David G. Nagel wrote:
Ian Thompson-Bell wrote:
Green Xenon [Radium] wrote:
Hi:

What would be heard on a 40 Hz AM DX receiver that uses the most
sensitive type of magnetic loop antenna? I doubt there would be any
hissing since that artifact would involve high-frequency sounds and a
40 Hz carrier cannot transport modulation-signals higher than 40 Hz
[violation of Nyquist theorem].

Not strictly true I think - depends on the modulation scheme - how
else do you think we manage to get 56K bps down a 3KHz phone line.

Cheers

IAn

Ian;

If it were possible to utilize 40hz at a high data rate it wouldn't take
15 minutes for the US Navy to send a 3 character message to it's
submerged submarines.

That's becuase the technology was developed ages ago.

As for the phone companies sending 56kbps down a
3khz phone line. They use something like 4 descrete tones and phase
shifts to do that and not at the rated speed either but something less
as a practical matter.
.

Multiphase AM schemes are common place; they are used in mobile phones
for instance. It's the old trade off between bandwidth and S/N just like
in FM but in reverse. Check out VPAM.

Cheers

Ian