Hello everyone,
I'm writing my first post in hope of shedding some light on rational resampling in FM radio. My idea was to get started with something easy to get me into the SDR programming and I chose FM radio. I have watched HAK5 SDR episode where the FM radio is built. I have also found similar graph on instructables (links below). I have tried to duplicate their graphs (with my own working frequencies), but without much success. My finished graphs produce loud cracking noise in my headphones. What I noticed, though, is what looks to me a strange discrepancy in the rational resampling and sample rate settings. Now, I'm quite sure I have some basic concept wrong, because it obviously works for both of them. Here's what is strange:
1. Sample rate starts at 2M at RTL-SDR Source
2. It is resampled with interplation 1 and decimation 4, which should produce the sample rate of 500k ("2M * 1/4", if I undestand correctly)
3. Low Pass Filter has also Sample Rate 2M (Shouldn't it be 500k?)
4. WBFM Receive has Quadrature rate of 500k and Decimation 1
5. It is resampled again with Interpolation 48 nad Decimation 500. If we take 2M sample rate from Low Pass Filter, it should produce 2M * (48 / 500) = 192 kHz. But if we take 500k sample rate, it should produce 48kHz, which is also the setting of the audio sink.
The problem is, if I put 500k as the Sample Rate of Low Pass Filter, instead of cracking noise I hear only silence. So my understanding is probably very much off. Can anyone shed some light on these problems? Is there another problem present, which is not tied to the GNURadio Companion and resampling? Here are the links:
HAK5 episode I watched:
Instructables:
http://www.instructables.com/id/RTL-SDR-FM-radio-receiver-with-GNU-Radio-Companion/?ALLSTEPS
Graph from instructables similar to HAK5's:
http://cdn.instructables.com/FFO/PKAD/HNE45JZQ/FFOPKADHNE45JZQ.LARGE.jpg
Rational resampling basics I read:
http://www.dspguru.com/dsp/faqs/multirate/resampling
Any help will be greatly appreciated.