From d88a051aff15fdf5c57e1e5a4083bbd9635af3ad Mon Sep 17 00:00:00 2001
From: Mathias Agopian <mathias@google.com>
Date: Tue, 30 Oct 2012 12:49:07 -0700
Subject: fix another issue with generating FIR coefficients

the impulse response of a low-pass is 2*f*sinc(2*pi*f*k), we were
missing the 2*f scale factor. This explains why we were seeing
clipping and had to manually scale the filter down.

Change-Id: I86d0bb82ecdd99681c8ba5a8112a8257bf6f0186
---
 tools/resampler_tools/fir.cpp | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

(limited to 'tools')

diff --git a/tools/resampler_tools/fir.cpp b/tools/resampler_tools/fir.cpp
index acd9911..ea3ef50 100644
--- a/tools/resampler_tools/fir.cpp
+++ b/tools/resampler_tools/fir.cpp
@@ -222,7 +222,7 @@ int main(int argc, char** argv)
     if (!polyphase) {
         for (int i=0 ; i<N ; i++) {
             double x = (2.0 * M_PI * i * Fcr) / (1 << nz);
-            double y = kaiser(i+N, 2*N, beta) * sinc(x);
+            double y = kaiser(i+N, 2*N, beta) * sinc(x) * 2.0 * Fcr;
             y *= atten;
 
             if (!debug) {
@@ -247,7 +247,7 @@ int main(int argc, char** argv)
             // generate a FIR per phase
             for (int i=-nzc ; i<nzc ; i++) {
                 double x = 2.0 * M_PI * Fcr * (i + p);
-                double y = kaiser(i+N, 2*N, beta) * sinc(x);
+                double y = kaiser(i+N, 2*N, beta) * sinc(x) * 2.0 * Fcr;;
                 y *= atten;
                 if (!format) {
                     int64_t yi = floor(y * ((1ULL<<(nc-1))) + 0.5);
-- 
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