[RTTY] My questions

[Kok Chen]

On Mar 31, 2011, at 3/31    9:31 AM, James E. Wells wrote:

> When and where are the best times and frequencies to find RTTY,  
> PSK31 (I think that is what it is called), and CW. I am not sure  
> what to list for except CW since that is a no brainer. But I am not  
> sure what RTTY sounds like or any other mode.

James,

Soundcard "AFSK" digital mode software works by producing tones in the  
"baseband," i.e., audio frequencies in the computer, send them through  
a sound card to an SSB transmitter, and then let the transmitter shift  
the tones (i.e., "modulate") the audio signal up into the RF spectrum  
as an RF signal.

At the far end, the SSB receiver shifts (i.e., "demodulate") the RF  
signal back to the audio baseband so it can be sampled by the input  
sound card into a numerical stream and then processed (further  
demodulated and "decoded") into the 1s and 0s that form the codeword  
for a character.

Thus, the audio that you feed to your rig will sound exactly like the  
audio that you hear from a receiver that is tuned to a signal.  (The  
pitch will depend on the VFO tuning offset, known as a "tone-pair" in  
RTTY.)

Most software modems today allow you to select the output sound card.   
So, if you make the modem select the computer speakers (instead of  
some other sound card that is connected to the rig) as the output  
device, you will hear the "digital signal" on your speakers when you  
place the software into transmit mode.

(As extra safety, turn off your rig when you do the following, so you  
won't be accidentally putting out a signal on the air in the wrong  
subband :-)

The pitch of the sound will be the same as that of a "real" signal  
that is properly tuned in.

On RTTY, select AFSK instead of FSK (since you asked about HRD, I  
don't think you need to worry, since it is fixed for using AFSK, as  
far as I know).

With RTTY, you can change the "tone-pair" for what you will regularly  
use to hear what an off-tuned RTTY signal sound like.  If the software  
supports waterfall clicking for RTTY (I think HRD supports changing  
the transmit RTTY tone-pair by clicking on the waterfall, I know that  
cocoaModem does), just click on different parts of the waterfall to  
change the tone-pair (you may have to stop transmission before you  
change the transmit frquency -- again, I am not sure about HRD since I  
don't use it).

PSK31, DominoEX, Olivia and other modes (even Hellschreiber nowadays)  
are implemented as "AFSK" so you should be able to hear them too on  
the speakers without having to do any "FSK" vs "AFSK" shifting.

"AFSK" is the mathematical way of generating a nice clean RF signal.   
FSK is kind of a hardware kludge (it used to be relays switching a  
capacitor in and out of a VFO tuned circuit, but often done nowadays  
in the direct digital synthesizer of a rig -- still a hardware  
kludge).  Even more mathematically elegant is to generate an in-phase  
and quadrature pair (I and Q) baseband signal and let the transmitter  
modulate the quadrature pair up to RF, like what an SDR transmitter  
would do.

Just like RTTY, you can click on different parts of the waterfall and  
then engage transmission to hear what PSK31 and other modes sound like  
when they are located at different parts of the waterfall.

Once you are familiar with signals, you can identify them with how  
they look on a waterfall or panadapter, instead of from how they sound  
to your ears.

As to where to find signals, a very reliable place is 20m in the  
daytime or early evening (depending on when 20m closes down for you).   
For PSK31, listen in the 14070 kHz to 14072 range.  Between 14072 and  
14080 (more or less), you will find the lesser used modes, ranging  
from Olivia to DominoEX to Hellschreiber and even the high speed PSK  
modes (PSK63 and PSK125) usually hang out up there.  RTTY stations  
usually stay between 14080 and 14095, but you won't find many RTTY  
signals nowadays unless there is a contest going on.  Wideband digital  
mode signals can be found above 14100.

73
Chen, W7AY