Recording rtl_fm output Solved

Martin McCormick martin.m at suddenlink.net
Tue Mar 14 01:59:23 UTC 2017


	I made a mistake, all right. We have a radio station
affiliated with a university, here. There is a transmitter meant
for this city on 88.3 MHZ and another transmitter on 91.7 MHz
serving Oklahoma City. Both transmitters are easily receivable
here but the 88.3 MHZ transmitter is much stronger. If I use that
transmitter and attach any kind of antenna to the dongle, the
audio is distorted, apparently because it overloads the device.
There was also an audible whine on either signal which wasn't too
loud, but was distracting.

	I finally discovered that I could get an almost perfect
signal from the nearer 88.3 MHZ transmitter and the whine went
away if I changed the 200 KHZ output sample rate to 215 KHZ.

	FM stations in North America often transmit a digital
stream containing a digital version of their analog signal plus a
second unrelated digital stream of classical music or alternative
program material.

	The carrier for this digital stream is near 200 KHZ and I
think the 200 KHZ sampling frequency heterodynes with the data
carrier.

	If one sets this sampling rate to 500 KHZ, for example,
you can hear the data carrier mixed in with the audio. What you
hear is a noise like an electric motor running in the background.

	I set the shell script that directly feeds the stream to
the sound card to listen to our 88.3 MHZ signal and forgot to
change the recording script also to 88.3. It was still on 91.7
and I just missed that omission since I see both 88.3 and 91.7 a
lot when experimenting with the scripts.

	After discovering the mistake and setting the record
script to 88.3 MHZ, the data recorded are producing sound that is
the same, now as one would expect.

	I was using a very limited antenna for reception of the
local signal so the more distant signal was audible but just
barely.

Martin


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