GNURADIO 3.7.8: identify a part of a byte stream

Question

I am feeling Stream Tags, Message Passing, Packet Data Transmission are a bit of overkill, and I have hard time to understand.

I have a simple wish: starting from a stream of bytes I would like to "extract" only a fixed number of bytes) starting from a known pattern. For example from a stream like this: ...01h 55h XXh YYh ZZh..., it should extract XXh YYh ZZh.

I utilized Correlate Access Code Tag block -- Tagged Stream Align -- Pack K Bits to convert a bit stream into a byte stream and synch to the desired Access Code (01h 55h), but how do I tell gnuradio to only process 3 bytes after every time the code is found? Likely OOT block would solve, but is it there some combinatino of standard GRC block to do this?

Answer 1

I think with correllate_access_code_tag_bb you can actually build this, with a bit of brain-twisting, from existing blocks alone. (Note: this does rely on stream tags, because those are the right tool to mark special points in a sample flow.)

However, your simple case might really not be worth it. Simply follow the guided tutorials up to the point where you can write your own python block.

Use self.set_history(len(preamble)+len_payload) in the constructor of your new block to make sure you always see the last samples of the previous iteration in your current call to work, and simply search for the preamble in your sample stream, outputting only the len_payload following bytes when you find it, not producing anything if you don't find it.