rtl: remove residue DC component introduced by RTL chip

received from Juha Vierinen:

A student here noticed that there is dc bias even with the rafael tuner.
We looked into this issue and found that using 127.4f instead of 127.5f
removes this bias. I assume this is associated with a bug in the digital
downconversion of the RTL chip. This change fixes the problem.

2 files changed, 5 insertions, 5 deletions

diff --git a/lib/rtl/rtl_source_c.cc b/lib/rtl/rtl_source_c.cc
index 15bde78..6479b28 100644
--- a/lib/rtl/rtl_source_c.cc
+++ b/lib/rtl/rtl_source_c.cc
@@ -170,11 +170,11 @@ rtl_source_c::rtl_source_c (const std::string &args)
   // create a lookup table for gr_complex values
   for (unsigned int i = 0; i <= 0xffff; i++) {
 #ifdef BOOST_LITTLE_ENDIAN
-    _lut.push_back( gr_complex( (float(i & 0xff) - 127.5f) * (1.0f/128.0f),
-                                (float(i >> 8) - 127.5f) * (1.0f/128.0f) ) );
+    _lut.push_back( gr_complex( (float(i & 0xff) - 127.4f) * (1.0f/128.0f),
+                                (float(i >> 8) - 127.4f) * (1.0f/128.0f) ) );
 #else // BOOST_BIG_ENDIAN
-    _lut.push_back( gr_complex( (float(i >> 8) - 127.5f) * (1.0f/128.0f),
-                                (float(i & 0xff) - 127.5f) * (1.0f/128.0f) ) );
+    _lut.push_back( gr_complex( (float(i >> 8) - 127.4f) * (1.0f/128.0f),
+                                (float(i & 0xff) - 127.4f) * (1.0f/128.0f) ) );
 #endif
   }
 
diff --git a/lib/rtl_tcp/rtl_tcp_source_f.cc b/lib/rtl_tcp/rtl_tcp_source_f.cc
index 4d38ddb..211cc95 100644
--- a/lib/rtl_tcp/rtl_tcp_source_f.cc
+++ b/lib/rtl_tcp/rtl_tcp_source_f.cc
@@ -130,7 +130,7 @@ rtl_tcp_source_f::rtl_tcp_source_f(size_t itemsize,
   d_temp_buff = new unsigned char[d_payload_size];   // allow it to hold up to payload_size bytes

   d_LUT= new float[0xff+1];

   for(int i=0; i <=(0xff);++i){

-    d_LUT[i] = (((float)(i&0xff))-127.5f)*(1.0f/128.0f);

+    d_LUT[i] = (((float)(i&0xff))-127.4f)*(1.0f/128.0f);

   }

   // create socket

   d_socket = socket(ip_src->ai_family, ip_src->ai_socktype,