> Skipp,
> I find your comments interesting in that the purpose that Motorola
> had in using "reverse burst" of the PL tone was to quickly damp the
> mechanical reed in the PL decoder to close the squelch and eliminate
> the user from hearing the noise burst. But of course you knew that.
> However, in later model radios there is no mechanical vibrating reed
> to abruptly dampen and stop the vibrating from being detected. So
> where is the need for a inverted burst if there are no receivers
> using mechanical reeds as PL tone decoders?
Because the software-based decoders respond the same way as a mechanical
reed to the reverse burst. When the phase shift occurs, the correlation
routines "lose lock" causing the processor to signal the audio gates to
close. Eventually the software routine would regain lock at the new
phase, but the transmitter drops before that can happen.
Hardware-based designs that don't look for change of phase but rather
just the presence or absence of tone are another story, but fortunately
those aren't very common nowadays. Some of the early non-reed
non-software decoders used what were basically very narrow bandpass
filters followed by a detector. They could care less what the phase of
the incoming tone was as long as the frequency was right - you always
heard the squelch crash when the transmitter unkeyed. "Chicken burst",
or transmitting no PL before transmitter drop, was invented as a means
of getting those types of decoders to mute before the transmitter
dropped.
> I can remember many years ago that some hams used a circuit which
> they refered to as "polish PL" which turned off the PL tone before
> the xmtr dropped and had no "reverse burst".
Yeah, "chicken burst", same thing.
--- Jeff
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