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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: |
Sun, 27 Aug 2017 16:40:32 -0700 |
User-agent: |
Mozilla/5.0 (X11; Linux i686; rv:52.0) Gecko/20100101 Thunderbird/52.3.0 |
On 08/26/2017 05:36 PM, Robert McGwier wrote:
> At one time rational resampling meant that the interpolation and
> decimation were each integers so the rate change is the ratio of two
> integers and thus rational.
All integers are rational. But not all rationals are integers.
1,2,3, are integers and rational.
The quotient of rationals is rational.
Hence
1/2 = .5000000......
1/3 = .3333333......
2/3 = .6666666......
are all rational.
>
> If one or more of your inputs is fractional>
I can input 1/2 in the rational resampler which satisfies the "rate
change is the ratio of two integers and thus rational" - and the machine
will reduce it to .5000000....... - which is also rational.
> I believe you need the
> arbitrary rate resampler.
I've never used an arbitrary or fractional resampler.
Maybe by an arbitrary resampler they mean it can used with either
rational or irrational resampling ratios?
And maybe a fractional resampler is to make a small adjustments to
sampling rate - like on the order of a couple of percent?
-- Cinaed
>
> Bob
>
>
> On Aug 15, 2017 10:16 PM, "Cinaed Simson" <address@hidden
> <mailto:address@hidden>> wrote:
>
> 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:
> >
>
> UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
> >
>
> UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
> >
>
> UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
> >
>
> UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU
> >
> > 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
>
> or
>
> 15.625/3
>
> 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>
> <mailto:address@hidden
> <mailto:address@hidden>>
> > 07875 813 745
> >
> >
> > _______________________________________________
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> >
>
>
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