>From the introduction to Section 16.2.1 (RTTY) of Chapter 16 in the 2010 and
>2011 ARRL Handbook for Radio Communications:
"RTTY consists of a frequency shift keyed signal that is modulated between two
carrier frequencies called the mark frequency and the space frequency. The
protocol for amateur RTTY calls for the mark carrier frequency to be the higher
of the two in the RF spectrum."
Then, under "FSK vs AFSK Modulation":
"AFSK can be generated by using either an upper sideband transmitter or a lower
sideband transmitter. With a USB transmitter, the mark tone must be the higher
of the two audio tones in the Audio FSK signal. The USB modulator will then
place the corresponding mark carrier at the higher of the two FSK carrier
frequencies. When LSB transmission is used, the mark tone must be the lower of
the two audio tones in the Audio FSK signal. The LSB modulator will then place
the corresponding mark carrier at the higher of the two FSK carrier
frequencies."
Under "'Spotting' An RTTY Signal,"
"By convention, RTTY signals are identified by the frequency of the mark
carrier on the RF spectrum. "Spotting" the suppressed carrier frequency dial
of an SSB receiver is useless for someone else unless they also know whether
the spotter is using upper and lower sideband and tone pair the spotter's
demodulator is using."
When Ward N0AX asked me to contribute to the 2010 and 2011 Handbooks, I had
very carefully included those paragraphs just because these topics keep getting
asked on this reflector every year or so.
(Of course cocoaModem has a menu to select between USB or LSB for RTTY, PSK31,
MFSK16, ASCII RTTY, DominoEX and SITOR-B. I was surprised to see so many
software modems in Joe's list that don't do it, since it really doesn't take
more than a few minutes of mostly clerical work.)
73
Chen, W7AY
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