[FedCom] About the "digital intermod"

[hithel]

I have been hearing this "digital intermod" noise for
maybe 4 or 5 years on Boston FD channels. The noise
typically appears after a mobile unit unkeys. (Boston
FD uses repeaters at 483Mhz with 486 input freqs). No
FD personell ever seem to complain about this noise -
and I have never seen any scannerists post about this
noise. I listen without PL/DPL - maybe that is why I
hear it and few others do.

Come to think of it - I think I did a hear a blast of
this "digital intermod" on a California scanner feed
once.

Recently Somerville MA had a large fire. They were
using 483.2625R. I heard a blast of the digital
intermod - and at the same time I heard the MetroFire
dispatcher talking. The MetroFire dispatcher uses
483.2875 or 483.3125. Maybe this is a problem caused
by repeaters on adjacent channels. (Boston FD has 4
adjacent freqs.)

Maybe the CAP intermod on 148.15 came about due to
adjacent channel interference from 148.125.

When I mentioned a "digital repeater" in my previous
post - I was thinking of any repeater that had any
sort of digital audio processing. I think I have read
that some analog repeaters actually do a 'analog to
digital to analog' conversion between their receiver
and their transmitter. (For example - on 154.01, 
Methuen MA FD sometimes has a distinct echo from
radios on their new repeater - I think the repeater is
putting a tiny time delay into the repeated message.

Maybe I can get a tape recording of this noise some
day. Peter S

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