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.

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.