What is a DCC decoder?

To understand what a DCC decoder does, we must first take a look at how DC and DCC work.

DC stands for Direct Current, this is the way modellers have been controlling their model railways for years. You have a controller which sends electric to the metal rails of your track, your engine sits on those rails and picks up the electric through its metal wheels and feeds it directly to the motor. The more you turn up the controller, the more electric is fed to the engine's motor and consequently, the engine moves faster. DC, also known as analog, is very simple and very effective but not without limitations, for example, if you put two engines on the same track they will both pick up the electric and therefore both move.

DCC stands for Digital Command Control. It works by the controller sending an AC power supply through the tracks, not DC. Unlike DC, the amount of power that a DCC controller sends through the tracks doesn't vary to make the train go faster or slower, a DCC controller will send full power all the time! so why doesn't your model train just whizz around the layout uncontrollably?

Firstly, your train's motor cannot use AC. It would just buzz loudly and probably wouldn't thank you for the experience. This is the first job for the DCC decoder: it takes AC power and converts it into DC which your locomotive's motor can use.

Secondly, although the decoder receives full AC power all the time from the controller, it will vary the amount of DC power that it sends to the motor to make the engine go faster or slower just like with traditional DC control. But how does the decoder know how fast or slow you (the operator) wants the engine to go?

The decoder can cleverly receive hidden messages carried along inside the AC power supply from the controller. AC stands for Alternating Current and it travels through wires (or tracks in this case) by oscillating its way along (analogy not science). The decoder doesn't much care what pattern the oscillations make just so long as it gets its power, it's happy. However, different patterns of oscillation can be used to send messages a bit like Morse-code and that is how the decoder receives its commands from you the boss!

Because the decoder receives full power all the time whether the train is moving or not, it can use some of that power to operate auxiliary functions such as lights or sound, again the prompt to turn such functions on or off comes from the controller in the form of hidden messages. 

The main advantage of having DCC (and therefore a decoder) is its ability to understand messages, specifically, messages that are meant for that locomotive and only that locomotive. The implication of this is that you can now operate your engines independently even when they are on the same track. It's fair to say that playtime just keeps getting better!