Humm? Those DF9IC modems I know of use FSK (well the modem is actually
a PAM modem, the transceiver converts that to FSK), and I know
DF9IC pretty well :-)
On the contrary, these modems aren't very bandwidth-efficient,
if I recall correctly they use about 1.2 times the bandwidth
of a WA4DSY modem. But their biggest advantage is simplicity.
They're constant amplitute, so that you can use C class amplifiers
(unlike WA4DSY), and they allow for simple noncoherent detection.
And besides, there's now a 200kHz duplex (i.e. 2 times 200kHz)
channel allocated on 70cm for 76.8kbit/s user access. This was
demonstrated last weekend at the Friedrichshafen fair.
73s
Tom