[Elecraft] RTTY vs PSK for DATA A question K3

[Richard Ferch]

Don,

The choice of sidebands depends more on your software than on the mode. 
If you are using software that was written to support a wide variety of 
digital modes, and particularly if it does not support FSK keying, it is 
likely that this software expects the radio to be in upper sideband, 
whether you are in RTTY or any other digital mode. This kind of software 
quite likely also expects the radio's displayed dial frequency to be the 
suppressed carrier frequency, not the actual transmitted frequency.

To put it another way, with this kind of software and with many 
transceivers, the expectation is that the radio is in USB mode. On the 
K3, you should use DATA A sub-mode for this kind of software. DATA A is 
upper sideband and the dial displays the suppressed carrier frequency, 
the same as in USB, but in DATA A compression and equalization are 
turned off and the microphone is muted (using the normal audio input 
configuration settings). The exact same default software settings that 
work with other radios in USB should also work with the K3 in DATA A.

On the other hand, if the software was written for RTTY only, and 
especially if it also supports FSK keying, then it is likely that this 
software expects the radio to be in lower sideband, and it is likely 
that it also expects the radio's displayed dial frequency to be the 
actual transmitted mark frequency. With this kind of software, the 
appropriate sub-mode on the K3 is AFSK A (unless you are using FSK 
keying, of course, in which case it is FSK D).

With either kind of software, it is usually possible to select the 
opposite sideband (often with a switch or button called "Reverse"). Note 
that if you are using the wrong sideband for RTTY, you will not decode 
anything meaningful at all. If you get even scraps of sensible-looking 
text or callsigns, you are almost certainly using the correct sideband.

If you are having poor decode when using AFSK A with software that is 
set to expect the correct sideband, the next thing to look at is whether 
your audio frequency settings in the software and on the radio are aligned.

In DATA A, the K3's filter bandpass is centred on 1500 Hz, so you should 
use audio frequencies near this value (a centre frequency of 1500 Hz in 
USB corresponds to mark/space frequencies of 1585/1415 Hz). In AFSK A, 
you can set the K3's expected Mark frequency to be any one of 2125 Hz, 
1445 Hz, 1275 Hz or 915 Hz. The DSP filter bandpass settings are 
automatically adjusted to match this choice, so that the filter bandpass 
centre is at 2210 Hz, 1530 Hz, 1360 Hz or 1000 Hz respectively. You 
should choose an audio frequency in your software that is consistent 
with the K3's mark pitch and filter settings.

Until you are familiar with how to use these software capabilities, you 
should also avoid using AFC and clicking in the waterfall to tune in 
RTTY signals, because either of these moves the software's audio 
frequency away from the optimal frequencies near the centre of the 
filter bandpass. If the audio frequencies are near or beyond the edge of 
the filter bandpass, your software's ability to decode will be degraded, 
especially on weaker signals.

The convention for spotting amateur RTTY signals is to spot the actual 
mark frequency. In AFSK A, this is the same frequency that is displayed 
on the K3's dial. In DATA A, on the other hand, the actual mark 
frequency is higher than the displayed dial frequency by an amount equal 
to the audio mark frequency (e.g. 1585 Hz if the audio centre frequency 
is 1500 Hz). Many software programs are capable of doing this arithmetic 
for you automatically, provided the software is configured correctly for 
the particular sub-mode and sideband you are using.

73,
Rich VE3KI


KD8NNU wrote:

> RTTY is supposted to be LSB and PSK on USB.
>
> If you have the K3 in DATA A mode does it matter.  Once you tune into
> the signal and my system decodes I would suspect that I am fine.
>
>  From what my brain is telling me is that a RTTY signal that is posted
> if you are in DATA A and its in USB then all I need to do is turn lower
> in FRQ to find the signal and all shoud be good.
>
> What am I missing if anything with the RTTY.   Will it have a big effect
> on RX decode or TX to someone else.
>
> The reason that I am asking is I just realized the LSB issue yesterday
> and now I am thinking that may be part of my problem where I have poor
> RTTY decode and ability to make weak contacts.    Other digital modes
> like PSK and JT65 work just fine with the system.
>
> I am using the HRD suite of software with DM780 in case that matters.
>
> Otherwise everything works fine, so its more of an education issue for
> operator not a K3 problem.