_LENGTH = 85.0;
+	f_cnt_t total = n->totalFramesPlayed();
+	if (total == 0)
+	{
+		const fpp_t frames = n->framesLeftForCurrentPeriod();
+		const f_cnt_t offset = n->offset();
+
+		// We need to skip the first sample because it almost always
+		// produces a zero crossing; it's not helpful while
+		// determining the fade in length. Hence 1
+		int maxZeroCrossings = countZeroCrossings(buf, offset + 1, offset + frames);
+
+		fpp_t length = getFadeInLength(MAX_FADE_IN_LENGTH, frames, maxZeroCrossings);
+		n->m_fadeInLength = length;
+
+		// apply fade in
+		length = length < frames ? length : frames;
+		for (fpp_t f = 0; f < length; ++f)
+		{
+			for (ch_cnt_t ch = 0; ch < DEFAULT_CHANNELS; ++ch)
+			{
+				buf[offset + f][ch] *= 0.5 - 0.5 * cosf(F_PI * (float) f / (float) n->m_fadeInLength);
+			}
+		}
+	}
+	else if (total < n->m_fadeInLength)
+	{
+		const fpp_t frames = n->framesLeftForCurrentPeriod();
+
+		int new_zc = countZeroCrossings(buf, 1, frames);
+		fpp_t new_length = getFadeInLength(MAX_FADE_IN_LENGTH, frames, new_zc);
+
+		for (fpp_t f = 0; f < frames; ++f)
+		{
+			for (ch_cnt_t ch = 0; ch < DEFAULT_CHANNELS; ++ch)
+			{
+				float currentLength = n->m_fadeInLength * (1.0f - (float) f / frames) + new_length * ((float) f / frames);
+				buf[f][ch] *= 0.5 - 0.5 * cosf(F_PI * (float) (total + f) / currentLength);
+				if (total + f >= currentLength)
+				{
+					n->m_fadeInLength = currentLength;
+					return;
+				}
+			}
+		}
+		n->m_fadeInLength = new_length;
+	}
+}
 
 void Instrument::applyRelease( sampleFrame * buf, const NotePlayHandle * _n )
 {
@@ -109,6 +201,3 @@ QString Instrument::fullDisplayName() const
 {
 	return instrumentTrack()->displayName();
 }
-
-
-