[OZAPRS] TNC capture range

[Darryl Smith]

Hi Owen

Great responseŠ I will look at it in more detail, and you should send a
copy to Kenneth. 

FYI, I will probably be heading back to Braidwood on Friday (or possibly
Thursday) if you wanted to compare journeys. I do this journey every few
weeks

Darryl


---------
Darryl Smith, VK2TDS POBox 169 Ingleburn NSW 2565 Australia
Mobile Number 0412 929 634 [+61 4 12 929 634 Int] - 02 9618 6459





On 11/10/2014 8:50 pm, "Owen Duffy" <owen at owenduffy.net> 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
>_______________________________________________
>OZAPRS mailing list
>OZAPRS at aprs.net.au
>http://lists.aprs.net.au/mailman/listinfo/ozaprs