[Discuss-gnuradio] Improve FSK4 decodes in OP25

[Luke Berndt]

I am using the C++ OP25 libs to capture and record the P25 transmissions from a trunked radio system ( https://github.com/robotastic/trunk-recorder ).

Transmission can either use 4-Level FSK or CPSK. I am getting great decodes from a system using CPSK but I am having a lot of trouble with a system using 4FSK.

At a high level, here is my 4FSK flow graph (https://github.com/robotastic/trunk-recorder/blob/master/trunk-recorder/recorders/p25_recorder.cc):

Xlating + LPF to roughly capture the channel

ARB Resampler to get the system sample rate

Quad FM Demod

Multiply Const - to get the right levels, -3 to 3

FSK4 Demod - from OP25

FSK4 Slicer - from OP25

OP25 Decode

it then converts it to what is needed to record it as a Wav file

Are there some obvious blocks I should add in to help condition the input better? I think the incoming levels may not be constant. There also seems to be occasional interference. Is there a block that will discard values above a certain range? It would be great to throw-away samples above 3.5 or below -3.5.

Attached is a picture with what I think is an example of the interference.

Here is an example of a very clean recording: https://beta.openmhz.com/wmata?call-id=587652495833790100186198

Here is an example of a bad decode: https://beta.openmhz.com/wmata?call-id=58765250583379010018619c

Any tips would be greatly appreciated!!

- Luke

peaks.jpeg
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