Fix the DC offset computation. The previous computation would occassionally get stuck with an incorrect offset.

input_adc.cpp

@@ -31,7 +31,8 @@
 DMAMEM static uint16_t analog_rx_buffer[AUDIO_BLOCK_SAMPLES];
 audio_block_t * AudioInputAnalog::block_left = NULL;
 uint16_t AudioInputAnalog::block_offset = 0;
-uint16_t AudioInputAnalog::dc_average = 0;
+int32_t AudioInputAnalog::dc_average_hist[16];
+int32_t AudioInputAnalog::current_dc_average_index = 0;
 bool AudioInputAnalog::update_responsibility = false;
 DMAChannel AudioInputAnalog::dma(false);
 
@@ -50,7 +51,9 @@ void AudioInputAnalog::init(uint8_t pin)
 	for (i=0; i < 1024; i++) {
 		sum += analogRead(pin);
 	}
-	dc_average = sum >> 10;
+	for (i = 0; i < 16; i++) {
+		dc_average_hist[i] = sum >> 10;
+	}
 
 	// set the programmable delay block to trigger the ADC at 44.1 kHz
 #if defined(KINETISK)
@@ -133,7 +136,7 @@ void AudioInputAnalog::isr(void)
 void AudioInputAnalog::update(void)
 {
 	audio_block_t *new_left=NULL, *out_left=NULL;
-	unsigned int dc, offset;
+	uint32_t i, dc, offset;
 	int32_t tmp;
 	int16_t s, *p, *end;
 
@@ -178,18 +181,23 @@ void AudioInputAnalog::update(void)
 	block_offset = 0;
 	__enable_irq();
 
-	// find and subtract DC offset....
-	// TODO: this may not be correct, needs testing with more types of signals
-	dc = dc_average;
+	// Find and subtract DC offset... We use an average of the
+	// last 16 * AUDIO_BLOCK_SAMPLES samples.
+	dc = 0;
+	for (i = 0; i < 16; i++) {
+		dc += dc_average_hist[i];
+	}
+	dc /= 16 * AUDIO_BLOCK_SAMPLES;
+	dc_average_hist[current_dc_average_index] = 0;
 	p = out_left->data;
 	end = p + AUDIO_BLOCK_SAMPLES;
 	do {
+		dc_average_hist[current_dc_average_index] += (uint16_t)(*p);
 		tmp = (uint16_t)(*p) - (int32_t)dc;
 		s = signed_saturate_rshift(tmp, 16, 0);
 		*p++ = s;
-		dc += s / 12000;  // slow response, remove DC component
 	} while (p < end);
-	dc_average = dc;
+	current_dc_average_index = (current_dc_average_index + 1) % 16;
 
 	// then transmit the AC data
 	transmit(out_left);

input_adc.h

@@ -41,7 +41,8 @@ public:
 private:
         static audio_block_t *block_left;
         static uint16_t block_offset;
-	static uint16_t dc_average;
+        static int32_t dc_average_hist[16];
+        static int32_t current_dc_average_index;
         static bool update_responsibility;
 	static DMAChannel dma;
 	static void isr(void);