[RTTY] Modes and Sidebands

[Kok Chen]

On Nov 9, 2012, at 7:48 AM, rick darwicki wrote:

> I have been using USB for PSK and RTTY even on 160/80/40

SSB is just a mathematical method to take a baseband signal and literally move it up into the RF domain.

In our case, an AFSK or audio PSK31 signal from a sound card.

With USB, the AFSK signal is shifted up, as is.  If the VFO is set to 10 MHz, and the AFSK signal has a 2125/2295 Hz tone pair, the resultant RF signal appears at 10 MHz + 2125/2295 Hz.

With LSB, the resultant RF signal appears at 10 MHz - 2125/2295 Hz.

Notice there is just a sign change.

However, this sign change has a profound implication.

If your AFSK Mark is 2125 Hz, and AFSK space is 2295 Hz, the USB case will put your Mark at 10.002125 MHz and your Space at 10.002295 MHz.

In the USB case, using the same AFSK signal, your Mark is at 9.997875 MHz and your space will be at 9.997705 MHz.

Notice that everyone else will be copying the USB signal as an inverted signal since Space is a higher frequency than Mark.  The RTY protocol says Mark is the higher of the two RF carriers.  The LSB signal agrees with the RTTY protocol (Mark is at a higher frequency).

This is zero problem in out day and age.  If you want to use USB instead of LSB, sinply flip the two AFSK tones before you feed it to a transmitter.

The above holds true for receiving also.  Except that the SSB receiver translates the RF down to audio.  But the same signs occur.  All that has to happen is that when you receive RTTY with a USB receiver, the modem has to know that Mark is the higher of the two audio tone.  In software, that is one line of code.

Most software modems has the provision for you to choose USB or LSB for either transmitting or receiving.

You can think of PSK31 as something at a finer scale.  Instead of shifting things at 170 Hz, you are shifting things at a fraction of a Hertz.

In the case of BPSK, you are "shifting" the phase by 0 or by 180 degrees.  It is not an absolute phase shift, but a differential phase shift (i.e., the phase is either the same as the phase at the previous bit time, or it is 180 degrees away from the previous bit time.  So, the PSK31 family is by textbook a differential PSK method.

When you apply USB, 180 degree phase shift remains as 180 degree phase shift.  But with LSB, if you start in audio as a 180 degree phase shift, the RF output looks like a -180 degree phase shift.

Do we have the same problem as the "inverted" nonsense in RTTY?  Guess what, we don't.  Because +180 degrees is the same as -180 degrees since a cycle of a sine wave has 360 degrees.

Voila!  BPSK31 does not care if you use USB or LSB.  You will be able to send in either USB or LSB and no one would know the difference, and you can copy using USB or LSB and your demodulator does not know the difference (try it if you don't believe me, switch your rig between USB and LSB and you can copy the PSK31 signals; they just fall at a difference position of the VFO dial).

Now, QPSK31 is different.  In the QPSK case, you modulate with 4 phases.  The "Q" of QPSK comes from "quadrature."  The "B" of BPSK comes from "binary.")

A QPSK31 signal is transmitted with 0, +90, +180 and +270 degrees.  An audio signal at +90 degrees will still be +90 degrees on RF when you use a USB transmitter, but it will turn into -90 degrees with a LSB transmitter.

So, with QPSK31, just like RTTY, the software modem has to know which type of transmitter/receiver you are using.  Again, this is one line of code in software. 

Pretty much every multitoned FSK modes (MFSK16, DominoEX, etc) has a reference tone that assumes that you use USB fro transmission and reception -- however, the same thing holds, it is easy for the software to reverse the tones for you.  If the software author had planned ahead, it is again a single line of code.

If your modem does not support USB/LSB switching, you can ask the author to include it.  It is really quite trivial to add.  All that the user has to do is to make sure the software is switched to agree with what you have set your transceiver up to use. 

73
Chen, W7AY