Why don't voice radio communications use FM?

On Thu, 07 Sep 2006 07:41:55 +0200, Mxsmanic wrote:

Jim Logajan writes:

Just tell people they would operate it like a telephone: the pilot would
direct her call to a particular listener (e.g. ATC) and ATC gets a signal
(like a phone ringing!) and can let it ring until they have time to answer
the call. But in a pinch, the system could also act like a party line system
and after hitting the emergency transmit button in her aircraft, the pilot's
distress call would automatically cut in over less-urgent calls to not only
ATC, but to any aircraft who have set their receivers to automatically accept
emergency calls.

In essence, digital systems provide multiple virtual private circuits if
needed, but still allow broadcast or "party" line equivalents for situations
where that communication mode is more useful.

What about analog users? What if an analog user transmits while a
queued message is being transmitted?

Normally, the duration between keying and the call grant is very. It's
measured in milliseconds. This means, the analog users PTTs and starts
talking, assuming he doesn't already hear someone talking. The grant
comes back to the repeater and it starts repeating. That means the
digital portion is some number of milliseconds behind the analog portion.
If someone else on the analog side walks on your analog call, I imagine
the DSP will do its best to filter it out. That probably means your call
it terminated before you intended or it's removed. If an analog user
talks over the P25 repeater, repeating from the digital side (xmiting
analog), then you have the classic walked on radio call problem. That is,
after all, one of the problems of analog radio.

And from my first posting on the topic:

How do you make this work in parallel with analog systems that cannot
queue?


The repeater initiates the call on your behalf. The repeater is queued
rather than the analog radio. Likewise, the reply goes to the repeater,
which then re-RXs ("repeats") as analog. For this to work, the analog
and digitial systems must have their own frequencies.

Hopefully this clears things up.

Greg