A group of keyboard modes that send text on many tones with built-in error correction. They are built to survive the conditions that wreck RTTY and PSK31: fading, interference, static crashes and signals below the noise. They are slower and typically wider, but they keep a conversation going when the simple modes cannot.
How they work
MFSK is multiple frequency-shift keying: instead of two tones (mark and space), the signal picks one tone out of many each symbol, so each symbol carries several bits. Olivia and MT63 go further and send each character spread across many tones and over time, with error-correcting code across the whole group.
Schematic. Robust modes buy their resilience with extra symbols, which cost time or bandwidth.
If a burst of static or a fade wipes out some symbols, there is still enough left to reconstruct the character.
ARQ asks again. FEC repairs it on the spot.
This is forward error correction (FEC), the second approach in the diagram above: the receiver fixes errors using the redundancy it was sent. There is no return channel, so it works for one-way broadcast and group chat as well as two-way conversation.
What it costs
Speed. Text is slow. Olivia in particular is far slower than PSK31.
Delay. Characters arrive in blocks, only after the whole block has been received. A reply feels like a short wait.
Bandwidth. Robust configurations take wider slices of the band than PSK31. MT63 is wide; Olivia lets you pick between narrower and wider settings, and the narrower ones are slower.
Choosing one
Situation
Leans toward
Fading, flutter, noisy HF paths
Olivia or MFSK16
Lots of static crashes or impulse noise
MT63
Clean, strong, narrow-band signals
PSK31 or RTTY
Both stations must agree on the mode and its settings, so operators arrange them in advance or announce them on the air.