Do I Run Big Carrier, Little Audio or Big Audio, Little Carrier?
submitted by Tom K1JJ
Cormac Thompson wrote:
|With all due respect to the fans
of plate modulation, I would opine that one is better off with
maximum carrier power and whatever "clean, minimum
distortion" modulation scheme results in not exceeding the
1500 watt limit.
There's many varying opinions on this, but I'll give ya
First, it doesn't matter what type of modulation scheme you
use, the end result will sound pretty much the same if it is
set up correctly. The efficiency is usually the only difference.
A lot depends upon your voice: Is it very asymmetrical or
not? Take a look on the scope and phase it correctly.
Your audio system: Whether it can handle big peaks
The receiver detection system on the other end: What kind
Detector: With a sync detector on the other end, one can
hear 300% positive modulation cleanly. With a standard
diode detector, above 100% appears to be where they
start generating distortion sometimes.
The voice: Some guys naturally have very asymmetrical
voices. I've seen it in tests we all made at Steve/QIX's
house one time. At -95% negative peak limiting, I saw
one guy hit 140% positive! The average was around
120% and my voice was the most symmetrical at 110%.
I wish my voice was more asymmetrical, but it's not.
Your transmit audio system: Can you put a sine wave
tone in and see the peaks go up past 100% into the
140% range [overmodulation and pinch-off] and have
the peaks still stay rounded at the tops and clean looking?
Well, they MUST be able to do this if you run big asymmetrical
audio at 140%, etc.
So here's how I handle all of those problems above... All of
my rigs can do 140% positive easily. It took some doing.
I have no control over the detector on the other end, so
this means 120% positive is about the limit before some
guys start complaining of detector overload. My voice is
110% positive max, so I don't try for big audio peaks like
others can get away with. I've tried 200%+ modulation systems -
they look great on the scope, but were a bust on the air!
The bottom line is this: The PEP limit on AM is 1500 W. I have
a Bird PEAK reading wattmeter. I feel the lower I run my audio,
the cleaner it will sound thru the detector, and the less splatter
it will cause to my adjacent channel neighbors. So under
GOOD conditions, I usually run my carrier up between 600-700
watts and let the peaks run up to 1500W or so. I find I really
like the lazy scope pattern and cleanliness the on-air signal has
for certain receivers. When conditions get lousy, I can always
reduce carrier and slam the audio hard using a processor, but
the peaks are still 1500W.
There's something about a big carrier [an extra 3db in this case]
that quiets the adjacent channel garbage, static crashes, giving
more of a quiet FM quality sound. When we run full fidelity
audio, especially the deep low end, it is fragile to interference
and the big carrier helps - UNDER GOOD CONDITIONS.
Flexibility under varying conditions is the key here.