[Chronic Philharmonic]

"Don Pearce" wrote in message
...
On Sun, 18 May 2008 17:01:02 +0100, "Serge Auckland"
wrote:

Thanks for that, most interesting. I assume that if it was demodulated
with
a standard envelope detector the distortion would be horrendous.

As you are probably aware, my background is in Broadcast, and the idea of
greater than 100% negative mod just goes against the grain.

S.

Welcome. I was in measurement and signal generation, so stopping at
100% just wasn't an option.

Carrier cut-off defines 100% modulation for continuous carrier double
sideband AM as used in the AM broadcast services. With the kind of
modulators used in this service, negative overmodulation is analogous to
(and indistinguishable from) clipping the baseband on negative peaks (the
carrier does *not* become inverted in phase). AM broadcast transmitters can
be overmodulated in the positive direction if they have adequate headroom to
pull it off. In the US, the FCC allows asymmetric modulation: 100% on
negative peaks, and 110% on positive peaks (the last time I checked).

The FCC does not allow negative overmodulation because of the sideband
splatter caused by the carrier cutoff, and because most AM receivers use
envelope demodulators, which cannot accommodate negative overmodulation,
even if the transmitters could do the carrier phase reversal.