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Re: [Discuss-gnuradio] Help understanding AX.25 encoding and tagged stre

From: Cinaed Simson
Subject: Re: [Discuss-gnuradio] Help understanding AX.25 encoding and tagged streams
Date: Tue, 15 Aug 2017 19:14:08 -0700
User-agent: Mozilla/5.0 (X11; Linux i686; rv:52.0) Gecko/20100101 Thunderbird/52.2.1

On 08/14/2017 07:14 AM, Jack White wrote:
> Hello, knowledgeable ones,
> I need to put together testbed satellite uplink using AX.25 encoding and
> GMSK modulation. The flowgraph source is an Ettus USRP X310 with a
> WBX-120 daughterboard.
> The flowgraph is a modification of one used with a USRP B210. That radio
> had handle different sample rates to the X310, so some level of
> resampling is required - in this case from the proposed 614.4ksps to
> 400ksps.
> I understand the premise of constructing the AX.25 packet, but I'm
> having real difficulty understand tagged streams and how the tagging and
> the PDU format is affecting my transmission.>
> The disabled low-pass filter and the enabled and encircled rational
> resampler are my additions. The AX.25 line coding hierarchical block is
> below.
> ​So, when I run the flowgraph and try to set a command from the
> groundstation, I get a huge load of buffer underrun errors in the GRC
> log:
> If I try sending the command again, I get a tP error, followed by more
> underruns.
> In my head, it looks like the USRP is expecting a combined sample of a
> certain length, but it doesn't get it. The buffer is more than half
> full. Then the next packet comes in, of the same length, and you get one
> buffer filled with half of one packet and half of another.
> The USRP /is/ transmitting something, because I checked the transmitting
> antenn400k/614.4a on a spectrum analyser.
> Can anyone help me understand the flowgraph, especially the tagged
> streams and PDU input, and get this working?
> Cheers,

I don't really understand what you're doing but I have a couple of drive
by comments.

The produce of 2 rationals is always rational - and the quotient of two
rationals is always rational.

I would replace the 2 serial rational resamplers with one, namely,

  (8/1)*(400000/614400)=(8/1)*400/614.4) = 5.208333333333333/1



That is, interpolate by 15.625 decimate by 3.

Since the sampling rate of the sink is 400000 Hz, then the sampling rate
coming out of the GFSK Mod needs to be 76800 Hz.

And the baud rate out of the GFSK Mod would then be 76800 Hz/8 sps = 9600.

If reducing the 2 serial rational resamplers to one doesn't help,  I
would poke the front side of GFSK Mod - add a throttle in front of the
GFSK and set it to 76800 Hz - see what happens- not recommend for
general use.

And I would add a Message Debug to Tagged Stream Multiply Length Tag and
set the Length scalar to 1 - 512 times the packet_length seems high -
but then I don't know the packet_length either.

-- CInaed

> -- 
> Jack White
> address@hidden <mailto:address@hidden>
> 07875 813 745
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