RTTY

fldigi can operate on a wide range of RTTY symbol rates and bandwidths. The selection of symbol rate and bandwidth is made on the RTTY configuration tab. The three most common in amateur radio use can be selected from the mode menu. These are

Mode	Symbol Rate	Typing Speed	Bandwidth
RTTY 45	45.45 baud	6.0 cps (60 wpm)	270 Hz
RTTY 50	50.0 baud	6.6 cps (66 wpm)	270 Hz
RTTY 75	75.0 baud	10.0 cps (100 wpm)	370 Hz

These modes were a result of mechanical and electrical designs of the early TTY machines. The 45.45 baud and 75 baud machines were for the US / Canadian market and used 60 Hz synchronous motors. The 50 baud machines were for the European market and used 50 Hz synchronous motors.

fldigi can encode and decode many other symbol rates and bandwidths. "Custom" combinations are set up on the RTTY configuration tab. You probably will never have to do that unless you like experimenting with unusual RTTY modes.

AFSK is not FSK

All of the modem signals that fldigi produces are audio signals. That includes the RTTY signal. fldigi can encode and decode an RTTY signal that is anywhere within the passband of the sideband transceiver. It is not limited to the traditional tone pairs around 2100 Hz. The following screen captures clearly show three side-by-side RTTY-45 signals with the middle one being tracked correctly. These signals were generated using fldigi's ability to save audio waveforms. The three RTTY signals were combined and then white noise added to create the three signals with s/n of approximately 10 dB.


Waterfall centered at 1500 Hz	Spectrum centered at 1500 Hz	Digiscope Signal View	Digiscope X-Hair View

The decoding on any of the three signals was equal and very near 100% print. The decoder uses a hysterisis detector to help in noise burst rejection. It also uses AFC to track the signal. If a nearby CWI or RTTY signal drags the AFC you can disable it with the AFC button on the fldigi main dialog. Fldigi uses a DSP bandpass filter to reduce interference. The width of the filter can be set on the rtty configuration tab
or by positioning mouse pointer in the waterfall; pressing the control key; and rotating the mouse-wheel.

The slider and the red bar in the frequency scale correspond.

To start decoding a signal simply left click on the signal and the AFC should lock on to the signal.

DSP filtering is applied to the audio before decoding takes place. That process helps to reject nearby interference, other RTTY signals or CWI. When a baud rate / shift combination is selected the optimum filter bandwidth is computed and the Receive filter bandwidth is set to that value. You might find a different value gives better performance, especially if you are using a narrow band transceiver filter. You will need to reset the DSP filter each time you reselect the RTTY modem.

The digiscope display will extinguish when the Rx signal level falls below the squelch setting.

You must operate your transceiver in the USB mode for the RTTY signal to be the correct polarity. You must also observe the requirement to maintain linearity in the transmit path.

It is possible to use fldigi to generate the keying waveform for use with an FSK type of transmitter. See Pseudo FSK for a description of how this can be accomplished.

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