Receiving data using aux cable on GNU RADIO

Question

I am transmitting and receiving data using aux cable and GNU RADIO between two laptops.

I have implemented DQPSK using PSK mod block.

The problem is that while receiving I have to provide a delay, some integer value e.g 0,1,2 etc.

It is different every time.

Is there a way to dynamically check for the right delay value or any other workaround to this situation?

I have written 'start.' at the start of data being transmitted and 'end.' at the end.

I have to give a demo for this project and I dont want to manually change the delay at runtime.

I cannot find the .cc file of file sink in GNU RADIO, I can change the C++ code according my requirement but there is no such file.

Below is the screenshot of the grc file on the receive side.

Any help will be appreciated.

Answer 1

Since there's no way for the receiver to know when the transmitter started transmitting, it decodes stuff before there's actually anything to decode.

In essence, you need some kind of preamble or so to tell your receiver when to start – side effect of having something like that would be that you could correct some things (the two sound cards don't share the same oscillator, which leads to a symbol rate offset, and a center frequency offset).

You basically added that framing - your start. and end. strings.

I cannot find the .cc file of file sink in GNU RADIO, I can change the C++ code according my requirement but there is no such file.

It's in gr-blocks/lib; however, you shouldn't modify the file sink. Really, I'd recommend you take the time to go through the guided tutorials, use gr_modtool to generate a general block which has a state machine that looks for the bits of your start string and drops everything before and including those, and then passes everything till it sees the stop string. That all can be done with a single state machine, and a bit of python or C++ code.