Is DCC high frequency AC or alternating DC based?

DCC is a PWM signal, as to whether it's "AC" or "DC", you can debate that all day.  I'd say it's AC since it alternates negative and positive.  I think they're equating sine waves with AC and square waves with DC.

DCC being A/C or DC is like the great engines vs. motors debate, e.g. what defines and engine and what defines a motor .   In the case of DCC, both sides have a valid argument.  The signal crosses zero volts so it is alternating (A/C); however, at its heart, it is a bi-polar D/C signal, so it could be considered an alternating DC waveform.  It all comes down to which side of the tracks you come from .

Scott Kay 

Thanks Scott.  I get it and can see why it might be viewed as one or the other, when actually its neither.   I recently bought a RRampmeter so I can measure all three - AC, DC, and DCC amps and volts with something.  

I come from the engineering side, so it's AC.  It's really very similar to frequency modulated radio signals, they also have a variable frequency with the data encoded as frequency variations.  Just like any other AC, the net DC voltage on the rails with DCC is zero volts.  FWIW, a true RMS meter will measure DCC voltage as long as it has sufficient frequency response.

Time to put on my Nomex suit for the incoming flames.

John, love your sense of humor.

Ray

Glad I run Legacy/TMCC, not so much heat

I feel like I'm back in electronics school learning this stuff all over,  please STOP

superwarp1 posted:

I feel like I'm back in electronics school learning this stuff all over,  please STOP

We're just trying to fill in the missing parts of your education.

help me with my missing education i have a digitrax 150 that uses a ac power pack also a a zepher 51 that uses only a dc power and they both can be uses together that does confuse me

That's called Zero Stretching or Zero Bit Stretching and it's hard on DC motors. Some DCC manufacturers will let you operate 1 DC locomotive this way at address 00.

http://www.dccwiki.com/Zero_Stretching 

Sweet.............

gunrunnerjohn posted:

I come from the engineering side, so it's AC.  It's really very similar to frequency modulated radio signals, they also have a variable frequency with the data encoded as frequency variations.  Just like any other AC, the net DC voltage on the rails with DCC is zero volts.  FWIW, a true RMS meter will measure DCC voltage as long as it has sufficient frequency response.

Time to put on my Nomex suit for the incoming flames.

Now thats funny..i would like to borrow that phrase if i may...its either your phrase or "Foam down the runways"

Doug-Sr posted:

help me with my missing education i have a digitrax 150 that uses a ac power pack also a a zepher 51 that uses only a dc power and they both can be uses together that does confuse me

I think he is referring to these 2 units use either AC or DC as INPUT to the Digitrax command stations.

Some DCC systems will only accept AC input, some will accept AC or DC as a power source. It's a little simpler to make them AC only input. In my experience DC input is superior, because you can use a regulated DC supply. Finding a regulated AC supply is pretty much impossible for us. (they do exist).

There was an option, now fallen into disfavor, on some DCC systems to modify the DCC output to the rails to allow a DC only loco to run, and that was indeed called "zero stretching", basically "stretching" one "half" of the AC signal, so there was a lot more time on the positive part of the "AC cycle", so it was a long pulse of positive voltage, and then a short pulse of negative... so it was MOSTLY DC... still make motors buzz and heat up.

(DCC modifies the AC square wave "timing" to give the digital signal, so modifying the signal to a gross level, i.e. "stretching" a "zero" made it this "mostly" positive DC pulse with a little bitty "negative pulse". Picture below shows "normal" 1's and 0's

Picture below shows an example of zero stretching in the middle of a normal DCC sequence.

Hope that helps a bit.

Greg

I just wrote a paper and posted it at the Digitrax-Users group site all about various test equipment you might use to measure the DC signal (and also why what somebody is using might not be working).  Things from light bulbs to multimeters to oscilloscopes.  In doing that and poking around online, I found confusion all over the place with some arguing mightily that it is NOT an AC voltage.  Even a DCCwiki claiming that.

I think some of the confusion is that typically the DCC voltage is created from only positive voltages so people think no "negative" can exist.  But the NMRA does not force you to make the waveform in any particular manner.  They do "recommend" that there be a "common", but it is not required.  And of course, neither track rail is perpetually a "common".

The typical way is for one half cycle to hold one rail at a positive voltage and the other to "common".  And then on the other half cycle hold the other rail at a positive voltage and the first rail to "common".  So, you have two positive pulsating DC voltages that are the inverse of each other.  Some think no negative voltage can come from this.  But the voltage of interest is BETWEEN the rails and whether a voltage is positive or negative depends on your point of reference.  When one rail is positive, the other is negative relative to it.  The voltage BETWEEN the rails is most definitely AC.  There are other ways we know this.  We can connect a scope and see positive and negative half cycles with no DC offset.  We need bridge rectifiers in decoders to convert to real DC.  If we put a multimeter set to DC across the rails we read zero (or very close to it).

Also, you could certainly generate the voltage differently such as taking a positive and negative supply and, by a switching means, swap those voltages between the rails.  Or you could use only negative voltages instead of positive ones.  The result would be the identical AC waveform the decoder would not know the difference.

Another confusion is if the DCC voltage has "polarity".  Being an AC voltage, by definition, it does not have a polarity.  It has a phase.  NMRA correctly uses the term "phase" in various places in the standards and technical notes.  So that much is good.

