Whatever the individual configuration, a DCC decoder is really a miniaturised model railway controller, designed to be small enough to fit easily inside a loco or other item of rolling stock. Fitting each loco with it's own dedicated controller in this way is what makes DCC so versatile. Not only does it mean that you can control many separate locos from just one master controller, without a complicated mess of switches and wiring, but you can also fine-tune the performance of individual locos should you wish. You don't have to do this to get up and running, but it's nice to know the facility is there for the future.
Fitting DCC decoders to the locos you want to use on your DCC controlled layout is thus the biggest task you'll face when you're starting out. Fortunately this is a fairly easy procedure, especially for D&E modellers in 4mm scale where space is normally plentiful. The basic wiring principles are very simple indeed, involving just four connections as illustrated by looking at a typical model:
Lima's OO gauge Class 20 shows off it's conventional DC wiring, with all four connections conveniently close together for the camera. One wire joins the wheels/pickups on one side of the loco with one motor terminal, and a second wire does the same for the other side.
The same Lima Class 20 with a DCC decoder fitted, showing the four simple solder connections required. The decoder is effectively spliced-in between the pickups and the motor, with two wires going each way. The simple rhyme 'red and black to the track, orange and grey the other way' pretty much says it all. The wires for other functions such as lighting are bundled up out of the way as they're not required when dealing with the motor.
Most modern locos are DCC-Ready, meaning they come pre-fitted with a socket into which you can plug a DCC decoder. This recent Bachmann OO gauge Class 20 is a good example, conversion takes a matter of moments with none of the soldering needed to hard-wire the older Lima model.
Many models are supplied with capacitors as part of the circuitry, a cheap and easy way to minimise TV interference with plain old DC operation. Under DCC, however, these pesky things are surplus to requirements and often cause poor running or other reliability issues. Experienced modellers remove these items as a matter of course when upgrading a model to DCC - it's good practise to get into this habit right from the start.
While it's not true to say that capacitors will always cause problems, they frequently do. In mild cases there's often a small but worthwhile improvement in slow running, for example, when the capacitor is removed. In severe cases a loco can be almost uncontrollable until the capacitor is removed, taking off like a scalded cat and offering little in the way of slow-speed controllability. People in their hundreds experience this problem on a regular basis, something that can be quite off-putting when they're new to DCC. Fortunately the solution is simple - snip the capacitor(s) off! This cures more than 95% of running problems instantly, so make sure you learn this precaution from the outset.
The capacitors found on model trains are most commonly little orange blobs, as seen in the above photo of a Bachmann Class 37. Just snip both of the legs and throw the offending item in the bin. There's no need to remove the adjacent copper-coloured coils, if present, as they don't interfere with DCC operation. The capacitor, however, is your mortal enemy.
Bachmann locos in particular need close inspection. The lone capacitor on the circuit board is easily despatched, but be aware that there's frequently a nest of the little blighters clustered around the motor terminals, too. Seek them all out and show no mercy.
Not all capacitors are orange blobs. Heljan favour a flat, box-like type as seen on the above Western. Grip it with a pair of pliers and twist/wiggle until it comes free.