Getting Started on RTTY - Page 11
Diddles & Transmit Troubleshooting
Diddles are transmitted in RTTY when idling. When no text
characters are being sent, the RTTY modulator should send out a steady stream of
diddles. This normally happens when you key your transmitter and don't
type anything from the keyboard. Or if you are typing live in a QSO and
you pause. While you are paused, diddles are being sent in an attempt to
keep the receiving station's demodulator in sync. It's a timing device
that assists when receiving RTTY. It does nothing for your transmit.
It's a receive-only issue. Diddles are very important in RTTY. It is
highly recommended that you always transmit diddles. You could send out a
steady Mark or Space tone when idle, but this could alter the timing sequence of
the receiving station's demodulator in the event of a hit. If the signal
should fade or a burst of noise occur, it would take the receiving station's
demodulator a longer amount of time get get back in sync and more print will be
lost than if diddles were being used. To hear what diddles sound like,
click here. For a
more technical & excellent explanation of diddles, courtesy of Chen W7AY,
click here.
Diddles
are turned on by default in MMTTY. So when you first key your transmitter,
you should hear diddles in your side tone. If not, there is a
problem. Either diddles got turned off somehow in MMTTY or your signal is
not shifting between Mark and Space as it should be.
The first thing to
check is to make sure diddles is turned on in MMTTY. Go to Options, Setup
MMTTY and to the TX tab. Under DIDDLE in the upper left hand corner, LTR
should be checked. If not, click on LTR, then click OK.
If diddles were turned on and you still don't hear diddles when
you key your transmitter, then there is most likely a hardware problem
somewhere. If you are running AFSK, I can think of no reason why diddles
would not be sent if your radio is transmitting. With AFSK, you either
have audio or you don't. If your radio is in the LSB position, the radio
keys up when F9 is pressed and you hear a steady tone instead of diddles, then
something is happening I cannot explain. The only thing that comes to mind
is RFI on the audio line coming from the sound card. If you suspect RFI,
go to the RFI page here. You should
visit this page anyway as there is a lot of information here on protecting
yourself against RFI when operating RTTY.
If you are running FSK, then
there are a couple of things to look at. If you are running FSK and you
hear a steady Mark or Space tone when you key your radio, chances are that your
interface circuit is not working correctly. With your interface connected
into the FSK input of your transmitter, check the DC voltage on the FSK output
of the interface. It should be around +5VDC. If you don't see 5
volts, then your interface is not working. When you key your radio you
should see this the voltage fluctuate as diddles are being sent. The best
way to test your interface circuit is by using an oscilloscope. Place an
oscilloscope across the FSK output of your interface. Set your scope for 5
volts/division and a sweep of about 10 ms per division. You should see a
waveform something like this:
If you don't see a waveform or any change in state on your DC
voltmeter, then your interface is not working. There is a very remote
chance that your serial COM port is not working, but that is rare. You
could change COM ports to see if the problem still exists, but odds are your
interface is not correct.
If you do see this waveform but you still don't
hear diddles while monitoring your radio's transmit, it could be possible that
the FSK circuit in your radio is not working. This is also very
rare. You can test the FSK circuit in your radio by manual shorting and
opening the FSK input while monitoring your transmit. You won't hurt
anything by doing this. If the tone changes in frequency from 2125 to 2295
or vice versa, then the FSK circuit is working. If you do not hear a
change in frequency between an open and a short on the FSK input of the
transmitter, then the FSK circuit in the radio is indeed
inoperative.
Other Transmit Problems
I mentioned RFI.
RFI is a common problem in RTTY. If you are having any sort of "strange"
occurrences while operating RTTY, it could be RFI getting into audio cables,
your monitor, your keyboard or FSK line. Be sure to check the RFI
page.
Something I haven't covered yet, and it pertains to FSK
transmission only, is making sure your radio's FSK circuit is set for 170 hz
shift. Most radios that operate FSK have at least two shift
selections - 170 hz or 850 hz. 850 hz is called "wide shift" just as 170
hz shift is referred to as "narrow" shift. My Kenwood TS870 has four
choices - 170, 200, 425 & 850 hz. If you start transmitting and your
RTTY tones do not sound right, it could be possible that your radio's FSK
circuit is not set to the correct shift. Check your owner's manual to
determine how to set the correct shift in your radio. Modern radios have
menu selections that set the shift, while some older models, such as the Icom
IC-751A has a switch under the top cover inside the radio.
One of the
most common transmit problems is transmitting inversely or up-side-down.
RTTY is polarity sensitive. Transmitting inverted means you are sending a Mark
when you should be sending a Space and vice versa. This can happen if you
are using AFSK or FSK. With AFSK, it could mean you are using the wrong
sideband. LSB is standard but USB can be used if you have the polarity
correct. Yet, most stations who send inverted are using the wrong
sideband. If you are set up to transmit in LSB, sending RTTY while in USB
will result in your transmit being inverted.
When operating FSK, you
could transmit inverted if your radio is not set correctly. Most modern
transceivers today will have a menu option for setting the polarity of your Mark
tone. If you find yourself receiving RTTY OK but transmitting inverted,
change the setting in your radio. Some older radios will have a switch
setting to change the polarity of your FSK signal. Refer to your owner's
manual on how to change the polarity of FSK.
Unless you know what an
inverted RTTY signal sounds like, it can be difficult to tell if you are
transmitting inverted or not. The best way to test your RTTY operation is
to get on the air with someone you know who can tell you if you have it right or
not. If you are on the air and someone tells you that your transmit is
inverted, 9 times out of 10, you are. Eventually you will be able to
recognize an inverted RTTY signal quickly. Occasionally, I'll have someone
tell me I am inverted. But after 20 years on RTTY, I can guarantee you I
know what an inverted signal sounds like. Soon you will be able to tell
the difference as well.
If after all this, you are still having transmit
problems, you need to direct your questions to the MMTTY reflector.
Another excellent forum for RTTY operations is the RTTY
Reflector. I have been a subscriber to the RTTY reflector for many
years, first with WF1B and now with contesting.com and I can tell you there is
no finer group of people willing to help you with RTTY.
This concludes "Getting Started on RTTY". Click here to return to the AA5AU RTTY Page.
