[OZAPRS] TNC capture range

[Ian Bennett]

Sorry I asked!!

On 11/10/14 20:50, Owen Duffy wrote:
> Hi,
> 
> On 11/10/2014 18:51, Darryl Smith wrote:
>> Owen commented:
>>> Isn't that entirely about VHF APRS using AFSK and Ham202 (ham adaptation
>>> of Bell202) on FM LMR?
>>>
>>> It does not address the frequency accuracy issue that arises with FSK on
>>> HF.
>>>
>> Kenneth has done more, some of which was in his presentation at the DCC,
>> as opposed just to his paper. What he found was that there is that no two
>> implementations of the Bell 202 are the same, with how they want the
>> amplitudes of the tones, and their performance in noise, at base band.
> Most modulator / demodulator pairs assume a flat channel, flat amplitude
> and group delay response.
> 
> Group delay response is a bigger issue at band edges, and for AFSK, we
> operate well inside the band edges of the radio's response. My own view
> is that we have bigger problems with the amplitude frequency response.
> 
> FM LMR have preemphasis of nominally 6dB/octave from the bottom of the
> voice band (say 200Hz break point), and complementary de-emphasis. These
> characteristics are not perfect, and there is typically some residual
> slope across the voice band, but not nearly 6dB/octave.
> 
> The original concept of AFSK AX.25 on these type of radios is that it
> used an unmodified radio, ie audio in the mic and out the speaker
> jacks... or equivalent. I don't know that this was ever expressed in a
> standards document, even if it was, hams do not tend to pay attention to
> such things.
> 
> So, apart from some hams thinking they knew a better way an bypassing
> either or both of pre and de-emphasis... things worked ok. Failure to
> use matching pre and de-emphasis at both ends of the link does not
> prevent the link working, it just degrades performance (ie needs higher
> S/N for same error rate).
> 
> Some might argue that a channel without pre AND de-emphasis is flatter,
> and that is probably true, but a mixed channel is VERY poor. Now hams
> being hams have devised recipes for mixed channels, you will hear people
> say "we run our digi with no demphasis on the rx and we pre-emphasis the
> tx"... but I have never seen credible quantitative tests to show that is
> actually better.
> 
> So, as I said, most of the community had converged on a consistent /
> compatible use of pre/de-emphasis regime and it was very convenient
> because you could use the existing external jacks on a mobile or hand
> held to do AFSK AX.25.
> 
> Then Kenwood apparently saw no reason to conform, all Kenwoods that I
> have tested do NOT use pre/de-emphasis. They are not the only ones, the
> digis that use Argent T3-135 Alinco combination do NOT use pre/de-emphasis.
> 
> Darryl, the track you laid through the Southern Highlands a few days ago
> was excellent compared with some weeks ago when the area was flooded
> with iGate traffic from Orange via Canberra. I took the trouble to
> listen to the radio as you passed through and I decoded you well if you
> were 40% or more on the S meter. By contrast, I recieve hundreds of
> packets a day from VK1RGI as low as 20% S meter reading, even though
> VK1RGI has only 50% of the audio drive that it should. A likely
> explanation of why I note that Kenwood radios need to be stronger is
> that my rx uses de-emphasis.
> 
> So, it all works, but it works better if the channel is flat, either by
> not using pre/de-emphasis at both ends, or using matching
> pre/de-emphasis at both ends.
> 
> Is there specs on this parameter? Well modem chip specs often
> incorporate a "twist" parameter, and the maximim twist permitted is
> typically 6dB for a Bell202 modem. So failure to use matching
> pre/de-emphasis puts the system within a dB of its tolerance limit for
> that factor alone.
> 
> But, it works... hams prove it every day.
> 
> ================Picture removed, too large for the mail list ... it is
> Figure 2 from Kenneth's paper.==================
> 
> Kenneth gives the figure above, but does not mention that it is not a
> model signal, it is a classic result of de-emphasis after no
> pre-emphasis. Why would he show a defective signal as representing the
> modulation method?
> 
>>
>> If there is a lack of data about what the best relative amplitude of the
>> tones going into the modem decoder is for the best SNR, you have little
>> hope of working out what to use.
> It wouldn't matter, we hams will do what we like.
> 
> A fundamental question is whether the whole community should swing to no
> pre/de-emphasis to follow the lead of Kenwood, because Kenwood is
> unlikely to change?
> 
> We will not rid the system of incompatible stations... it is part of the
> environment, it always existed with more general AX.25 networks to some
> extent, but now to a greater extent.
> 
> We can't achieve consistent deviation level, and in an application where
> excess audio drive is much worse than being short, the tendency is to
> excess... all knobs to the right and some prominent digis lead by example.
>>  At least that was what I got out of his
>> presentation, and then spending too many hours in the bar that night
>> talking to him about this stuff
> That is good.
>> Darryl
>>
>> ---------
>> Darryl Smith, VK2TDS POBox 169 Ingleburn NSW 2565 Australia
>> Mobile Number 0412 929 634 [+61 4 12 929 634 Int] - 02 9618 6459
> 
> 73
> Owen
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