ORIGINAL MESSAGE:
On Sat, 23 Mar 2013 19:00:50 -0500, AC0C wrote:
>RTTY contests would be better for everyone if every guy's signal is as
>crispy clean as they can make it.
REPLY:
There is a tradeoff here. The laws of physics have something to say about a
"crispy clean" signal.
RTTY is a form of FM. Think of a carrier modulated by a square wave. If you
recall your FM theory, any FM modulation of a carrier results in sidebands
which are wider than the actual shift of 170 Hz. And with a square wave
doing the modulating, the sidebands theoretically extend to infinity on both
sides of the carrier.
If your receiver had a bandwidth of exactly 170 Hz with very sharp skirts,
you wouldn't be copying much. The sidebands are necessary to give the
demodulated signal the desirable square shape.
One can reduce the bandwidth by "rounding off" the corners of the square
wave, but the more rounding, the less energy is there to be demodulated.
Like I say, it's a tradeoff. Most experts say a bandwidth of about 300 Hz is
a good compromise, and if QRM allows, even wider out to about 500 Hz.
Most of the signals I hear during a contest are OK. Occasionally I hear one
which seems wide, but invariably they are very strong. Sidebands are
inevitable, and with a strong signal, even the weaker sidebands are audible.
I say don't worry about it. It's the nature of RTTY.
73, Bill W6WRT
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