And a bit of progress, I put the FFT sink in Float mode after the
demodulator and sure enough there is the audio 0 - about 15kHz and then a
nice big spike at 19khz (my pilot wave). I am of course missing the upper
end of the demodulated signal so next up we recover that.
On Mon, Dec 19, 2016 at 11:05 PM, Chuck McManis <chuck.mcmanis@xxxxxxxxx>
You know that feeling that you understand what you're doing and then you
apply your knowledge only to realize you don't really know what is going on
at all? :-) Trying to pull the 19khz pilot wave out of the signal. And I've
tried it two different ways and both did something mysterious. So in try
#1, we took the output of the low pass filter, multiplied it in the complex
domain by a -19khz cosine (shift the signal by 19khz) and low pass that.
But as I write it I realize that is in the frequency domain not in the
demodulated domain. Ok back to the drawing board.
I'm having a blast btw, this is *sooo* much more fun than doing this stuff
with just equations and an oscilloscope and a handful of discrete
components of dubious tolerances.
On Mon, Dec 19, 2016 at 9:18 PM, Chuck McManis <chuck.mcmanis@xxxxxxxxx>
Awesome, that is perfect. Off to build us a new grc file once we ingest
On Mon, Dec 19, 2016 at 9:03 PM, Michael Ossmann <mike@xxxxxxxxxxx>
On Mon, Dec 19, 2016 at 08:16:06PM -0800, Chuck McManis wrote:
So took the output of the 100khz low pass filter coming off
multiplier, and multiplied it by 19k and -19k and took those outputs
as the left and right channels. Not what I expected. Both sides
"sound" the same (I put them through my fader into the sound sink) and
even putting the subcarrier frequency all over the map it doesn't
change, so I'm clearly not doing what I think I should be doing.
Below 19 kHz is the sum of the left and right audio channels. Above 19
kHz is the modulated difference of the left and right audio channels.
(The reason for the sum and difference thing is backwards compatibility
with mono receivers.) To separate the two signals you have to split
them after the FM demodulation step. You should be able to detect (and
synchronize to) a pilot tone found at 19 kHz. This looks like a good
It is a lot more complicated than building a mono receiver!