On Tue, Feb 15, 2005 at 07:59:45PM -0500, Achilleas Anastasopoulos wrote:
I am working on extracting the audio FM signal from the NTSC
signal, and experimenting with the data file
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.
This code is known to work with that file...
from gnuradio import gr, eng_notation
from gnuradio import audio
from gnuradio.eng_option import eng_option
from optparse import OptionParser
# 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
# input: complex; output: complex
channel_filter = \
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,
# 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)
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: ')
if __name__ == '__main__':