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Diffstat (limited to 'gr-analog/python/analog/fm_emph.py')
| -rw-r--r-- | gr-analog/python/analog/fm_emph.py | 150 |
1 files changed, 150 insertions, 0 deletions
diff --git a/gr-analog/python/analog/fm_emph.py b/gr-analog/python/analog/fm_emph.py new file mode 100644 index 0000000000..2821f6e3cd --- /dev/null +++ b/gr-analog/python/analog/fm_emph.py @@ -0,0 +1,150 @@ +# +# Copyright 2005,2007,2012 Free Software Foundation, Inc. +# +# This file is part of GNU Radio +# +# GNU Radio is free software; you can redistribute it and/or modify +# it under the terms of the GNU General Public License as published by +# the Free Software Foundation; either version 3, or (at your option) +# any later version. +# +# GNU Radio is distributed in the hope that it will be useful, +# but WITHOUT ANY WARRANTY; without even the implied warranty of +# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +# GNU General Public License for more details. +# +# You should have received a copy of the GNU General Public License +# along with GNU Radio; see the file COPYING. If not, write to +# the Free Software Foundation, Inc., 51 Franklin Street, +# Boston, MA 02110-1301, USA. +# + +from gnuradio import gr, filter +import math + +# +# 1 +# H(s) = ------- +# 1 + s +# +# tau is the RC time constant. +# critical frequency: w_p = 1/tau +# +# We prewarp and use the bilinear z-transform to get our IIR coefficients. +# See "Digital Signal Processing: A Practical Approach" by Ifeachor and Jervis +# + +class fm_deemph(gr.hier_block2): + """ + FM Deemphasis IIR filter. + """ + + + def __init__(self, fs, tau=75e-6): + """ + + Args: + fs: sampling frequency in Hz (float) + tau: Time constant in seconds (75us in US, 50us in EUR) (float) + """ + gr.hier_block2.__init__(self, "fm_deemph", + gr.io_signature(1, 1, gr.sizeof_float), # Input signature + gr.io_signature(1, 1, gr.sizeof_float)) # Output signature + + w_p = 1/tau + w_pp = math.tan(w_p / (fs * 2)) # prewarped analog freq + + a1 = (w_pp - 1)/(w_pp + 1) + b0 = w_pp/(1 + w_pp) + b1 = b0 + + btaps = [b0, b1] + ataps = [1, a1] + + if 0: + print "btaps =", btaps + print "ataps =", ataps + global plot1 + plot1 = gru.gnuplot_freqz(gru.freqz(btaps, ataps), fs, True) + + deemph = filter.iir_filter_ffd(btaps, ataps) + self.connect(self, deemph, self) + +# +# 1 + s*t1 +# H(s) = ---------- +# 1 + s*t2 +# +# I think this is the right transfer function. +# +# +# This fine ASCII rendition is based on Figure 5-15 +# in "Digital and Analog Communication Systems", Leon W. Couch II +# +# +# R1 +# +-----||------+ +# | | +# o------+ +-----+--------o +# | C1 | | +# +----/\/\/\/--+ \ +# / +# \ R2 +# / +# \ +# | +# o--------------------------+--------o +# +# f1 = 1/(2*pi*t1) = 1/(2*pi*R1*C) +# +# 1 R1 + R2 +# f2 = ------- = ------------ +# 2*pi*t2 2*pi*R1*R2*C +# +# t1 is 75us in US, 50us in EUR +# f2 should be higher than our audio bandwidth. +# +# +# The Bode plot looks like this: +# +# +# /---------------- +# / +# / <-- slope = 20dB/decade +# / +# -------------/ +# f1 f2 +# +# We prewarp and use the bilinear z-transform to get our IIR coefficients. +# See "Digital Signal Processing: A Practical Approach" by Ifeachor and Jervis +# + +class fm_preemph(gr.hier_block2): + """ + FM Preemphasis IIR filter. + """ + def __init__(self, fs, tau=75e-6): + """ + + Args: + fs: sampling frequency in Hz (float) + tau: Time constant in seconds (75us in US, 50us in EUR) (float) + """ + + gr.hier_block2.__init__(self, "fm_deemph", + gr.io_signature(1, 1, gr.sizeof_float), # Input signature + gr.io_signature(1, 1, gr.sizeof_float)) # Output signature + + # FIXME make this compute the right answer + + btaps = [1] + ataps = [1] + + if 0: + print "btaps =", btaps + print "ataps =", ataps + global plot2 + plot2 = gru.gnuplot_freqz(gru.freqz(btaps, ataps), fs, True) + + preemph = filter.iir_filter_ffd(btaps, ataps) + self.connect(self, preemph, self) |