The only time you would need specific tones as Demetre mentions below,
is if you have filters that are preset for those tones. This is the way
it used to be in the past with one filter tuned for the mark frequency
and the other for the shift frequency offset. Mostly that was
standardized with a 170 Hz shift in later years. Early equipment had 850
Hz and commercial equipment often used 425 Hz. So having different
filter combinations from the standard ones was not practical since
they were tuned and fixed as best you could get them and each discrete
filter was relatively expensive.
In the late 1970's the integrated circuit made it much easier to build
filters that were easily adjustable with a potentiometer instead of
adding and subtracting capacitors from an array of 88 mH and /or 44 mH
toroids (remember those!). One of my early homebrew TU's used the
XR-2206 tone generator and XR-2211 tone decoder. Not as good as the
older equipment, but incredibly compact, inexpensive, low voltages, etc.
Until packet radio wiped out VHF RTTY in our area, I used that TU with a
Model 15 teleprinter for local contacts through our RTTY regenerative
repeater.
Today the filter is often in the software program you are running on
your computer and it no longer matters what the tones are. You move the
cursor along the waterfall to place your signal or to decode the signal
and the mark tone may be 500 Hz, 1214 Hz, 2001Hz, but it could even be
2125 Hz if you so choose and want to tune the rig so that the tone is at
that point. But it is not necessary to do this as long as the
relationship (the shift) remains proper at 170 Hz for most cases, and
with the mark tone high, as it relates to FSK. With AFSK and using a
different sideband things are a bit reversed, but as I mentioned
earlier, the programmers have mostly standardized on leaving the rig on
USB if using AFSK and they make the tones work correctly as if you were
actually transmitting FSK with mark high.
73,
Rick, KV9U
Demetre SV1UY wrote:
For RTTY whoever uses the American Tones (Mark 2125Hz - Space 2295Hz)
always uses LSB, whereus if you used the European Tones (Space 1275Hz
- Mark 1445Hz) you have to use USB so that the 2 systems are
compatible to each other.
All my old homemade terminal units were using the European Tones so I
always used USB.
I always favoured the European Tones because they were chosen more
wisely to be in the center of the passband of the crystal filter of
the old transceivers that only had an SSB crystal filter and also
because I liked to listen to the sound around 1360 Hz (the center of
the European Tones) while tuning and and afterwards during the QSO. It
is a more natural sound to me. Whenever I heard the higher tones that
the american terminal units used, I always ended up with a headache.
Of course f you did not follow this norm, you could always switch your
tones the other way around with the REVERSE SWITCH that all terminals
used to have incase someone was not using the correct combination. Is
you notice at the tones I described in the first paragraph, the
Americans tones are Mark-Space where the Europeans always are Space-Mark.
Now with the soundcard programs usually they tell you to use 1500 HZ
as a center frequency and use USB. Of course if everybody follows this
protocol there is no problem. Some soundcard programs have a SOFTWARE
REVERSE SWITCH so that you can change it in case there is any
incompatibility in the tone pairs.
73 de Demetre SV1UY
