AFSK and FSK
By convention, amateur RTTY uses the lower sideband, with the Mark frequency 170 hz higher than the Space frequency. The generation of Mark and Space frequencies required for Amateur Radio RTTY transmission is commonly accomplished in one of two ways:
Frequency Shift Keying (FSK), in which the transmitter is directly switched between Mark and Space frequencies via it's FSK Input (sometimes referred to as the RTTY keying signal); the FSK input must be driven by an FSK connection
Audio Frequency Shift Keying (AFSK), in which Mark and Space frequencies are generated from Mark and Space tones fed to the transceiver's audio input, typically driven by a soundcard's audio output
Also by convention, a RTTY QSO's frequency is that of its Mark frequency. Thus accurate DX spots for RTTY stations specify the station's Mark frequency.
FSK
FSK requires a transceiver support in the form of a RTTY or FSK mode in which the transmitter frequency switches between RTTY Mark and Space frequencies as dictated by the signal driving the transmitter's RTTY input. FSK's advantages include
the transceiver displays the Mark frequency on its front panel and reports this frequency via its transceiver control (CAT) interface
recent Icom transceivers provide a Twin Peak Filter for RTTY reception that is only available when the transceiver is in RTTY (FSK) mode
However, only a few transceivers provide RTTY output waveshaping.
AFSK
AFSK is accomplished by placing the transceiver in LSB mode (some transceivers provide an LSB DATA or PKT mode for use with AFSK) and directing the PC soundcard to generate Mark and Space tones that are fed to the transmitter's audio input to produce Mark and Space frequencies at the transmitter's output. AFSK's advantages include
- can be used with any transceiver capable of SSB transmission
does not require an FSK connection to drive the transceiver's RTTY input
the MMTTY engine can be configured to provide good RTTY output waveshaping
- disable compression in the transceiver
- configure the MMTTY engine's TX bandpass filter
set WinWarbler's Operating Mode to RTTY
on the RTTY tab of WinWarbler's Configuration window, click the MMTTY Setup button on the tab's right edge
in the TxBPF/TxLPF panel on the Setup window's TX tab,
check the Tx BPF box
set the Tap selector to 512
Setting up CW, Phone, PSK, and RTTY Operation