Ralph,
The symptoms you describe are those of CTCSS distortion due to limiting
of excessive deviation.
If your authorized emission is 20K0F3E, the sum of voice and CTCSS
deviations cannot exceed 5.0 kHz. The measurements you cited for your
"test bench" HT reveal excessive deviation at 5.15 kHz. But, that's not
the main issue here.
Ideally, the CTCSS modulation should be in the range of 500 to 700 Hz;
any more does absolutely nothing for improving detection, and it
increases the chances for a buzz to be heard in the received audio. The
majority of CTCSS decoders will reliably detect the tone with a CTCSS
deviation as low as 200 Hz.
If the CTCSS encoder in each user radio was adjusted to produce no more
than 700 Hz deviation, then the microphone audio limiter can and should
be set to limit voice deviation to 4.3 kHz. Unfortunately, some radios
are designed so that the CTCSS modulation is mixed with the voice ahead
of the limiter, and this almost guarantees that repeater talk-off will
occur. Nearly all Amateur portable radios are designed this way, and
there is seldom any means to separately adjust the CTCSS level.
Compounding this deficiency is the tendency for economy-minded
manufacturers to use far more CTCSS deviation than is necessary, just to
make sure there's enough!
The CTCSS modulation should be mixed with the voice modulation AFTER the
limiter. If you can adjust your portable and mobile radios for the
deviation levels noted above, your talk-off problems should go away.
73, Eric Lemmon WB6FLY
At 01:24 PM 12/24/2004, you wrote:
Working on an IDA control shelf equipped UHF GE M2 base/repeater.
Normally used to working with a lot of the older card rack style control
shelves. Never saw this happen on other GE M2 repeaters or mobiles.
Here's the situation: I am using the repeater system looking at RUS for
PL detect and CAS for OS. Either running repeater in stock IDA repeat
mode, or externally wired RC-85 controller.
Using an HT on the bench, if I hit the audio hard getting close to 4.5
kHz peaks the repeater would shut down in PL mode (127.3 Hz FWIW). If I
run in COS only mode, no issues....
This effect happens on several HT's on hand. Main 'test bench' HT
measured 650 Hz pl dev and 4.5 kHz dev voice peaks. Audio source, my
highly calibrated voice 'Ahhhhhh 1 2 3 W4XE' hihi...
Now I can fully understand a real distorted audio signal or one that's
way over deviated out of the receiver limits not being decodable. But, I
can't ever remember being able to talk off a pl decoder on innumerable
GE M2's I've had on the bench before...
Has anyone else seen this problem?
73's and happy holidays
Ralph W4XE
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