From 61f76d6b4680b7f1568cba90c1bdc9701362ade7 Mon Sep 17 00:00:00 2001
From: Steven Franke <s.j.franke@icloud.com>
Date: Wed, 24 Jun 2015 02:08:59 +0000
Subject: [PATCH] Compensate for LPF filter step response at beginning and end
 of transmission.

git-svn-id: svn+ssh://svn.code.sf.net/p/wsjt/wsjt/branches/wsjtx@5629 ab8295b8-cf94-4d9e-aec4-7959e3be5d79
---
 lib/wsprd/wsprd_exp.c | 20 ++++++++++++++++----
 1 file changed, 16 insertions(+), 4 deletions(-)

diff --git a/lib/wsprd/wsprd_exp.c b/lib/wsprd/wsprd_exp.c
index d9d004eb4..e0935ee47 100644
--- a/lib/wsprd/wsprd_exp.c
+++ b/lib/wsprd/wsprd_exp.c
@@ -391,6 +391,7 @@ void subtract_signal2(double *id, double *qd, long np,
     double dt=1.0/375.0, df=375.0/256.0;
     double pi=4.*atan(1.0), twopidt, phi=0, dphi, cs;
     int i, j, k, ii, nsym=162, nspersym=256,  nfilt=256; //nfilt must be even number.
+    int nsig=nsym*nspersym;
     
     double refi[45000],refq[45000];
     double ci[45000],cq[45000],cfi[45000],cfq[45000];
@@ -445,7 +446,8 @@ void subtract_signal2(double *id, double *qd, long np,
     }
 
     //quick and dirty filter - may want to do better
-    double w[nfilt], norm=0;
+    double w[nfilt], norm=0, partialsum[nfilt];
+    memset(partialsum,0,sizeof(double)*nfilt);
     for (i=0; i<nfilt; i++) {
         w[i]=sin(pi*(float)i/(float)(nfilt-1));
         norm=norm+w[i];
@@ -453,6 +455,9 @@ void subtract_signal2(double *id, double *qd, long np,
     for (i=0; i<nfilt; i++) {
         w[i]=w[i]/norm;
     }
+    for (i=1; i<nfilt; i++) {
+        partialsum[i]=partialsum[i-1]+w[i];
+    }
     
     // LPF
     for (i=nfilt/2; i<45000-nfilt/2; i++) {
@@ -467,12 +472,19 @@ void subtract_signal2(double *id, double *qd, long np,
     // (ci+j*cq)(refi+j*refq)=(ci*refi-cq*refq)+j(ci*refq)+cq*refi)
     // beginning of first symbol in reference signal is at i=nfilt
     // beginning of first symbol in received data is at shift0.
-    for (i=0; i<41472; i++) {
+    for (i=0; i<nsig; i++) {
+        if( i<nfilt/2 ) {        // take care of the end effect (LPF step response) here
+            norm=partialsum[nfilt/2+i];
+        } else if( i>(nsig-1-nfilt/2) ) {
+            norm=partialsum[nfilt/2+nsig-1-i];
+        } else {
+            norm=1.0;
+        }  
         k=shift0+i;
         j=i+nfilt;
         if( (k>0) & (k<np) ) {
-            id[k]=id[k] - (cfi[j]*refi[i]-cfq[j]*refq[i]);
-            qd[k]=qd[k] - (cfi[j]*refq[i]+cfq[j]*refi[i]);
+            id[k]=id[k] - (cfi[j]*refi[i]-cfq[j]*refq[i])/norm;
+            qd[k]=qd[k] - (cfi[j]*refq[i]+cfq[j]*refi[i])/norm;
         }
     }
     return;