Use GNU Radio emphasis filters, add filters generation scripts

FMControl.cpp

@@ -25,7 +25,7 @@
 #endif
 
 const float        DEEMPHASIS_GAIN_DB  = 0.0F;
-const float        PREEMPHASIS_GAIN_DB = 30.0F;
+const float        PREEMPHASIS_GAIN_DB = 0.0F;
 const float        FILTER_GAIN_DB      = 0.0F;
 const unsigned int FM_MASK             = 0x00000FFFU;
 
@@ -39,8 +39,8 @@ m_filterStage1(NULL),
 m_filterStage2(NULL),
 m_filterStage3(NULL)
 {
-    m_preemphasis  = new CIIRDirectForm1Filter(0.38897032f, -0.32900053f, 0.0f, 1.0f, 0.28202918f, 0.0f, PREEMPHASIS_GAIN_DB);
-    m_deemphasis   = new CIIRDirectForm1Filter(1.0f,0.28202918f, 0.0f, 0.38897032f, -0.32900053f, 0.0f, DEEMPHASIS_GAIN_DB);
+    m_preemphasis  = new CIIRDirectForm1Filter(8.315375384336983F,-7.03334621603483F,0.0F,1.0F,0.282029168302153F,0.0F, PREEMPHASIS_GAIN_DB);
+    m_deemphasis   = new CIIRDirectForm1Filter(0.07708787090460224F,0.07708787090460224F,0.0F,1.0F,-0.8458242581907955F,0.0F, DEEMPHASIS_GAIN_DB);
 
     //cheby type 1 0.2dB cheby type 1 3rd order 300-2700Hz fs=8000
     m_filterStage1 = new CIIRDirectForm1Filter(0.29495028f, 0.0f, -0.29495028f, 1.0f, -0.61384624f, -0.057158668f, FILTER_GAIN_DB);

Tools/DeEmphasis.py

@@ -0,0 +1,43 @@
+#based on https://github.com/gnuradio/gnuradio/blob/master/gr-analog/python/analog/fm_emph.py
+
+import math
+import cmath
+import numpy as np
+import scipy.signal as signal
+import pylab as pl
+
+tau = 750e-6
+fs = 8000
+fh = 2700
+
+# Digital corner frequency
+w_c = 1.0 / tau
+
+# Prewarped analog corner frequency
+w_ca = 2.0 * fs * math.tan(w_c / (2.0 * fs))
+
+# Resulting digital pole, zero, and gain term from the bilinear
+# transformation of H(s) = w_ca / (s + w_ca) to
+# H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1)
+k = -w_ca / (2.0 * fs)
+z1 = -1.0
+p1 = (1.0 + k) / (1.0 - k)
+b0 = -k / (1.0 - k)
+
+btaps = [ b0 * 1.0, b0 * -z1, 0 ]
+ataps = [      1.0,      -p1, 0 ]
+
+# Since H(s = 0) = 1.0, then H(z = 1) = 1.0 and has 0 dB gain at DC
+
+
+taps = np.concatenate((btaps, ataps), axis=0)
+print("Taps")
+print(*taps, "", sep=",", end="\n")
+
+f,h = signal.freqz(btaps,ataps, fs=fs)
+pl.plot(f, 20*np.log10(np.abs(h)))
+pl.xlabel('frequency/Hz')
+pl.ylabel('gain/dB')
+pl.ylim(top=0,bottom=-30)
+pl.xlim(left=0, right=fh*2.5)
+pl.show()

Tools/PreEmphasis.py

@@ -0,0 +1,51 @@
+#based on https://github.com/gnuradio/gnuradio/blob/master/gr-analog/python/analog/fm_emph.py
+
+import math
+import cmath
+import numpy as np
+import scipy.signal as signal
+import pylab as pl
+
+tau = 750e-6
+fs = 8000
+fh = 2700
+
+# Digital corner frequencies
+w_cl = 1.0 / tau
+w_ch = 2.0 * math.pi * fh
+
+# Prewarped analog corner frequencies
+w_cla = 2.0 * fs * math.tan(w_cl / (2.0 * fs))
+w_cha = 2.0 * fs * math.tan(w_ch / (2.0 * fs))
+
+# Resulting digital pole, zero, and gain term from the bilinear
+# transformation of H(s) = (s + w_cla) / (s + w_cha) to
+# H(z) = b0 (1 - z1 z^-1)/(1 - p1 z^-1)
+kl = -w_cla / (2.0 * fs)
+kh = -w_cha / (2.0 * fs)
+z1 = (1.0 + kl) / (1.0 - kl)
+p1 = (1.0 + kh) / (1.0 - kh)
+b0 = (1.0 - kl) / (1.0 - kh)
+
+# Since H(s = infinity) = 1.0, then H(z = -1) = 1.0 and
+# this filter  has 0 dB gain at fs/2.0.
+# That isn't what users are going to expect, so adjust with a
+# gain, g, so that H(z = 1) = 1.0 for 0 dB gain at DC.
+w_0dB = 2.0 * math.pi * 0.0
+g =        abs(1.0 - p1 * cmath.rect(1.0, -w_0dB)) \
+/ (b0 * abs(1.0 - z1 * cmath.rect(1.0, -w_0dB)))
+
+btaps = [ g * b0 * 1.0, g * b0 * -z1, 0]
+ataps = [          1.0,          -p1, 0]
+
+taps = np.concatenate((btaps, ataps), axis=0)
+print("Taps")
+print(*taps, "", sep=",", end="\n")
+
+f,h = signal.freqz(btaps,ataps, fs=fs)
+pl.plot(f, 20*np.log10(np.abs(h)))
+pl.xlabel('frequency/Hz')
+pl.ylabel('gain/dB')
+pl.ylim(top=30,bottom=0)
+pl.xlim(left=0, right=fh*2.5)
+pl.show()