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I'm pretty new to using GNURadio and I'm having trouble recovering the data from a signal that I've saved into a file. The signal is a carrier frequency of 56KHz with a frequency shift key of +/- 200hz at 600 baud.

So far, I've been able to demodulate the signal that looks similar to the signal I get from the source:

Demodulated signal

I'm trying to get this into a repeating string of 1s and 0s (the whole telegram is 38 bytes long and it continuously repeats). I've tried to use a clock recovery block in order to have only one byte per sample, but I'm not having much luck. Using the M&M clock recovery block, the whole telegram sometimes comes out correct, but it is not consistent. I've tried to adjust the omega and Mu values, but it doesn't seem to help that much. I've also tried using the Polyphase Clock sync, but I keep getting a runtime error of 'please specify a filter'. Is this asking me to add a tap? what tap would i use?

So I guess my overall question would be: What's the best way to get the telegram out of the demodulated fsk signal?

Again, pretty new at this so please let me know if I've missed something crucial. GNU flow graph below:

Flow Graph

Marcus Müller
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D Kains
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1 Answers1

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You're recovering the bit timing, but you're not recovering the byte boundaries – that needs to happen "one level higher", eg. by a well-known packet format with a defined preamble that you can look for.

Marcus Müller
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  • Thanks for the response Marcus. I have an idea what the preamble would be (not 100%, but it's something i can start with), is there a block that I could use to recover the preamble and telegram structure or is this just a matter of parsing the binary file afterwards? – D Kains Oct 19 '16 at 13:41