Lighting and other non-DCC devices.

DCC is AC voltage with digital info. Those who do not understand DCC will say it is not AC voltage.

You can power many items on a layout if you prefer with DCC. There are different amperage systems. Many prefer to use DC to power accessories. Some use AC, some DC.

Running a DC loco on a DCC system. Two or three ddifferent systems can do this.

There is no best, only what you like best. NCE, Digitrax, MRC, Lenz are a few of the DCC systems. There are about six or more decoder companies. There are also two or three companies that have a form of DCC and can run on DC layouts and DCC layouts. You will have to do a lot of searching for different companies on the Internet. The digital control of trains is rapidly evolving. Many DCC on board, sound and none sound can run on a DC power pack. Just do not use a DC power pack that has pulse power option. From you first request, you will have to do a lot of reading to catch up with the latest in model railroading or you may go down a path you will not care for. Stuff in other countries I am not familiar with. Only the USA stuff.

It can get quite heated at times :-)

Some systems can be adjusted, some you can adjust by using a different voltage power supply.

If converting locos with filament bulbs designed for 12V it's worth considering changing to LED as the higher voltage of some DCC systems will reduce the bulb lifetime.

I would arrange seperate supply(ies) for accessories such as lineside lighting and point motors. Keep the DCC for loco control.

MBQ

I suppose it depends on what you call an 'AC voltage'. It is not an AC voltage in the sense of being like the mains or the auxiliary AC output of a conventional controller ie a sine wave. Also is is not a control signal superimposed upon another voltage. It is purely square-wave like digital data sent with a high enough voltage and with sufficient current capability so as when rectified it will run motors etc.. So it is an AC signal in as much as it is a pulsed signal with varying mark space ratio, but it is not a continuous sine wave like the mains.

The contentious bit is calling a pulsed signal AC, when does a pulsed signal become AC rather than DC that is switched on and off, at what pulse rate does that change occur. Is turning DC on and off once a second AC? To some it is, depending on the application and over what period you are observing the signal!!

Jeff

The definition of AC has nothing to do with frequency, waveshape or having control superimposed on it.

DCC is not a "pulsed signal" it is AC.

A decoder sees two connections to the track, lets call them A and B. During one half of the cycle, A is more positive than B and current flows from A to B. During the other half of the cycle, B is more positive than A and current flows from B to A. The current flow reverses twice every cycle. The current alternates. It is an alternating current.

DCC is AC. It is not a "pulsed signal", nor is it "bipolar DC" as some would have it.

Contrast this with a real pulsed DC controller where the pulses simply turn the track on and off. The current only ever flows in one direction (unless you flip the reversing switch!). That *is* a pulsed DC signal and not AC [1].

I think the confusion arises due to people looking at the output of a command station with reference to the local ground. If you measure the two outputs A and B independently then they do look like pulsed outputs. The point is that DCC is defined by the two track signals alone, there is no ground reference. It is the differential signal between the rails that is DCC and that is AC.

MBQ

Being really pedantic you could show that it is a combination of an AC signal and a DC offset but that is taking things to a ridiculous extreme.

It is most certainly is a pulsed signal, with pulse width modulation that carries the data. The fact that it is applied to the track in a differential manner is not really relevant to that fact.

The fact that it is a pulsed signal is what causes the confusion. Towards the limit if the pulse rate was low enough it would tend to being called DC (if for example the state changed say only once a minute), but because the pulse rate is high it would normally be refereed to as AC.

If you viewed the spectrum of the DCC signal you would observe a classic pulse envelope, but there again so would you if you observed a DC signal being reversed every minute, if you viewed it over a long enough sample time!!

Jeff

DCC is *frequency* modulated.

The fact that it is applied differentially means it is AC as seen by the decoder.

Are you sure you are not getting confused with the motor drive output of the decoder, which is PWM and is DC, only reversing when the direction of the loco changes.

MBQ

It most certainly is _not_ a pulsed signal.

From Wikipedia, definition of "pulse":

The basic feature of pulsed voltage is the existence of the baseline value.

In DCC, there is no baseline value. It has two possible values, +X volts and -X volts, between of which it alternates. Hey, it is an alternating voltage!

I think it is you that is confused with the definitions of AC and DC. A pulse modulated signal is AC, whether that be pulse width modulated or otherwise modulated. DC is an unchanging constant voltage. Perhaps Pulse width modulation is not quite the clearest term for the DCC signal as it is really a pulse train that caries data, however, the pulses are of different fixed widths for a one and a zero and so it is the width of the pulse that encodes the information. There may only be 2 allowable widths but that does not alter the fact that it is PWM. It is not frequency modulated as such, although the spectral content for a string of zeros would be half of that for a series of ones.

Any signal that varies with time is considered to be AC; mathematically DC is nothing more than a special case where the time component is zero. Even the PWM motor signal is AC as it is a pulse train and it is not constant with time.

If you are being really pedantic even the output of a conventional DC controller is really AC as soon as you make any change to the voltage!!

Jeff

Of course there is a baseline, a DCC pulse consists of an excursion from

-X to +X and then back to -X, it is merely the length of the excursions that determine the data. Another way of looking at it is that the source

Yes pulse is AC as it has a time varying component.

Jeff

PWM implies a fixed frequency variable mark/space ratio.

DCC is fixed mark/space ratio (discounting stretched zeroes for driving analogue locos) with variable frequency. A '1' is sent as a single cycle of approx 8.6KHz nominal, a '0' as a single cycle of 5KHz nominal.

The width of the pulse *and* the width of the idle period are critical in decoding a DCC bit stream. Both are significant and it is the overall period that matters. Hence DCC is frequency modulated, not PWM.

This is getting into the real of AC with DC offset, which I did mention in an earlier post.

MBQ

A decoder has no explicit reference. All it has is the differential signal on the rails, which is AC. Generally the decoder will full wave rectify the AC signal to give an on-board DC supply. It is then creating it's own virtual reference point at the zero-crossing of the input.

Some specialised DCC circuits half-wave rectify and use one rail as reference but you need to be very careful if such modules are connected together.

MBQ

I am not disputing that, but it does not get away from the fact that the AC signal is Pulse width modulated, albeit that there are only 2 positions that the data can sit at. A pulsed signal IS an AC signal.

Jeff

No, there is no such constraint on PWM, DCC is PWM with 2 discrete positions. PWM is AC.

It is merely sent as RTZ with a fixed width for 1 and a different fixed width for a '0'; that may equate to 5kHz (sic) for a '1' but it is not frequency modulation. It would be FM if the data stream were used to modulate a carrier.(Actually '1' is nearer 8.3kHz, 60us per bit 120us total period).

Any data stream of this type will have frequency components that are related to the bit widths, that actual spectral components of the DCC waveform will not just contain 8.33kHz, and its harmonics, due to the unequal bit periods there will be other components in there as well depending on the data sent (particularly if there are long highs for non DCC powering.

Jeff

No, the period changes since both the mark and space are changed.

I don't know where you are getting you information, but I *was* correct, the nominal bit time, for a '1' is 58us per *half-bit*, 116us total per bit, or approx 8.6KHz. See

MBQ

OK, I accept your agreement that DCC is AC, which you seemed to be disputing.

Now you just have to understande that DCC is FM, not PWM but that's in the other sub-thread.

MBQ

Maybe it is single side band :>)

No it's not. AC has alternating polarity, DC doesn't. Pulse width modulated DC is still DC, as is DC with a superposed digital signal. Guy

That is utter rubbish, do an FFT on the DCC signal and see what components go to make it up!!!!

UJeff