Gareth's Downstairs Computer wrote:
On 30/03/2017 15:01, Rob wrote:

5-level codes are typically used with FSK or AFSK modulation so there
is a lot of efficiency to be gained by implementing a more efficient
modulation method. When doing that, you can do some compression as well.

GMSK?

No, not by far. Typical RTTY is sent with 170Hz shift FSK at a rate
of 50 baud, so well above GMSK.