[RTTY] Some basic RTTY questions

[Kok Chen]

On May 29, 2014, at 5:54 PM, Michael Rapp wrote:

> But, I'm set up for FSK.  My software (MMTTY) doesn't decide what my
> transmitted tones are.  My radio does.  My Yaseu FT-950 allows me to pick
> 2125 or 1275 and that's it.  So it looks like I can't set my mark tone to a
> lower frequency even if I wanted to.
> 
> Moreover, changing the mark tone makes no sense on receive as one is
> receiving the tone frequency of the transmitting station.  Am I correct or
> am I missing something really basic here?

When you set your FT-950 up to transmit with "2125," what it does is to send the Mark carrier 2125 Hz lower than the receiver's SSB LO frequency.  It sends the Space carrier at 2295 Hz lower than the receiver's SSB LO frequency if the FT-950's shift is set to 170 Hz.

What this does is that when you change from transmit to receive, the another person's Mark carrier that is zero beat to your RTTY signal will be demodulated into a 2125 Hz tone at the receiver output, and with 170 Hz shift, the other person's Space carrier will be demodulated to a 2295 Hz tone.

When you change to "1275,"  your transmitter will transit the Mark carrier at a location that is 1275 Hz lower than the receiver's LO, etc.

As a result, your software modem must also know what Mark/Space tones you have selected on the rig.  The two must agree, otherwise the modem you won't decode anything.

I don't know about the FT-950, but on the Yaesu transceivers that preceded the FT950 (FT990, 1000D, 1000MP), you can also make the dial reading display the Mark carrier instead of the LO frequency.  If you have this option, please use it, since that is where others spot an RTTY signal,  and expect an RTTY signal to be spotted at.  Unlike SSB voice, the LO frequency is not longer at the dial frequency.

Some software modems can remodulate the audio tones from the receiver to any tone pair that you choose and output that the computer speakers.  I.e., it could be set to demodulate and decode 2125/2295 tone pair, but output 1000/1170 audio tone pair to your ears.  If your favorite software modem does not allow that, just pester the developer to implement it.

> My second question is on optimizing my receive audio.  My radio PC is an
> old server and does not have a built-in sound card  I'm using a cheap
> little USB sound card that has a mic in not a line in.  I receive my RTTY
> audio by taking the REC OUT of my FT-950 into a mixer (for sideband work)
> and then the tape out of the mixer that goes into my USB sound card's mic
> jack.

My recommendation is to ignore that sound card.  You can buy usable USB sound cards with line level input for around $5 at Amazon. 

If you insist on saving the $5, you would need to build a 2-resistor voltage divider (or use a potentiometer) that brings the receiver output to within the dynamic range of the existing sound card.

73
Chen, W7AY