RTTY Operation

RTTY Operation

Overview

RTTY (Radio Teletype) is a digital mode that encodes text as two alternating audio tones — a Mark tone and a Space tone. AetherSDR supports RTTY operation using the radio's built-in RTTY demodulator via the RTTY mode selector.

When you switch to RTTY mode, AetherSDR displays two dashed vertical lines on the panadapter labeled M (Mark) and S (Space). These lines show exactly where the two RTTY tones sit in the audio passband, and they move with your VFO frequency.

Mark and Space Explained

RTTY transmits binary data by switching between two audio frequencies:

Tone	Meaning	Default Frequency
Mark (M)	Binary 1 — the "key down" tone	2125 Hz above the carrier
Space (S)	Binary 0 — the "key up" tone	2125 − 170 = 1955 Hz above the carrier

The difference between Mark and Space is the Shift — by default 170 Hz, which is the standard amateur RTTY shift.

How to read the panadapter lines

When you tune to an RTTY signal, you want to position the two dashed lines so that the Mark line sits on the higher-frequency tone and the Space line sits on the lower-frequency tone. When both tones fall on their respective lines, the signal is properly tuned and the radio can decode it.

Panadapter view (RTTY mode):

    S           M
    |           |        ← dashed lines on panadapter
    ┊    ╱╲     ┊╱╲
    ┊   ╱  ╲    ╱  ╲
    ┊──╱────╲──╱────╲──
    ┊ ╱      ╲╱      ╲
  Space     Mark
  tone      tone
  (1955 Hz) (2125 Hz)

  ←── 170 Hz shift ───→

Settings

Mark Frequency

The Mark tone frequency (offset from carrier) can be set in Radio Setup → Phone/CW. The default is 2125 Hz, which is the worldwide amateur RTTY standard. Other common values:

Mark Frequency	Usage
2125 Hz	Amateur radio standard (default)
2295 Hz	Commercial / maritime RTTY
1275 Hz	Some European stations

The radio stores a global default (rtty_mark_default) that applies to all new RTTY sessions. AetherSDR reads this from the radio on connect.

Shift

The shift is the frequency difference between the Mark and Space tones. The standard amateur RTTY shift is 170 Hz. Other shifts exist (e.g., 850 Hz for military/commercial) but are uncommon on the amateur bands.

The shift is set per-slice via rtty_shift in the radio's slice status.

Using RTTY with External Software

Most operators use external RTTY software (MMTTY, fldigi, WSJT-X, etc.) connected via AetherSDR's DAX virtual audio and CAT control:

1. Enable DAX — In the DIGI applet, enable DAX on a channel (e.g., DAX 1). This creates a virtual audio device that routes RX audio to your RTTY software.
2. Set up CAT — In the DIGI applet, enable rigctld or a PTY serial port for CAT control. Your RTTY software uses this to key the radio and read frequency.
3. Select RTTY mode — Switch the slice to RTTY mode. The radio applies the correct filter bandwidth and enables its RTTY demodulator.
4. Use DIGU/DIGL for AFSK — If your RTTY software generates audio-frequency shift keying (AFSK) rather than expecting the radio to do FSK, use DIGU or DIGL mode instead of RTTY. The Mark/Space lines will not appear in DIGU/DIGL since the demodulation is handled by your software.

Recommended RTTY Software

Software	Platform	Notes
fldigi	Linux, macOS, Windows	Full-featured, supports many digital modes
MMTTY	Windows	Classic RTTY-specific decoder
WSJT-X	Linux, macOS, Windows	Primarily FT8/FT4, but connects via same DAX/CAT path

See the DAX Virtual Audio and CAT Control wiki pages for setup details.

Typical RTTY Operating Frequencies

Band	Frequency Range	Notes
80m	3.580 – 3.600 MHz	RTTY/Data segment
40m	7.040 – 7.080 MHz	RTTY activity center ~7.080
30m	10.130 – 10.150 MHz	RTTY ~10.143
20m	14.080 – 14.100 MHz	RTTY activity center ~14.085
15m	21.080 – 21.100 MHz	RTTY ~21.085
10m	28.080 – 28.100 MHz	RTTY ~28.085

Troubleshooting

Mark/Space lines are in the wrong position

The Mark frequency may not match the station you're receiving. Most amateur stations use 2125 Hz Mark / 170 Hz Shift. If the lines don't align with the signal's two tones, check Radio Setup → Phone/CW for the Mark default setting.

I don't see the M/S lines

The Mark and Space lines only appear in RTTY mode. They do not appear in DIGU, DIGL, or other modes. Make sure the slice mode selector shows RTTY.

Audio sounds like two alternating tones but no decode

Make sure the RTTY signal's tones align with the M and S dashed lines on the panadapter. Tune your VFO until both tones sit on their respective lines. If using external software, verify the DAX channel is routing audio correctly.