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Digital Net
The digital net has been suspended until further
notice. KC2RLM
************************
Every Wednesday at 9:00 p.m. (night
and time change starting May 25, 2005), club members hold a "net"
(on-air meeting) using digital modes instead of voice. This net is
perfect for beginners and others who want to experiment with various
digital modes including RTTY, PSK-31, packet, and slow scan TV.
Even though many of these modes are commonly run on HF frequencies,
this net is run on a VHF frequency so that Technician-class hams can
also participate. Everyone -- including non-club members -- are welcome to
join the digital net!
The net is run in 'round table' fashion,
meaning that every ham who checks in is given an opportunity to
transmit in a specified order (usually the 'check in' order). The transmitted reports are usually personal news or
activities that might be of interest others in the net, or responses to
comments made previously on the net. Transmissions do not need to be related to ham radio
topics. The nets also allow for operating
assistance and troubleshooting, usually by voice.
Currently the net is following a scheduled "mode of the month"
(see chart below). This schedule may change as the year progresses so check back
periodically.
Quick Links for this page:
mode schedule
software
net frequency
net protocol
'help' for
sound card digital modes
This is the current schedule of digital modes for 2005.
This mode schedule can change based on input from club members to
the webmaster. The scheduled
mode for a night is
not set in stone -- net participants can agree on an alternative mode for any evening's net
(or part of it), but
they should recognize that some net participants may not have the necessary
software for that mode downloaded, configured, and ready-to-go. It is
suggested that net participants have RTTY and PSK31 software available,
since these two modes are the most popular alternative modes (useful
when the scheduled mode is not working well).
1 Mode:
In the chart above, "Mode" refers to the basic modulation scheme. Some
modes have more than one variation. The primary variation is listed
in parentheses beneath the mode and it will be the first
variation used in the first net of the month. Other variations may be
explored during the month.
2 "Elmers" are individuals experienced in this
mode who have volunteered to be a resource for anyone who needs
guidance with the mode or its software.
Contact the webmaster if you
would like to volunteer.
3
Software:
Generally, all of the digital modes above can be run with free Windows
software that uses the computer's sound card to transmit and
receive the mode's modulation scheme. Net
participants are encouraged to download the suggested software well in advance of
the first net that will use that mode. This will give them time to
configure and learn to operate the software before the net.
A few modes, such as RTTY and packet, can be run without a sound card if
you have the
special radio modem (a hardware device) that is required for that mode.
Thus, some net
participants may be using these hardware devices, instead of using a sound card program.
For some popular modes, such as
PSK31, there are several programs that can be used. The first software
listed is the program that most club members seem to use. For beginners,
this means it is the program for which you are mostly likely to find
help from other club members. Still, net participants are encouraged to experiment with
different programs, since you may find a program better suited for you
and maybe even for the group! Not listed above are "multi-mode" programs
-- programs that can handle several digital modes.
Hamscope is a
free multimode program. Another is MixW,
but it is NOT free; there is a $50 fee for this program after a
15 day test period.
Unfortunately, sound card digital mode
software for Apple/Mac computers is
very limited. In fact the only known Apple/Mac program is "Multimode for
Macintosh": http://www.blackcatsystems.com/software/multimode.html . It
handles Morse code, RTTY, FAX, SSTV, PACKET, ACARS, PSK31, ALE, and many
other modes, but this program is NOT free. (Site
with other ham software for MacIntoshes.)
There is more digital mode software for
LINUX users, but maybe not for all the modes we
will be exploring. Try
N9NU's page for some possibilities. Then do a web search to UNIX information pages
for the mode in which you are interested.
Most of the modes that will be used on this net
were designed specifically for keyboard-to-keyboard/ "live" conversations
rather than transmissions of previously composed copy or files.
'Conversation' modes such as RTTY, PSK31, MFSK16 and Hellschreiber work
well for 'round table' nets since everyone can 'listen in' to the
transmitting station, and transmission rates are not slowed by protocols
that check to make sure each station received perfect copy. In fact, it
is a given that when using conversation modes, you may not receive
perfect copy. Even if the transmitting station typed perfectly (rare!),
other factors such as noise can produce errors. So please understand
that what you receive is not necessarily what was sent. Still, you can
usually make out the meaning of the sender's message.
Other modes such as packet, PACTOR, and AMTOR
were designed primarily for error-free transmissions. These 'error
checking' modes are generally used in one-to-one situations where it is
important to verify that the receiving station has received all data
perfectly from the sending station. These modes are generally
not used
in 'roundtable' nets, since error-checking protocols are difficult to
use where there are many receiving stations. (Note: Packet does have a
special 'roundtable' mode called 'unconnected' or 'UI' mode and we will use that for our net).
Most of the modes used on this net were
developed for use on HF frequencies. The wide variety of modes stems
from the objective of achieving the ideal mode for various situations.
Each of the modes has advantages and disadvantages in the areas of
bandwidth, speed, accuracy, and resistance to the noise, fading and
distortion that is common on HF frequencies. We won't be able to fully
see those advantages and disadvantages on a VHF FM net, but we should
gain some operational experience that may be useful if we use these
modes on HF. At minimum, we should have fun communicating in new and
different ways.
The default digital net frequency is 145.75 MHz FM simplex
(the "club frequency"). A suggested testing or backup frequency is
145.70 MHz. Although some of the modes used on the net were originally FSK modes
used on HF frequencies
(such as RTTY), sound card software
uses AFSK (Audio Frequency Shift Keying) which can be used in either FM mode on a VHF frequency
or SSB mode on HF frequencies.
Unfortunately, FM signals do not travel nearly
as far as HF signals. This means that stations away from the net
'center' of New Providence, NJ may not be
able to hear all of the net participants. Nevertheless, by using a relay
to a station you can hear, you can participate in the net. Your
relay station can tell you when it is your turn to transmit and, at minimum,
you should be able to receive your relay station's digital transmissions,
and perhaps others.
At the start of the net, interested net participants
will use voice mode to find other participants. Participants may appoint
a "Net Control" station for the evening or they may elect to run the net
informally without a "Net Control". (It is suggested that, at least for
the first net of the month, a member experienced in the selected mode
step forward to volunteer as the Net Control station.)
In voice mode, the net participants will
confirm the digital mode of choice (see suggestions above), determine who is participating (the roll), the order in which stations will
transmit during the net, and also work out a plan if some members of the net
can not hear other members (for example, a net control or a relay
station may be needed to
indicate when it is someone's turn to go).
The first part of the net can be used to work
out any technical difficulties. For example, the first person to
transmit might give a relatively brief transmission and then ask in voice mode
if any of the net participants had any problems. After any problems are
resolved, the first person can continue with a longer transmission and
then pass the frequency to the next person in the list. Try to use the
selected mode to orchestrate this transition if possible. (In Slow Scan
TV mode, for example, it may be more efficient to simply announce the order of transmissions and let the transition
proceed automatically after each person has sent one picture; or else use
voice to coordinate the transition after each picture is sent.)
At the end of
one round (i.e. after everyone has transmitted once), the net should pause and ask
by
voice (in case someone who wants to join is having trouble with
the chosen digital mode) if there is anyone else who wishes to join the net. Anyone joining
the net at that time should be given a turn to transmit and work out
problems. A second round of transmissions can then start.
The net can continue for additional rounds as the
members determine, but typically the net lasts 30-60 minutes depending
on the number of participants.
If you wish to drop out of additional rounds, as a
courtesy, try to remember to let the net know "this will be my last
round". If someone drops out without announcement, the next person on
the roll after the missing person should automatically pick up the
frequency, after waiting a short time for the missing person.
