On Fri, 2016-03-25 at 11:07 -0600, Tom Golden wrote:
> AX.25 data is NRZI encoded (so inverse bits and then NRZ). I hand > calculated the HDLC frame start byte and first few address bytes so > the corr_est only triggers at the start of the packet. I've added NRZI decoding and encoding and adjusted the correlation pattern for the HDLC flag in the attached. > Seems to trigger but I had to lower the threshold down to .390. Something seems wrong. Look at how a build the preamble samples in my flowgraph. Since I use the GMSK Modulator to build the preamble samples, it has a bit of a delay, and I have to discard the first 1.5 symbols or so of samples. (GMSK with L=4). Also note, that unlike PSK, which will correlate to both the normal and inverted preamble (since it's just a 180 phase shift), for FSK the corr_est block will only correlate to the version of the preamble you specify, and not the inverted one (different frequencies don't correlate). > > I have a separate program I'm using to check for the bit pattern I'm > sending (0xA5, 0xA5, ...). It checks against raw, nrz, and nrzi to > see if the pattern is present. It's present in 2 bytes and the data > after those 2 bytes is wrong but consistent - which I think points to > a timing issue. > > > I think the fact that I had to reduce the threshold down to 0.390 > points to the same issue as the bits not coming out correctly. I'm not sure. I added the HDLC deframer in the flowgraph to decode the packet bytes for you. It won't spit out anything though, if the CRC check fails. You may want to hack the block to output data even when the CRC fails; just to see if things look sane. The HDLC deframer was originally used for AIS, but it will generically deframe any HDLC (hopefully!). -Regards, Andy > > On Fri, Mar 25, 2016 at 10:51 AM, Andy Walls > <a...@silverblocksystems.net> wrote: > On Fri, 2016-03-25 at 12:22 -0400, Andy Walls wrote: > > On Fri, 2016-03-25 at 12:00 -0400, > discuss-gnuradio-requ...@gnu.org > > wrote: > > > FWIW, I couldn't get the attached flowgraph (which looks to > correlate to > > the HDLC flag) to crash. I was probably just lucky. > > I forgot to mention, in the flowgraphs I winged up: > > a) I might have the sense of 0 and 1 symbols reversed. Right > now I have > 0 symbols as the low frequency and 1 symbols as the high > frequency, but > that guess could be wrong. (The convention for AFSK systems > is usually > the reverse of what I have, for example.) > > b) I'm assuming the data bits are not differentially encoded > or NRZI > encoded. That could be wrong. AIS uses NRZI, for example. > > -Regards, > Andy > > > > >
gmsk9600_test2.grc
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