From 5b0087554964eedc50175fec8ce75600de938b4f Mon Sep 17 00:00:00 2001
From: wb2osz <wb2osz@comcast.net>
Date: Sat, 16 Dec 2017 10:10:36 -0500
Subject: [PATCH] Accumulate DC average for audio signal.

---
 multi_modem.c | 14 ++++++++++++++
 multi_modem.h |  2 ++
 2 files changed, 16 insertions(+)

diff --git a/multi_modem.c b/multi_modem.c
index fd53b48..2d50f90 100644
--- a/multi_modem.c
+++ b/multi_modem.c
@@ -292,6 +292,14 @@ unsigned char is_crc_in_queue(unsigned int chan, unsigned int crc) {
  *
  *------------------------------------------------------------------------------*/
 
+static float dc_average[MAX_CHANS];
+
+int multi_modem_get_dc_average (int chan)
+{
+	// Scale to +- 200 so it will like the deviation measurement.
+
+	return ( (int) ((float)(dc_average[chan]) * (200.0f / 32767.0f) ) );
+}
 
 __attribute__((hot))
 void multi_modem_process_sample (int chan, int audio_sample) 
@@ -300,6 +308,12 @@ void multi_modem_process_sample (int chan, int audio_sample)
 	int subchan;
 	static int i = 0;	/* for interleaving among multiple demodulators. */
 
+// Accumulate an average DC bias level.
+// Shouldn't happen with a soundcard but could with mistuned SDR.
+
+	dc_average[chan] = dc_average[chan] * 0.999f + (float)audio_sample * 0.001f;
+
+
 // TODO: temp debug, remove this.
 
 	assert (save_audio_config_p->achan[chan].num_subchan > 0 && save_audio_config_p->achan[chan].num_subchan <= MAX_SUBCHANS);
diff --git a/multi_modem.h b/multi_modem.h
index 77c5b6d..0c096ef 100644
--- a/multi_modem.h
+++ b/multi_modem.h
@@ -14,6 +14,8 @@ void multi_modem_init (struct audio_s *pmodem);
 
 void multi_modem_process_sample (int c, int audio_sample);
 
+int multi_modem_get_dc_average (int chan);
+
 void multi_modem_process_rec_frame (int chan, int subchan, int slice, unsigned char *fbuf, int flen, alevel_t alevel, retry_t retries);
 
 #endif