High-Frequency Decoders

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Short Definition

This makes your engines run quieter

This is an advanced topic, and is not required to understand for operation of a DCC layout or mobile decoder.

Contents 1 A name is a name 2 Under the Hood 3 But mine doesn't buzz... 4 Some warnings

[edit] A name is a name

Lets get the marketing stuff out of the way. The various manufacturers have come up with various marketing names for their high frequency decoding:

• NCE calls it Silent Running.
• Train Control Systems calls it Quiet Drive.
• Digitrax calls it SuperSonic Drive.

There are probably other names that we don't know of. No matter what the manufacturer calls the technology, it's mostly all the same technology - sending power to the motor in a way that people can't hear it.

[edit] Under the Hood

Decoders control the locomotive's speed through Pulse Width Modulation (PWM). A square wave is fed to the motor, and the duty cycle (relationship of time between 0 and full voltage applied) is varied to control the motor speed.

When frequency of these pulses are within the range of human hearing, it can cause a buzzing or humming sound in locomotives. The PWM can be increased to frequencies above the range of human hearing, so the buzz can't be heard by us mere humans. (Your dog may not like it...) Unfortunately, increasing the frequency can also reduce the motor's torque (the twisting force produced by the motor). Decoder manufacturers provide various methods for adjusting the PWM frequency without losing too much torque.

The buzz starts in the motor, but can be amplified by the resonance of the shell. The buzz appears to be more common in diesels than steam, and brass compared to plastic. There are a few ways to reduce the noise, but increasing the frequency to above what most people can hear makes it inaudible.

However, when the pulse frequency increases you lose motor torque. Some manufacturers of high-frequency decoders allow the user to adjust the frequency for a balance of torque and quiet operation. Other manufacturers set it for an optimum frequency then allow for minor adjustments from there - one way or another.

To help compensate for loss of motor torque, manufacturers have started implementing dithering technologies which further manipulates the PWM.

Running an analog locomotive (a unit not equipped with a DCC decoder) on a DCC layout can also produce this effect. This is a result of an AC square wave driving the motor, and the Zero Stretching technique used to operate a direct current motor. The sound will change as the throttle is opened. Some direct current motors are a lot more sensitive to signals that are not pure direct current than others.

[edit] But mine doesn't buzz...

Not all locomotives suffer from the buzzing problem. Results can vary between the same manufacturer and model. There appears to be no reason or why one loco will, yet another won't. Some decoder motor control circuitry can send a very dirty signal to the motor, and no amount of tweaking can fix that.

[edit] Some warnings

• Some Bachmann locos have a capacitor across the motor leads that will drastically affect the operation of high-frequency-type decoders. You must disconnect that capacitor.