Decoders control
the loco's speed through Pulse Width Modulation
(PWM). When these pulses are within the human hearing range, it can
cause some buzzing in locos that aren't "tight". The PWM
frequency can be increased to that above the human hearing range so
the buzzing can't be heard. Unfortunately, increasing the frequency
willy-nilly can cause a motor to lose torque. Decoder manufacturers
provide various methods for increasing the frequency without losing
too much torque.
Train Control Systems calls it Quiet
Drive.
NCE calls it Silent Running.
Digitrax calls it SuperSonic Drive.
There are probably other high-frequency
decoders, by other manufacturers, that we are not aware of. But whatever
the manufacturer calls it, it's a decoder that has a pulse frequency
higher than most people can hear.
The need for this feature stems from
the fact that pulse power can cause some locos to buzz at slow speeds.
This buzz does not occur in all locos, just some. There is no rhyme
or reason as to why one loco will buzz and another won't. With two
identical locos, one can be super quiet while the other will buzz.
For those that do buzz, the buzz can be anywhere from barely audible
to very audible.
The buzz starts in the motor, but
can be amplified by the resonance of the shell. The buzz is more common
in diesels than steam, and more common in brass than plastic. There
are several ways to reduce the buzz, but increasing the frequency
to above what most people can hear makes it inaudible. |
When the pulse frequency increases,
motor torque is lost. Some manufacturers of high-frequency decoders
allow the user to adjust the frequency for a balance of quietness
and torque. Other manufacturers set it for an optimum frequency then
allows for minor adjustments from there - one way or another. |
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NOTE: Some Bachmann locos have a capacitor
across the motor leads that will drastically affect the operation
of high-frequency-type decoders. For proper performance, you
must disconnect that capacitor. |
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All current Train Control Systems
decoders have Quiet Drive. The frequency is set at 16125 Hz - an optimum
frequency for a balance of quietness and torque. Train Control Systems
then provides another feature, Dither, to
overcome the natural loss of torque at that frequency.
Dither allows you to program in a
little extra power at varying frequencies. The extra power is sort
of like Kick Start, but this extra power
lasts for the entire first 1/3 of the power curve. Since the frequency
of this extra kick is varied from the base frequency, it helps overcome
the natural "stiction" of motors and drive trains to make
the loco start easier and run smoother at slow speeds. This is especially
important for lower-cost locos such as Athearn. In addition, V-Start
and Kick Start can be used in conjunction
with Dither for even better operation. |
