Decoders with this feature allow slow speed operation without a distracting buzz from the motor.
This is an advanced topic, and is not required to understand for operation of a DCC layout or mobile decoder.
A Name is a Name
Lets get the marketing stuff out of the way. Various manufacturers have come up with various marketing names for their high frequency decoding:
- NCE calls it Silent Running.
- Train Control Systems uses Quiet Drive.
- Digitrax calls their implementation SuperSonic Drive.
There are probably other names that we don't know of. No matter what the manufacturer calls their technology, it's mostly all the same technology - sending power pulses to the motor in a way that people can't hear it.
Under the Hood
The Duty Cycle describes the relationship between ON and OFF during one complete cycle. For example, at 100 Hertz, a cycle is 1/100 S or 10mS (0.010 seconds) At 50%, the on/off relationship is 50/50, or 5mS on and 5mS off. As the duration of the ON pulse increases or decreases, the duty cycle changes with it.
When the 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 and is transmitted to the frame. 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. There is also the possibility of resonance with the body, meaning that at a certain frequency the body will vibrate and amplify the sound. Various options are available to reduce the resonance of the body.
As 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 a DCC waveform 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. See the page on Zero Stretching for important information and warnings.
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.
Direct Current Powerpacks with Pulse Power
Many of the better Direct Current powerpacks offer a feature called "Pulse Power". Short high amplitude pulses are sent to the motor to get it moving, and allow realistic slow speed operations. This evolved into the high frequency Pulse Width Modulation used on DCC decoders to drive the motor for slow speed operations.
- 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. There may also be inductors in series with the motor connections, which also must be removed.
These components are added to reduce Radio Frequency Interference, but are not compatible with pulse wave modulation.