[op25-dev] Re: OP25 GRC - Almost but not quite working

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Matt Robert matt.robert80 at yahoo.com
Thu Apr 26 15:06:06 UTC 2012

Hi Bob,

Its more than likely a result of a bad signal. The FSK4 demod tends to output zeros when it loses sync which is why the DUID is erroneously coming up as 0x00.

LDU1/2 are the two types of voice frames. LDU1 has signalling data embedded and LDU2 has encryption sync data embedded.

The kludge diff posted below will work - but only for valid frames with a corrupt DUID value.

I'm sure the root cause is the FSK4 demod isn't locking properly and causing a bad DUID value to be outputted. I have seen similar behaviour in other areas of OP25 - for example I was looking at the IV/MI value (72 bits) on a local P25 system here. Whenever the demodulator ran out of talent, it would output steams of constant zeros.

Ohio MARCS is a Type 2 Smartzone Omnilink System with CAI compatible ASTRO voice channels - so its essentially P25 compatible. The P25 CAI spec was based on the ASTRO CAI from the same era, so its essentially the same thing.

Is the system running any form of simulcasting? 


 From: rrgsti <bobrich at gmail.com>
To: op25-dev at yahoogroups.com 
Sent: Thursday, 26 April 2012 10:59 PM
Subject: [op25-dev] Re: OP25 GRC - Almost but not quite working

Quick update from my end.

It looks like every frame coming out of the fsk4 demodulator (I'm assuming, still a n00b here) is marked with a 'duid' of 0x0.  Consequently, when data_unit.cc initializes a new data_unit from the frame, it is always creating it as an HDU (P25 header) type.  This then prevents the IMBE decoder from being executed b/c it's not a voice data unit type (LDU1/LDU2 (no idea what these mean)).

I figured maybe it has something to do with our system (Ohio MARCS) not being full P25 compliant and not including metadata of any sort, so I just made the following change to data_unit.cc and re-ran it:

--- op25-orig/blocks/src/lib/data_unit.cc    2012-04-24 10:31:29.139694592 -0400
+++ op25/blocks/src/lib/data_unit.cc    2012-04-26 08:12:35.183962129 -0400
@@ -39,7 +39,8 @@
uint8_t duid = extract(frame_body, 60, 64);
switch(duid) {
case 0x0:
-      d = data_unit_sptr(new hdu(frame_body));
+      //d = data_unit_sptr(new hdu(frame_body));
+      d = data_unit_sptr(new ldu1(frame_body));
case 0x3:
d = data_unit_sptr(new tdu(frame_body, false));

This seemed to sort of work as I now get rather garbled, but intelligible, audio from the decoder.

I've uploaded the baseband capture (1Msps) and resulting audio .wav file that I get at the following URLS:


Not sure if this is of any use, but it is encouraging to hear voices at least.  :)



--- In op25-dev at yahoogroups.com, Andy Knitt <andyknitt at ...> wrote:
> It looks like imbe_decoder_factory.cc in OP25 defaults to
> 'software_imbe_decoder'.  I manually changed the IMBE environment variable
> to "soft" and confirmed it with printenv, but I'm still getting a flat line
> at the output of the OP25 block.  Any other ideas?
> Thanks,
> Andy
> On Mon, Apr 23, 2012 at 11:47 PM, Balint <balint256 at ...> wrote:
> > **
> >
> >
> > I *still* haven't checked out the latest code, but in my old code the
> > default voice frame output was (null?).****
> >
> > There are options for file output, null, external (hardware) decoder and
> > internal decoder. You used to be able to spec this on the command line as
> > an environment variable:****
> >
> > export IMBE=soft****
> >
> > I changed my default to be the internal decoder (see
> > `imbe_decoder_factory.cc').****
> >
> > ** **
> >
> > *From:* op25-dev at yahoogroups.com [mailto:op25-dev at yahoogroups.com] *On
> > Behalf Of *Andy Knitt
> > *Sent:* Tuesday, 24 April 2012 12:45 PM
> > *To:* op25-dev at yahoogroups.com
> > *Subject:* [op25-dev] OP25 GRC - Almost but not quite working****
> >
> > ** **
> >
> >   ****
> >
> > I have the OP25 GRC demo that Balint provided up and running.
> > Everything seems to working except I'm not getting any audio out of
> > the OP25 block. I'm getting the "four line" output from the dibit
> > output port when there is traffic on the channel, and the autotune
> > output is outputting data. However, no audio. I put a scope on the
> > audio output and it's a flat line at zero, even when the dibit output
> > is "four lines". Any tips on how to further troubleshoot?
> >
> > Thanks,
> >
> > Andy****
> >
> > ****
> >
> > 
> >

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