The asymmetric propagation case is impractical to address, whether
the stations involved are attended or unattended; fortunately, its
not common. The case we can address is that of the unattended station
that could, if suitably equipped, detect an already busy frequency
and thereby avoid QRMing an ongoing QSO.
The busy frequency detector in SCAMP was a first-cut "see how it
works" implementation -- yet expectations were exceeded. From my
email exchanges with Rick, it was clear that there remains plenty of
opportunity for improvement.
Appended below is Rick's post to the ARRL bandwidth committee, in
which he characterizes SCAMP's busy detector. This was originally
made available on the SCAMP reflector, to which you may not have
access.
73,
Dave, AA6YQ
Rick Muething KN6KB's post to the band width committee:
I want to take this opportunity to update the bandwidth committee on
recent progress made in the testing of a new digital mode called
SCAMP (Sound Card Amateur Message Protocol). This sound card mode
incorporates integrated ARQ (Automatic Retry reQuest) and dynamic
encoding levels to deliver error-free digital data at respectable
speeds (3-4 Kbytes/minute) over 1.9 KHz HF channels. One objective is
to provide performance comparable to Pactor II and III using low cost
sound card/PC technology and standard voice grade radios (HF and VHF).
On March 19, 2005 we began initial beta testing of SCAMP with Winlink
2000 with the exchange of test messages on 17 meters from the client
program Paclink SCD W5SMM (Vic Poor) to WL2K SCAMP Server KN6KB.
This will be continuing for the next few months using KN6KB's SCAMP
Server and one or two additional WL2K SCAMP Servers in selected
areas. This marks the the third phase of on-air SCAMP testing which
started in November 2004
( http://www.arrl.org/news/stories/2004/12/07/6/?nc=1 )
Since SCAMP is a "wide band" digital mode (1.9 KHz) SCAMP servers are
operating only in the narrow HF forwarding sub bands shared with HF
Packet, Pactor III and other automatic/semiautomatic wide band
modes. These sub bands were envisioned many years ago as a
compromise to permit unattended HF forwarding between HF packet
stations. Clearly the abundance of new digital modes including
digital voice, data and images has exceeded the narrow limitations
of these sub bands (only 5-10 KHz on some bands)
The SCAMP clients and servers also incorporate an effective channel
busy detector to significantly reduce QRM from either the careless
operator or the hidden transmitter (3rd station not heard by the
station manually initiating the connection but detected by the
automated server).
While there is and continues to be much comment from groups that
would like to banish all automated (full or semi) transmissions these
automated modes have proven to be a very useful and popular. These
modes have also proven to serve best and be most efficient and
reliable in times emergency when sufficient control operators are not
always be available.
The attached screen capture GIFs from the WL2K SCAMP server show that
while not perfect the state of the art in automated busy detectors
has improved considerably.
The following GIFs were all made at fairly weak signals...Signals
barely moving the S meter above the background noise. The SSB signal
is about 1 S unit over the noise.
Clear channel display (reference)
Weak CW (about 1000 Hz on the display)
PSK 31 signal at about 1000Hz with a weaker CW carrier below
Pactor II signal near the bottom of the Pass band
Pactor III signal showing mode transitions
SSB voice at about 1 S unit over the noise
There is no question that the integrated use of these types of busy
detectors can substantially mitigate QRM from automated or semi
automates stations even in difficult "hidden transmitter" scenarios.
This combined with reasonable partitioning by bandwidth (clustering
like bandwidth signals in band segments) will allow a peaceful co
existence of the myriad of modes now in use in amateur radio
including the semi automatic transfer modes now so popular. It will
also foster an environment to experiment with and expand the use of
digital technology...an important aspect of keeping Amateur radio
healthy.
I think it is also important for the committee to consider that the
US is only one country and that several other countries have adopted
a policy of minimal regulation of bandwidth and modes. Canada for
example permits all HF digital mode < 3 KHz (1 KHz on 30 meters) with
virtually no restrictions as to mode, or automation level.
I would suggest the committee consider the following in developing a
band plan to submit to the FCC.
1) Generally minimize the complexity of band restrictions by
mode,bandwidth and level of automation. The Canadian model is a good
example of such simplicity.
2) Allow semi automatic operation while encouraging the use of
technologies like smart busy detectors that reduce QRM both for the
manual and automatic station.
3) Use a simple metric (e.g bandwidth) to segregate modes if
required yielding a regulation that is flexible and viable for many
years to come.
4) Move with leadership toward a fast resolution of the current
antiquated band plan and propose it for fast track adoption with the
FCC.
Thanks for your consideration. I am at your disposal to answer any
technical details of the SCAMP effort and its implementations.
73,
Rick Muething KN6KB
--- In digitalradio@yahoogroups.com, "Jose A. Amador" <[EMAIL PROTECTED]>
wrote:
>
>
> Dave,
>
> Well, I was aware of SCAMP, but maybe not well informed about the
results.
> I have never seen it in action, as I was not one of the beta
testers.
>
> Nevertheless, it is a formidable task, and I know that Rick did
work
> hard on it.
>
> But as SCAMP has not been in public distribution and not released
after
> the tests,
> it is still only hope, as far as I can see. Besides what Rick and
team
> did, I know of
> no other similar efforts.
>
> And you are right, the perfect is the best enemy of good. But even
then,
> good operating
> practices are needed, as assymetric conditions will not
disappear
> and people with
> good detectors may be run over by those who don't use them or
defeat them.
>
> So, let's wait and see, hoping for the best.
>
> 73, Jose
>
