On a conventional electrically controlled layout, the speed and direction of a loco is controlled by varying the voltage and polarity of the DC voltage applied to the track. This means that each loco must be on a section of track which is electrically separated from other locos.

With DCC, the entire layout is fed with a constant AC voltage supply, onto which is superimposed a signal that tells each loco individually what to do, each loco having a unique identifying electronic address programmed into a decoder (electronic device ) fitted into the loco. Thus, if the controller is set to 1090, for example, only a loco with the electronic address 1090 will respond, even though all other locos on the layout are receiving the same signal and AC voltage supply. If the setting on the controller is then changed to 0520 only a loco with the electronic address 0520 will respond.

To achieve this, each loco must be fitted with a decoder, which can be set (once it is in the loco) to recognise the loco number (or address) and to respond only when that address is called up by the controller. For ease of remembering, most layouts use the last four digits of the loco number as the address – for example 1090 is loco number 61090, and 0520 is loco number 60520 . The electrical code (or protocol) is an international standard to which most manufacturers comply. Thus, any decoder can be controlled by any make of control set. However, it is sometimes easier to programme the decoder on a loco if it is the same make as the control system, since there are a number of differences in terminology between the different manufacturers (and between American, English, and German translated to English!).

As well as the basic purpose of controlling the loco’s speed and direction, the more expensive decoders have a number of ‘functions’. These are simply electronic switches which are turned on or off from the controller and can be used to control lights, sound systems or other effects.

The main advantage of DCC is that a layout is simpler to wire up, and operation is easier since it is not necessary to have section switches to control power to all the various track sections, or to pass the loco from one controller to another as it moves along the layout. However, on large layouts it is an advantage to break the layout into electrical sections to help ‘localise’ problems if they occur.

Another advantage is that you can have several controllers operating different locos at the same time without the need for any section switches.

There are also less-obvious benefits. If you want to double head (or bank), both locos can be set to work from the same controller. If you have a reversing loop, this can be controlled automatically using an extra module called a “frog juicer”. Operations such as attaching an assisting loco or using a shunting loco to add vehicles to the end of the train can be carried out realistically without the train engine moving during the process. In addition, when sound is fitted, a (diesel) loco can be left ‘ticking over’ for example, in a siding whilst another loco is being driven. Because of the Back EMF (ElectroMotive Force) control built into good decoders, locos can be run at a scale walking speed, so that movement is barely visible. The top speed, and rates of acceleration and deceleration can be set individually for each loco so that operation can be much more realistic. Most types of decoder allow the loco to be used on conventional DC, although with some types the speed is reduced, Note that conventional analogue DC locos should not be placed on a track which is live with DCC, as serious damage to the motor may occur.

The main disadvantage is the extra cost. Each loco must be fitted with a decoder and this cost can be significant, depending on the current consumption and number of functions required. The actual fitting is fairly simple in O gauge where space is not usually a problem, although very old motors with earthed frames such as motors by Bonds and Leakey, need to be modified because they have one brush holder earthed to the motor frame; it is essential that the output wires from the decoder to the motor are insulated from the input wires from the track to the decoder. Another disadvantage is that most control systems have sensitive overloads which will trip the power if there is a momentary short circuit. A common cause of this can be the backs of wheel flanges touching on the open blades at turnouts; to avoid this it is wise to wire turnouts so that the open blade is the same polarity as its adjacent stock rail.

Most modern decoders have overload protection so that a short circuit or overload on the motor itself will not damage the decoder.

DCC is not a panacea for a poor standard of construction, and when building a new loco it is always wise to ensure that it runs smoothly on DC before fitting the DCC decoder.

A basic knowledge of electrical principles is an advantage, but no computer knowledge is necessary, and don’t be put off by all the talk of ‘programming’ in the text books. This is quite simple and nothing to do with computers. The only real operational disadvantage is that there is nothing to stop you driving two trains into each other head on if you want to. If you want to run full length trains at scale speeds, some form of signalling or operating discipline is advisable if your workshops are not to spend a lot of time repairing collision damage! Before choosing a control system, it is wise to try out some of the different makes of DCC controller. Some have knobs for speed control, others a series of buttons, and the display screens come in various forms. If the layout is large and you want drivers to walk round near their train and obey signals, you might consider radio handsets which are a bit like a TV remote control. At this stage it is well worth visiting one of the main DCC suppliers if they have a demonstration layout (not necessarily Gauge 0) which you can try out.

The ‘icing on the cake’ is to fit locos with sound systems and smoke units. Provided the system is set up correctly, in particular with the correct number of beats per wheel revolution on a steam loco, it brings a whole new dimension to the hobby. A wide range of sound decoders is now available for both steam and diesel locos from various suppliers – see the GOG website Product Directory for details.

Although various suppliers programme the decoders with their own sound files, the basic decoders for British sounds come from two main suppliers, ESU and Zimo.

One of the most important features which governs the quality of the sound is the size of the speaker, the bigger the better. The back of the speaker must be enclosed in a chamber which is usually supplied with it. If the speaker can be mounted directly under the chimney this also improves the depth of the sound, but to do this needs planning at the build stage and can obviously be difficult with RTR locos. A 1” speaker can be fitted in the smokebox of most large locos, figure 1 shows an example.

Figure 1 - A sound decoder (with stay alive capacitor), 1” speaker, and firebox ‘glow lamp being fitted to a DJH A2 pacific loco

Although the slightly higher track voltage with DCC might suggest that dirty track would be less of an issue than with DC, this is not always the case, simply because the excellent slow speed running properties and controlled acceleration of DCC fitted locos mean they are likely to spend more time running at slow speed, when the risk of a stall due to dirty track is greater. A recent development is the use of stay alive capacitors that enable a loco to continue with both traction and sound (if fitted) when the supply is momentarily interrupted (a sort of electronic flywheel). These are often built into higher current decoders, or they can be added as an extra to smaller ones. They are most effective at slow speeds unlike traditional flywheels which are of limited use until speed has risen.

Points and signals can be controlled by traditional means, but alternatively, if you are starting a new layout, these can be controlled by the DCC system. On larger layouts this can save a considerable amount of wiring. However, the wiring needs to be such that a short circuit on the rails (such as a loco running through trailing points the wrong way) does not prevent operation of the points to rectify the problem.

Like most model railway subjects, there is a vast amount of information available to guide you through DCC, such as:

• Gazette articles ( https://www.gaugeoguild.com/gazette_archive/search_archive.aspx ),
• The Guild Forum ( https://www.gaugeoguild.com/xenforo/index.php ),
• RMWeb ( https://www.rmweb.co.uk/community/index.php ),
• Western Thunder Forum ( http://www.westernthunder.co.uk/index.php ),
• Youtube, and books.
• Particularly recommended is “Aspects of Modelling – Digital Command Control” by Ian Morton ISBN 0 7110 3152 5.

There is much further detailed information regarding DCC on both the Internet and in books, and the technical team is always grateful to receive recommended material to add to the relevant section in the Guidance manual