[RTTY] What is the sideband convention

[Kok Chen]

It really does not matter how you generate FSK for RTTY,
whether it is LSB, USB or direct, as long as Mark (binary 1)
appears as the higher of a pair or frequencies at the
antenna.

The usual U.S. convention for AFSK is to use 2125 Hz as the
audio Mark frequency and 2295 Hz as the Space (binary 0)
frequency.  When this is passed through an LSB generator,
both tones are below the suppressed carrier.  The lower
audio tone is closer to the suppressed carrier, thus,
appears as the higher frequency on the RF spectrum (if
this is confusing, see example below).

An example of what happens when using AFSK on LSB.

Imagine that you are generating AFSK with 2125 Hz Mark and
2295 Hz as space.

Also imagine that your LSB suppressed carrier is placed
at 14085 kHz.

When you send Mark, 2125 Hz will be sent to the modulator,
and the LSB transmitter will put out 14085.00 - 2.125 kHz, or
14082.875 kHz.

When you send Space, 2295 Hz is sent to the modulator,
and the LSB transmitter will put out 14085.00 - 2.295 kHz, or
14082.705 kHz.

Notice that the Mark frequency is the higher of the two
frequencies when it hits your antenna.

The same hold for receiving. It does not matter whether you
use LSB, USB or direct RF demodulation, as long as you treat
the higher of the two frequency pair at the antenna as binary 1
(Mark) and the lower of the pair as binary 0 (space), you will
copy a standard RTTY signal correctly.

Good practice for spotting on PacketCluster is to spot the Mark
frequency.  Spotting the SSB suppressed carrier frequency is
not very useful unless you also tell people the AFSK tones
you are using and whether you are using LSB or USB.

73
Chen, AA6TY