[aprssig] APRS low-power-local ALT input channel

AE5PL Lists HamLists at ametx.com
Mon Sep 27 10:50:55 CDT 2004

Bob, you just can't help yourself, can you.  Ok, time to put this flawed
argument to rest. 

> -----Original Message-----
> From: Robert Bruninga
> Posted At: Monday, September 27, 2004 10:13 AM
> Subject: Re: [aprssig] APRS low-power-local ALT input channel
> Not really, every single user of ANY voice repeater does it
> all the time.   It is such a standard pratice that most people
> appear to not even think about it. That is what the +600 or 
> -600 shift is doing on your radio.  Users are transmitting in 
> the *blind* on the input of the voice repeater (without 
> listening there first) every time they key their radio to talk.

Not true.  They are monitoring the frequency via the output of the
repeater (see below for further debunking of your argument).

> Why is it ok to use TX offsets on 2m voice and cause 
> collisions (doubles) that last for minutes at a time while 
> then saying it is "bad practice" and "poor network"
> design when we do it on APRS with a +600 offset?
> Ah, one might claim that a repeater is full duplex real time 
> and so a user can tell if the channel is busy or not.  Well 
> this simply is not so.  Here are 4 examples of why not:
> 1) If it was, we would not hear people *doubling*

The occurrence of collisions does not mean that collision avoidance was
not attempted.  Because collision avoidance is attempted is why doubling
seldom occurs.

> 2) ALL modern repeaters have 1 second KERCHUNK
>     delays, that delay the repeater coming up by

Not true.  Maybe your repeater has a 1 second kerchunk delay, but most
in this area do not.

>     about a second.  This is NO-DIFFERENTto what 
>     happens with a 1 second APRS packet between the 
>     input and output of an alt-input channel digipeater.

The delays introduced by your alt-input channel digipeater, if using CD
on 144.39, can be extensive AND if the stations using your alternate
channel are trying to use CD on 144.39 for collision avoidance, then
they will see little to no benefit because they will have the same
bandwidth restrictions they have today.

> 3) Many repeaters have delay lines that further delay 
>     the output again, making the output not a true 
>     copy of what is on the input.

Yes, there is always a delay introduced by any repeater.  Unlike a
digipeater which is a store and forward repeater, a FM repeater on two
meters is a real-time repeater with less than 1 second delays unless
they are poorly designed.  Even your 1 second kerchunk delay does not
affect over all delays.  It simply prevents the repeater from
transmitting until it sees a transmission of at least one second (and
this is usually only active when the repeater transmitter is not
currently transmitting).  This is why it is good practice to delay
speaking after keying your microphone to allow the repeater to activate.
What is transmitted by the repeater is a true analog copy of what it
receives on input.

> 4) Many repeaters use PL on the input.  Thus guaranteeing
>     that they are NOT hearing other users on the input
>     channel.

This is the biggest fallacy that you promote.  Coordinated repeaters on
the same frequency use different CTCSS tones to ensure that the users of
one repeater do not interfere with the users of another.  The entire
goal of CTCSS and the reason it is used so successfully in Amateur Radio
is BECAUSE it prevents interference between users of different
repeaters.  Also, take a look at how the FCC looks upon people that use
the input frequency of a coordinated repeater for use other than being
repeated by that repeater.  You can find numerous filings by looking
under "license revoked".

> None of these techniques which support the concept of 
> operating SPLIT frequencies are "bad practice" and in fact 
> they benefit the "design" of the networks on which they
> are implemented.   After 12 years of APRS growth it is time
> to consider adding the split frequency concept to APRS to 
> drastically improve local performance.

As I have shown, split frequency with a FM repeater is far different
than the albatross that you have proposed for APRS.  You have made the
same mistake many first year mathematics students make when told "prove
this theorem".  They set out to prove the theorem correct, creating
invalid premises for the proof without realizing that there is also a
second part to the statement from the professor (which the professor
purposely doesn't tell them): "or disprove this theorem".

Bob, you won't change your way of thinking and I don't even try to
attempt that.  But I hope that people reading this thread do get a
better comprehension of how far off base your arguments really are
concerning this idea.



Pete Loveall AE5PL
mailto:pete at ae5pl.net 

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