Well , may be old ,
But, if you have 128 orthongonal / close spaced psk carriers and a little supporting coding, you can send data at 6k or faster in a ssb channel .... its the basis for mt63, (the first in) pactor 111 , ardop and now vara )
And the ultra low s/n vlf data mode also uses narrow psk modulation, the reason Opera uses ask and not psk , no one could send psk on the low bands back then......and still very few have linera amplification .
NB - The technical issues , where/are
Audio [1500 hz] to RF on air carrier frequency translation , very few Tx/Rx where able to send on 500/477 KHz , , which prevented , the use of conventional data modes , I used a e-bay spacial, Racal MA1723 exciter , which covered [out of range] 500 K
Now sets like the IC7100 do function on MF and provide on-frequency drive out
Amplification , There was / is a trend to design MFET 'amplifiers' using a dividing , logic drive , using the same channel FET devices , this required a bi-stable drive , which in turn , at the minimum , divided the drive frequency by '2' , requiring the drive frequency to be x2 , but , then dividing the tone spacing of any mmfsk data mode by '2' , thus preventing any of the conventional modes of the time
Using P&N channel devices , overcomes this issue , by providing on-frequency 'carrier power'
73 Graham ..