>>>AA6YQ comments below
--- In digitalradio@yahoogroups.com, "Robert Thompson"
<[EMAIL PROTECTED]> wrote:
> Two years ago, SCAMP demonstrated a multi-mode busy detector for
> HF that proved highly effective, despite the fact that it was a
> quickand dirty first attempt.
I would *love* to see either code or a large-scale test, including
prearranged intentional usage on a busy channel. If both false
positive rate and false negative rate are good enough, this could
easily be the way forward for many different projects.
>>>Agreed
> For the same reason that most of us don't run with 10 KW output
> power or 10 kHz of bandwidth on HF -- because it would be a
> violation of both amateur radio regulations and operating practice.
> We should not allow our equipment to transmit over top an existing
> QSO if its possible to prevent it.
This is only true if both parties respect the other party's rights and
trust the other party to reciprocate.
>>>Agreed. No technology can overcome a total breakdown in civility.
In general, non-automated users resent the automated stations and
often regrettably attempt to interfere.
>>>There is no excuse for intentional interference of any kind. The
resentment of automated stations is rooted in their perceived lack of
civility: they transmit over ongoing QSOs, and there is no way to ask
them to QSY. The installation of effective busy detectors in
automated stations would dramatically improve their civility, and
thus reduce both frustration and polarization among the different
communities.
Given this fact, the automated station operators are often
unwilling to enable features that can be used by others to destroy the
automatic stations' functionality. Note that I am not saying this
*should* happen, but that given human nature, it *does*.
>>>I am an incurable optimist, and so believe that the amateur
community's reaction to the installation of busy detectors in
automated stations would be a collective sigh of relief. Those lids
who previously vented their frustration by QRMing automated stations
would most likely find more constructive uses of their time.
> I suggested that a busy frequency detector with an 80% success rate
> would reduce QRM to ongoing QSOs by a factor of 5; you say this
> statistic is misleading, but offer no explanation; please elaborate.
The problem is that a one-shot accuracy of 80% is trivially
achievable, but real use isn't one-shot. My probability-math reference
is at home out of reach at the moment, so I can't just quote you the
formula to determine the actual probability of failure given repeated
one-shot attempts. The bottom line, however, isn't good.
>>>That would be true if the trials were independent, but they are
not. For example, If the busy detector detects modulation on three
successive samples, then it can conclude that a QSO is in progress,
and set a substantially higher threshold for inactivity before
assuming the frequency is clear. One would not implement a busy
detector as a simple binary device that returns busy or not busy; its
a context-driven state machine.
An example follows:
There is an ongoing SSB QSO on a given frequency. The automated
station begins listening for a couple of seconds, and (correctly)
detects activity on the frequency. It initiates a hold-off for N
seconds. N seconds pass. It listens again, and (correctly) detects
activity, initiating another hold-off. This sequence repeats for a few
times until statistics catch up with us and it (erroneously) detects a
clear channel and begins transmitting.
>>>This would be a suboptimal design. Once a QSO is known to be in
progress on the frequency, the busy detector should not return "not
busy" based on a single modulation-free sample. At the first
modulation-free sample during a "QSO in progress" condition, the busy
detector might begin sampling every 15 seconds and only change state
to "not busy" after seeing, say, 3 minutes without any modulation on
the frequency.
Thus, assuming an 80% success rate and a hold-off algorithm that is
reasonably balanced for both avoiding interference *and* efficiently
using a shared channel (5 seconds < N < 60 seconds), over the course
of a ten minute QSO, the >chance that the automatic station will
interfere approaches unity.
>>>As pointed out above, this need not be the case.
If you pick too big of an N, the automatic station wastes lots of time
waiting on the channel to clear. If you pick too small of an N, your
busy detector's error rate increases the chance of a failure. In
either case, one party suffers unduly.
>>>Waiting 3 minutes for a QSO to clear doesn't seem like undue
suffering for a message-passing system --- except during emergency
conditions, during which the busy detector would be disabled.
This is ignoring that in the above SSB case, even a continuous
detector is going to misdetect periods of silence or other reduced
modulation as "channel now empty".
>>>A "universal QRL" signal would allow busy detectors to be less
conservative -- if an automated station's transmitter was given a
premature go-ahead, one of the participants in the QRM'd QSO could
send QRL (just as happens today between attended stations), which the
automated station would instantly respect.
On the wider discussion of automated stations and non-automated
stations sharing spectrum, I suspect that the way forward will
actually involve changes on both sides.
An example: If the initiating member of the automatic conversation
would, after the busy detector says the frequency is clear, use some
standard modulation to ask "Is this frequency clear", listen for say
30 seconds, and if the busy frequency detector trips during this time,
move onto an alternate frequency. Note that the automatic station only
needs to do this when it hasn't been using the frequency for long
enough that the frequency looks clear to some other party. Also note
that the automatic station does not need to be able to decode any
answer, it just needs to watch for a transmission.
>>>Agreed. I have suggested the use of 10 wpm CW to convey QRL?. A
positive response could simply be 1 second of 600 to 1000 hz CW
within 5 seconds of the query.
Combine this with listen-scanning on several alternate frequencies and
both automated and non-automated stations could share the spectrum.
Of course, the human QSO partners would need to tolerate occasional
automated channel query messages, but it at least gives them a way to
tell the automated stations to move frequency.
>>>I completely agree.
73,
Dave, AA6YQ
