Getting Started on RTTY - Page 5
Transmitting RTTY
Maybe by now, you have
read something or experienced something when receiving RTTY to give you an idea
of how you want to transmit RTTY. Again, your choices are AFSK or
FSK. When receiving RTTY perhaps you noticed that you copied better with
your radio in the FSK or RTTY position than in the LSB position. Or it's
possible your transceiver does not have the capability of using narrow filters
in the SSB position. If this is the case, you may want to use FSK
transmission. Or maybe your radio does allow use of narrow filtering in
the SSB mode, thus permitting satisfactory AFSK operation using LSB.
Before making your decision, read the next two topics covering both AFSK and FSK
transmission.
AFSK Transmission
For AFSK transmission, you
have two things to consider. You have to get audio (the RTTY tones) from
your sound card to either the mic input or audio input on an accessory jack to
your radio. And you must be able to key PTT on the radio to turn the
transmitter on. With AFSK, there are several ways of getting both of these
criteria done. AFSK is easier to cable-up but it also has some
pitfalls.
You could run a single cable from the output of the sound card
to the microphone input of your radio (with an audio transformer in line, of
course) and use VOX to key the transmitter. This is the easiest of all
scenarios. With MMTTY, no audio is sent out of the sound card until you
key the TX button or hit F9 (F9 toggles TX on and off). When TX is enabled
within MMTTY, audio is sent out of the sound card and your radio is keyed by
VOX. The main problem with using VOX, and especially if you use the
speaker out jack on the sound card for RTTY, is that other sounds from your
computer, such as sounds generated by Windows, will also key your radio and
these other sounds will be transmitted out over the air. Not only is this
very poor operating, it's illegal.
One way to keep Windows sounds from
keying your radio is to turn Windows Sound off. You do this by going to
the Windows Control Panel and then to Sounds. Under Schemes, choose "No
Sounds". However, this does not totally eliminate sounds generated by your
computer. For instance, the beep sound used in many programs will still be
generated.
Better alternatives to using VOX are to either use a separate
PTT circuit from a spare serial COM port or to use computer control of
your radio (Icom radios do not support PTT via computer control). You will
need to key your transmitter by one of these two methods if you have wired into
the audio input of your radio via an accessory plug instead of the microphone
jack since the VOX circuit is normally part of the mic input circuitry.
And you also need to use one of these methods if you decide to use FSK instead
of AFSK. VOX will not key PTT when using FSK because audio is needed to
key VOX and audio is not sent to the radio when using FSK. More on keying
the radio when using AFSK on the next page.
Getting back to the audio
feed from the output of the sound card to the input of your transmitter, great
care must be taken to ensure this audio is not too high in level to overdrive
your transmitter. This also will be discussed further on the next page
(Running AFSK with MMTTY).
FSK Transmission
FSK
transmission with MMTTY requires an interface circuit coming from a serial COM
port. This interface circuit takes high and low RS-232 signals, commanded
by MMTTY, on pin 3 (TXD) of a 9 pin serial port or pin 2 of a 25 pin serial port
and converts them to on/off (short/open) keying to the FSK input of your
transmitter. The simplest interface circuit is shown here.
Your radio does the frequency shift keying inside so all you
have to worry about is providing on/off keying. To show how simple this
concept is, plug an open ended cable (2 conductor) into the FSK input of your
transmitter. With your transmitter connected to a dummy load, key the
radio manually and listening to the side tone. It will be a steady tone,
either 2125 hz or 2295 hz. Now take the open ended cable plugged into your
FSK input and short the two wires together. The tone frequency will
change. If an open produced 2125 hz (mark tone), a short should produce
2295 hz and vice versa. You will not hurt your radio by shorting the FSK
input. That is how FSK is keyed anyway! This simple procedure is
used to test the FSK circuit in your transmitter in case you suspect you have
problems with your interface or with the FSK circuitry of your
transmitter. A few years ago my station was struck by lightning. It
damaged the FSK circuit in my Kenwood TS870. This is how I tested
it. When I shorted the two wires going into the FSK input, the tone stayed
the same frequency.
It's always a good idea to listen to your transmit
tones while sending RTTY, to make sure the tones are shifting correctly high and
low. You can monitor your transmit RTTY tones whether you are using
AFSK or FSK by using the Monitor function of your transceiver. However, I
do know that the Kenwood TS570 transceiver does not allow you to monitor your
RTTY tones when using FSK. I'm not sure why Kenwood missed this, but don't
be alarmed if you don't hear tones going out while transmitting RTTY in FSK with
a TS570.
In FSK, you also must key the transmitter PTT circuit.
Again, you have two choices, use an external interface to key PTT or use
computer control of your radio (Radio Command). I'll cover this on Page 7 for FSK
transmission. And by the way, you should use the exact same circuit shown
above for PTT as well as FSK and better yet, you can put them on the same COM
port! More on that later.
Page 6 will discuss setting up MMTTY to
run AFSK. Page 7 will be for running MMTTY in FSK. If you still
haven't made up your mind, read both pages and maybe you can then make your
decision.
