From 7b4d0677a1c1aa95a68d59d0c4286a92beb1be11 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?S=C3=A9bastien=20Bourdeauducq?= <sb@m-labs.hk>
Date: Sat, 28 Dec 2024 19:04:35 +0800
Subject: [PATCH] average input channels

---
 sndlock.cpp | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/sndlock.cpp b/sndlock.cpp
index 4b01291..d7983b4 100644
--- a/sndlock.cpp
+++ b/sndlock.cpp
@@ -14,6 +14,7 @@
 
 #define SND_BITS 16
 #define SND_PCHAN 2
+#define SND_RCHAN 2
 #define SND_RATE 44100
 #define SND_BUFLEN 4096
 
@@ -38,7 +39,7 @@ static void dsp_thread()
     par.sig = 1;
     par.bits = SND_BITS;
     par.pchan = SND_PCHAN;
-    par.rchan = 1;
+    par.rchan = SND_RCHAN;
     par.rate = SND_RATE;
     par.le = SIO_LE_NATIVE;
     par.xrun = SIO_ERROR;
@@ -96,13 +97,17 @@ static void dsp_thread()
         }
 
         if(revents & POLLIN) {
-            int16_t buf_in[SND_BUFLEN];
+            int16_t buf_in[SND_BUFLEN*SND_RCHAN];
             size_t read = sio_read(hdl, buf_in, sizeof(buf_in));
-            double scale = pow(0.5, SND_BITS-1);
+            // input channels are averaged together to reduce uncorrelated noise
+            double scale = pow(0.5, SND_BITS-1)/SND_RCHAN;
             for(int i=0;i<SND_PCHAN;i++)
-                for(int j=0;j<read/sizeof(buf_in[0]);j++) {
+                for(int j=0;j<read/(SND_RCHAN*sizeof(buf_in[0]));j++) {
+                    double sample = 0.0;
+                    for(int k=0;k<SND_RCHAN;k++)
+                        sample += (double)buf_in[SND_RCHAN*j+k];
                     std::complex<double> rotated;
-                    rotated = (double)buf_in[j]*std::polar(scale, phase_in[i]*2.0*M_PI/(double)UINT32_MAX);
+                    rotated = sample*std::polar(scale, phase_in[i]*2.0*M_PI/(double)UINT32_MAX);
                     phase_in[i] += ftw[i];  // wraps on underflow
 
                     lpf_y[i] += (rotated - lpf_y[i])*lpf_k[i];