Pulse Width Modulation is the method used by decoders to control the speed of the loco.

With traditional DC Analog control, the speed of the loco is controlled by the amount of voltage put on the track. For example, if you want the loco to go at half speed, you would put 6 volts (HO scale) on the track. With DCC, the decoder provides full voltage 50% of the time. That is, a pulse of full voltage for a short time, then no voltage for the same amount of time.

This pulsing happens so fast that the motor tends to see the average voltage. One benefit of this is that when the pulse happens at full voltage, it provides more power to help overcome the "stiction" of the motor and drive train. Therefore, locos (especially ones that are not of the highest quality) tend to run better with DCC than with DC analog - especially at slower speeds. The major reason PWM is used is that it generates less heat than actually regulating the amount of voltage. This could be done, but the decoder would be much larger and need a large heat sink.

One of the problems with PWM is that it can cause buzzing. The buzzing starts in the motor, caused by the power cycling on and off. If the on/off frequency matches the resonant frequency of the loco's shell, the shell can amplify the buzzing enough to be heard. There are several ways to reduce or eliminate this buzzing.

First, be sure that the shell is secured to the chassis tightly. A loose shell can rattle.

Another solution is sticking thin lead weights to the inside of the shell. Lead will change the resonant frequency of the shell and it will not vibrate. This is one of my favorite because it also adds weight to the loco. If there's not room for thin lead, use duct tape - anything to change the resonant frequency of the shell.

The magnets in Athearn motors are not secured. They rely upon their magnetism to hold them in place. The result can be a magnet that vibrates. The solution is to take the motor apart and secure the magnets, each with a small drop of silicon-based glue, such as Goo.

Another help is using Conducta Lube on all moving metal parts that pass electricity: axles, bolsters, even the motor's commutator if you can get to it. This won't eliminate the buzzing, but it can reduce it in some cases.

Some decoders have a feature that allows you to modify the frequency of the pulses, using CV9. By modifying the frequency so that it doesn't match the resonance of the shell, it won't be as noisy. Other decoders use a frequency that is so high it is inaudible. Train Control Systems, for example, calls this Quiet Drive. Digitrax calls it SuperSonic. NCE calls it Silent Running. Whatever they call it, they all do it the same way - high frequency.

PWM is handled solely by the decoder. The only thing needed is a system or other method to program whatever CVs, if any, that are used in conjunction with it.

All of Digitrax's current systems can program any value into any CV..

Digitrax decoders do not provide for high-frequency PWM - yet.

Train Control Systems decoders do not have modifiable PWM, but do have "Quiet Drive" decoders - which is what PWM control is for.

Throttle Up! sound decoders have PWM programming with CV9. It can be used to help quiet a noisy loco, but is not totally effective for the worst cases.

NCE and Lenz decoders have PWM programming with CV9 with frequencies high enough for "Silent Running" - NCE's term for high-frequency decoders

Pulse Width Modulation control is not intrusive. Unless you get a decoder that has some way to modify it, there is nothing to do. Even then, you do not need to bother with it unless you have a loco that buzzes and you want to try to quiet it.