Increase AudioEffectEnvelope numerical resolution

fixes #102

effect_envelope.cpp

 /* Audio Library for Teensy 3.X
- * Copyright (c) 2014, Paul Stoffregen, paul@pjrc.com
+ * Copyright (c) 2017, Paul Stoffregen, paul@pjrc.com
  *
  * Development of this audio library was funded by PJRC.COM, LLC by sales of
  * Teensy and Audio Adaptor boards.  Please support PJRC's efforts to develop
 void AudioEffectEnvelope::noteOn(void)
 {
 	__disable_irq();
-	mult = 0;
+	mult_hires = 0;
 	count = delay_count;
 	if (count > 0) {
 		state = STATE_DELAY;
-		inc = 0;
+		inc_hires = 0;
 	} else {
 		state = STATE_ATTACK;
 		count = attack_count;
-		inc = (0x10000 / count) >> 3;
+		inc_hires = 0x40000000 / (int32_t)count;
 	}
 	__enable_irq();
 }
 void AudioEffectEnvelope::noteOff(void)
 {
 	__disable_irq();
-	state = STATE_RELEASE;
-	count = release_count;
-	mult = sustain_mult;
-	inc = (-mult / ((int32_t)count << 3));
+	if (state != STATE_IDLE) {
+		state = STATE_RELEASE;
+		count = release_count;
+		inc_hires = (-mult_hires) / (int32_t)count;
+	}
 	__enable_irq();
 }
 
 				count = hold_count;
 				if (count > 0) {
 					state = STATE_HOLD;
-					mult = 0x10000;
-					inc = 0;
+					mult_hires = 0x40000000;
+					inc_hires = 0;
 				} else {
-					count = decay_count;
 					state = STATE_DECAY;
-					inc = ((sustain_mult - 0x10000) / ((int32_t)count << 3));
+					count = decay_count;
+					inc_hires = (sustain_mult - 0x40000000) / (int32_t)count;
 				}
 				continue;
 			} else if (state == STATE_HOLD) {
 				state = STATE_DECAY;
 				count = decay_count;
-				inc = ((sustain_mult - 0x10000) / (int32_t)count) >> 3;
+				inc_hires = (sustain_mult - 0x40000000) / (int32_t)count;
 				continue;
 			} else if (state == STATE_DECAY) {
 				state = STATE_SUSTAIN;
 				count = 0xFFFF;
-				mult = sustain_mult;
-				inc = 0;
+				mult_hires = sustain_mult;
+				inc_hires = 0;
 			} else if (state == STATE_SUSTAIN) {
 				count = 0xFFFF;
 			} else if (state == STATE_RELEASE) {
 			} else if (state == STATE_DELAY) {
 				state = STATE_ATTACK;
 				count = attack_count;
-				inc = (0x10000 / count) >> 3;
+				inc_hires = 0x40000000 / count;
 				continue;
 			}
 		}
-		// process 8 samples, using only mult and inc
+
+		int32_t mult = mult_hires >> 14;
+		int32_t inc = inc_hires >> 17;
+		// process 8 samples, using only mult and inc (16 bit resolution)
 		sample12 = *p++;
 		sample34 = *p++;
 		sample56 = *p++;
 		*p++ = sample34;
 		*p++ = sample56;
 		*p++ = sample78;
+		// adjust the long-term gain using 30 bit resolution (fix #102)
+		// https://github.com/PaulStoffregen/Audio/issues/102
+		mult_hires += inc_hires;
 		count--;
 	}
 	transmit(block);

effect_envelope.h

 /* Audio Library for Teensy 3.X
- * Copyright (c) 2014, Paul Stoffregen, paul@pjrc.com
+ * Copyright (c) 2017, Paul Stoffregen, paul@pjrc.com
  *
  * Development of this audio library was funded by PJRC.COM, LLC by sales of
  * Teensy and Audio Adaptor boards.  Please support PJRC's efforts to develop
 	}
 	void attack(float milliseconds) {
 		attack_count = milliseconds2count(milliseconds);
+		if (attack_count == 0) attack_count = 1;
 	}
 	void hold(float milliseconds) {
 		hold_count = milliseconds2count(milliseconds);
 	}
 	void decay(float milliseconds) {
 		decay_count = milliseconds2count(milliseconds);
+		if (decay_count == 0) decay_count = 1;
 	}
 	void sustain(float level) {
 		if (level < 0.0) level = 0;
 		else if (level > 1.0) level = 1.0;
-		sustain_mult = level * 65536.0;
+		sustain_mult = level * 1073741824.0;
 	}
 	void release(float milliseconds) {
 		release_count = milliseconds2count(milliseconds);
+		if (release_count == 0) release_count = 1;
 	}
 	using AudioStream::release;
 	virtual void update(void);
 	uint16_t milliseconds2count(float milliseconds) {
 		if (milliseconds < 0.0) milliseconds = 0.0;
 		uint32_t c = ((uint32_t)(milliseconds*SAMPLES_PER_MSEC)+7)>>3;
-		if (c > 1103) return 1103; // allow up to 200 ms
+		if (c > 65535) c = 65535; // allow up to 11.88 seconds
 		return c;
 	}
 	audio_block_t *inputQueueArray[1];
 	// state
-	uint8_t  state;  // idle, delay, attack, hold, decay, sustain, release
-	uint16_t count;  // how much time remains in this state, in 8 sample units
-	int32_t  mult;   // attenuation, 0=off, 0x10000=unity gain
-	int32_t  inc;    // amount to change mult on each sample
+	uint8_t  state;      // idle, delay, attack, hold, decay, sustain, release
+	uint16_t count;      // how much time remains in this state, in 8 sample units
+	int32_t  mult_hires; // attenuation, 0=off, 0x40000000=unity gain
+	int32_t  inc_hires;  // amount to change mult_hires every 8 samples
+
 	// settings
 	uint16_t delay_count;
 	uint16_t attack_count;