Yes .. but .. looking at it from over here .. Technically the situation looks a little quaint ..
What happens between 145 and 220 meg to enable the mode ..the concept is the same . so it cannot be based on intercept ability ..that only leaves bandwidth ?
Something must of driven the 'need' for the clause in the first place ? (my licence bans pulse tx below 10 GHz .. but all these mfsk systems transmit pulses .. long ones , but pulses just the same)
How old is the FFC 220 meg rule on SS , what is the actual text of the clause ?
Yes your right and as originally posted , the frequency allocation is randomised by a PG string added to the data , obvious way to go really .. but 'we' where locked into psk at the time … but data manipulation cannot be the reason for the 220 clause as the system may be used over this frequency ...
I suspect that in the mind of the 220 author , spread spectrum equated to high spectrum usage , hence the high vhf allocation to what i assume (in those days) was an experimenters band ?
To quote from wiki
Spread" radio signal over a wide frequency range several magnitudes higher than minimum requirement. The core principle of spread spectrum is the use of noise-like carrier waves, and, as the name implies, bandwidths much wider than that required for simple point-to-point communication at the same data rate.
Pure tone bursts within a audio channel 500 Hz , 100 Hz or 50 Hz , do not quite compare to what is normally associated with SS ? .. May be its time for some one to ask again ,? ,