Recording rtl_fm output

Martin McCormick martin.m at
Sat Mar 11 20:57:31 UTC 2017

	Something I am doing is not quite right. The following
example works almost perfectly but please read on.

rtl_fm -f 88.3e6 -M wbfm -s 215000 -r 44100 - | aplay -r 44.1k -f S16_LE

The audio from the sound card is nearly excellent.

	To record this stream, I do the following:

rtl_fm -f 88.3e6 -M wbfm -s 215000 -r 44100 testfile.bin

I get a file so it would appear that the following should sound
just like the stream I am listening to using the pipe in the
first example.

aplay -r 44.1k -f S16_LE testfile.bin

	The pitch of the audio is fine but instead of almost dead
silence between words, there is a lot of hiss and 
distortion. In theory, I should hear the same mostly noiseless
audio I hear in the first example when playing from the file.

	This is really not a DSP issue but maybe I have saved the
audio as unsigned and am trying to play it signed which would
preserve the pitch but be a cause for a lot of distortion.
Besides, it is apparently signed when piped in to aplay.

	I am mystified.


Martin McCormick

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