all signals shown in x2 waterfall mode, tick marks at 100 Hz intervals, waterfall palette is "blue-2"

General Description

The THROB family of modes are MFSK in nature, but are unusual in that (like DTMF) they use two tones at a time. Unique among other MFSK modes, the THROB family also uses amplitude modulation and sometimes sends just one tone!

The mode has no Forward Error Correction, is difficult to tune, but reasonably sensitive and moderately robust. Because of the limited character set however, the typing speed is reasonably good, one character per symbol. Tuning must be very accurate, and the software will not tolerate differences between transmit and receive frequency. THROB is essentially a curiosity mode.

Another unusual feature (which led to the name), THROB also includes raised cosine AM modulation of each symbol. This combined with two tones transmitted at the same time, means that a very linear transmitter is required.

For THROB, nine tones are used, spaced 8 or 16 Hz. For THROBX, 11 tones are used, spaced 7.8125 or 15.625 Hz. The THROB family was developed by Lionel G3PPT.


These are unconnected, manually controlled message asynchronous symbol synchronous simplex chat modes, with no Forward Error Correction. The most frequent calling mode is THROB2.

Reception is non-coherent, and sync is recovered from the amplitude modulation.

Coding and Character Set

A limited 44 character user interface is used, one directly mapped tone combination (symbol) per character, which contributes to the reasonable typing speed. There is no lower case. Modulation is symbol synchronous, and with 45 possible one- or two-tone combinations available, the remaining combination is used for the idle condition. (ThrobX has 53 characters, 11 tones, and never sends single tones).

Operating Parameters

Mode Symbol Rate Typing Speed1 Duty Cycle2 Modulation Bandwidth3 ITU Designation4
THROB1 1.0 baud 1.0 cps (10 wpm) 80% 1/2 of 9-FSK 72 Hz 72H0F1B
THROB25 2.0 baud 2.0 cps (20 wpm) 80% 1/2 of 9-FSK 72 Hz 72H0F1B
THROB4 4.0 baud 4.0 cps (40 wpm) 80% 1/2 of 9-FSK 144 Hz 144HF1B
THROBX1 1.0 baud 1.0 cps (10 wpm) 80% 2 of 11-FSK 94 Hz 94H0F1B
THROBX2 2.0 baud 2.0 cps (20 wpm) 80% 2 of 11-FSK 94 Hz 94H0F1B
THROBX4 4.0 baud 4.0 cps (40 wpm) 80% 2 of 11-FSK 188 Hz 188HF1B


1. WPM is based on an average 5 characters per word, plus word space. Values are approximate because a variable length code is used.
2. Transmitter average power output relative to a constant carrier of the same PEP value.
3. This is the "Necessary Bandwidth" as defined by the ITU.
4. A summary of the ITU Designation system can be found at

5. Default and normal calling mode.