I deactivated the three fftscopes that I had and now it works fine. The voice distortion was because the CPU could not handle the load. It is amazing how much CPU power the fftscopes consume...
Thanks for the help Achilleas
Eric Blossom wrote:
On Tue, Feb 15, 2005 at 07:59:45PM -0500, Achilleas Anastasopoulos wrote:
Dear all,
I am working on extracting the audio FM signal from the NTSC signal, and experimenting with the data file
ntsc-short-complex-baseband-8MS.dat
The idea seems simple: recenter the audio carrier to 0 frequency, LPF and decimate and then do standard fm demod. This is done in the attached file.
Unfortunately, although I hear something like
"winter storm will still going to stick around..."
the signal seems highly distorted as if the audio carrier is very unstable. I tried to fix this by tracking the video carrier, but the problem gets worse.
One possible reason for this is that the local oscilator used to generate the IQ signal is not stable, which unfortunately cannot be fixed...
Can someone verify this problem and/or suggest any solution.
Thanks Achilleas
This code is known to work with that file...
#!/usr/bin/env python
from gnuradio import gr, eng_notation from gnuradio import audio from gnuradio.eng_option import eng_option from optparse import OptionParser import sys import math
#
# return a gr.flow_graph
#
def build_graph (filename, repeat):
fm_demod_gain = 1100.0/32768.0
volume = 1.0
input_rate = 8e6
cfir_decimation = 25
audio_decimation = 10
quad_rate = input_rate / cfir_decimation # 320 kHz audio_rate = quad_rate / audio_decimation # 32 kHz
fg = gr.flow_graph ()
# file contains complex shorts
# output: short
src = gr.file_source (gr.sizeof_short, filename, repeat)
# deinterleave the shorts and convert them to complex
# input: short; output: complex
deinterleaver = gr.interleaved_short_to_complex ()
# Select only the FM audio channel.
# It's located at +2.75MHz
offset_freq = -2.75e6
channel_coeffs = \ gr.firdes.low_pass (1.0, # gain input_rate, # sampling rate 200e3, # low pass cutoff freq 200e3, # width of trans. band gr.firdes.WIN_HAMMING)
# input: complex; output: complex channel_filter = \ gr.freq_xlating_fir_filter_ccf (cfir_decimation, channel_coeffs, offset_freq, input_rate) print len (channel_coeffs)
# input: complex; output: float fm_demod = gr.quadrature_demod_cf (volume * fm_demod_gain)
# compute FIR filter taps for audio filter width_of_transition_band = audio_rate / 32 audio_coeffs = gr.firdes.low_pass (1.0, # gain quad_rate, # sampling rate audio_rate/2 - width_of_transition_band, width_of_transition_band, gr.firdes.WIN_HAMMING)
# input: float; output: float audio_filter = gr.fir_filter_fff (audio_decimation, audio_coeffs)
# sound card as final sink audio_sink = audio.sink (int (audio_rate))
# now wire it all together
fg.connect (src, deinterleaver)
fg.connect (deinterleaver, channel_filter)
fg.connect (channel_filter, fm_demod)
fg.connect (fm_demod, audio_filter)
fg.connect (audio_filter, audio_sink)
return fg
def main (): parser = OptionParser (option_class=eng_option) parser.add_option ("-i", "--input", type="string", default="ch4.dat", help="read data from FILENAME") parser.add_option ("-R", "--repeat", action="store_true", default=False) (options, args) = parser.parse_args ()
fg = build_graph (options.input, options.repeat)
fg.start () # fork thread(s) and return raw_input ('Press Enter to quit: ') fg.stop ()
if __name__ == '__main__': main ()
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