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

{play}http://www.pa4rm.com/attachments/073_throb-1.mp3{/play} Throb-1 test sample
{play}http://www.pa4rm.com/attachments/073_throb-2.mp3{/play} Throb-2 test sample
{play}http://www.pa4rm.com/attachments/073_throb-4.mp3{/play} Throb-4 test sample
{play}http://www.pa4rm.com/attachments/073_throbX-1.mp3{/play} ThrobX-1 test sample
{play}http://www.pa4rm.com/attachments/073_throbX-2.mp3{/play} ThrobX-2 test sample
{play}http://www.pa4rm.com/attachments/073_throbX-4.mp3{/play} ThrobX-4 test sample