filter: fixed default filter for rational_resampler.

Addresses issue gnuradio#745. When rate < 1 (downsampling), take into account the amount of downsampling (decimation) and adjust the LPF's passband to reduce aliasing.

Author: trondeau

gr-digital/lib/constellation.cc

@@ -1,6 +1,6 @@
 /* -*- c++ -*- */
 /*
- * Copyright 2010-2012 Free Software Foundation, Inc.
+ * Copyright 2010-2012,2014 Free Software Foundation, Inc.
  *
  * This file is part of GNU Radio
  *

gr-digital/python/digital/qa_constellation_soft_decoder_cf.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 #
-# Copyright 2013 Free Software Foundation, Inc.
+# Copyright 2013-2014 Free Software Foundation, Inc.
 #
 # This file is part of GNU Radio
 #

gr-digital/python/digital/soft_dec_lut_gen.py

@@ -1,6 +1,6 @@
 #!/usr/bin/env python
 #
-# Copyright 2013 Free Software Foundation, Inc.
+# Copyright 2013-2014 Free Software Foundation, Inc.
 #
 # This file is part of GNU Radio
 #

gr-filter/python/filter/rational_resampler.py

@@ -40,16 +40,25 @@ def design_filter(interpolation, decimation, fractional_bw):
     if fractional_bw >= 0.5 or fractional_bw <= 0:
         raise ValueError, "Invalid fractional_bandwidth, must be in (0, 0.5)"
 
-    beta = 5.0
-    trans_width = 0.5 - fractional_bw
-    mid_transition_band = 0.5 - trans_width/2
-
+    beta = 7.0
+    halfband = 0.5
+    rate = float(interpolation)/float(decimation)
+    if(rate >= 1.0):
+        trans_width = halfband - fractional_bw
+        mid_transition_band = halfband - trans_width/2.0
+    else:
+        trans_width = rate*(halfband - fractional_bw)
+        mid_transition_band = rate*halfband - trans_width/2.0
+
+    print trans_width, mid_transition_band
     taps = filter.firdes.low_pass(interpolation,                     # gain
-                                  1,                                 # Fs
-                                  mid_transition_band/interpolation, # trans mid point
-                                  trans_width/interpolation,         # transition width
+                                  interpolation,                     # Fs
+                                  mid_transition_band,               # trans mid point
+                                  trans_width,                       # transition width
                                   filter.firdes.WIN_KAISER,
                                   beta)                              # beta
+
+    print len(taps)
     return taps
 
 
@@ -90,13 +99,15 @@ def __init__(self, resampler_base,
         if taps is None:
             taps = design_filter(interpolation, decimation, fractional_bw)
 
-        resampler = resampler_base(interpolation, decimation, taps)
+        self.resampler = resampler_base(interpolation, decimation, taps)
 	gr.hier_block2.__init__(self, "rational_resampler",
-				gr.io_signature(1, 1, resampler.input_signature().sizeof_stream_item(0)),
-				gr.io_signature(1, 1, resampler.output_signature().sizeof_stream_item(0)))
+				gr.io_signature(1, 1, self.resampler.input_signature().sizeof_stream_item(0)),
+				gr.io_signature(1, 1, self.resampler.output_signature().sizeof_stream_item(0)))
 
-	self.connect(self, resampler, self)
+	self.connect(self, self.resampler, self)
 
+    def taps(self):
+        return self.resampler.taps()
 
 class rational_resampler_fff(_rational_resampler_base):
     def __init__(self, interpolation, decimation, taps=None, fractional_bw=None):