But most purveyors of things like auto reversers say "polarity".  This may be because people understand "polarity" better than "phase".  Or it harks back to the thinking of DC power pack control.  Digitrax says "polarity" in a data sheet, but in a technical note says, "...phase (or polarity if you will.)".  Like somebody who knew electronics jargon wrote the technical note, but had to tolerate the misnomer.  I'm pretty sure that any electrical engineer would feel "polarity" does not apply to AC other than in describing waveform crests or that the "polarity is constantly changing".

But I think the NMRA muddies the water some with some other terminology they have.

Such as "Bipolar DC".  I suppose this isn't technically wrong and it is used elsewhere in the world (only because it is square wave).  But "Bipolar" is another way of saying "AC".  And all AC is "bipolar".  Unfortunately, some note this as "proof" the waveform is "DC".

And the NMRA really makes things muddy with their term, "DCC Positive Polarity" which is “The wire or rail which has a positive voltage for the first half-cycle of the DCC bit”.  They are trying to define a rail (Rail "A" as opposed to Rail "B") in a sense of phase.

NMRA also uses the term "Digital Signal".  Again, not technically wrong if viewed broadly.  But in electronics a "Digital Signal" is typically voltage LEVELS such as 0 VDC being a "0" and +5 VDC being a "1".  Of course, besides voltage levels other than +5 (or even zero and a negative voltage) there are all manner of such encoding such as using zero and a positive and a negative voltage "duobinary" signal.  But it is always in terms of voltage levels.

Instead, the DCC AC voltage is modulated and might more correctly be called a carrier with data "transmission".

And the type of transmission is Frequency Shift Keying (FSK)  I see an example of that in a previous comment that is typical for radio (or radar) where some number of cycles of a frequency denote the state and have to be demodulated with fairly fancy electronics.  The DCC voltage can be handled easier by just timing zero crossings (and the NMRA uses that "zero crossings" term which is another BIG clue that it must be an AC waveform.)  Nothing about the concept of FSK says a "0" or "1" need be X number of cycles, so long as it is one cycle, that's enough.  And that's DCC.

A Lenz patent (the mother of the NMRA concept) even says, "...square wave voltage signal which is frequency and/or pulse width modulated”.  Lenz, being in the electronics design business, would certainly know.  Mentioning "pulse width modulated" is just to make the patent broader.  PWM is not used.  FSK is.  Although you see in blogs and elsewhere people saying the DCC signal is PWM which it is not because the duty cycle never varies.  The Lenz patent also says you could certainly use a sinusoidal wave, but essentially says why would anyone want to bother.

Being a square wave is 1) easier to create and 2) contains more power for a given voltage.  Otherwise, it is no less "AC" than the voltage at your electrical outlets.

And then there is the pesky "zero bit stretching" which was mentioned a little in a comment above.  This is NOT required by the NMRA and seems to have been accommodated (allowed) to soothe people concerned they couldn't run a DC locomotive.  Not every manufacturer supports it.  To explain it in a bit more detail (and what it means to the AC versus DC argument) is that either half cycle of the "0" bit can have a longer than normal duration.  Using the NMRA terminology for Rail A being when the first half cycle is positive, if you hooked up a scope with the probe on Rail A and ground clip on Rail B, you see the first half cycle of the "0" bits stretch longer and longer for more and more throttle forward.  And the negative half cycle of the "0" bits stretch longer for reverse throttle. (Using address 0000).  Not just some zero bit, but all the zero bits are affected.

"Zero bit stretching" does introduce a DC component to the waveform.  A DC offset.  So, while still an AC waveform, it will have a DC offset.  Does that mean it is now a DC voltage?  No.  That would require that all current go in the same direction.  It still has current in the negative direction and negative voltage on the lower half cycles.

I don't know if all this helps, or adds to confusion.  But I couldn't stop myself since I have recently taken a deep dive into all this.

My (non-technically edumacated) understanding is that the DCC system sends constant AC power to the track - usually around 14 volts - and then the control system sends a DC signal through the rails to tell the decoder what to do. The decoder then takes the AC power, converts it to DC, and then feeds it to the lights or motor etc. The constant power allows the headlight and sounds to continue even if the engine is stopped, unlike a traditional DC system where lights and sound stop when the engine stops because the throttle is set to zero.

Actually, the DCC signal is encoded in the track power signal, it's really a PWM signal that also supplies track power.

https://www.researchgate.net/f...t-0-4_fig2_318924580

In reality, DCC  is a DC voltage with a digital signal superimposed on it. I suppose one can argue that the square wave digital signal makes it "AC", but it really is a DC voltage carrying a square wave digital signal.

Note that this statement only applies to NMRA Standard DCC (Digital Command Control) and NOT to DCS or Legacy. Those systems use a different method of communicating with locomotives.

gunrunnerjohn is mostly correct and has a good attachment reference.  But technically it is not "pulse width modulation".  It is "Frequency Shift" modulation.  And there is a difference.  In PWM, the pulse repetition rate remains constant, but the width of the pulses vary and the duty cycle of the waveform changes (percentage of high to low).  In frequency shift modulation (or "frequency shift keying") the time of BOTH the positive and negative half cycles are changed.  So you get a complete cycle at one frequency or another and always with a 50% duty cycle.  However, there is one case in DCC called "zero bit stretching" which actually is a form of PWM, but that is only for systems that allow it to force the system to operate a DC motor by having the AC signal spend more time at one extreme than the other.  For all normal DCC activity, it is frequency shift only.

The NMRA DCC scheme was basically derived from a LENZ system.  It has the huge advantage that the "signal" is the full peak to peak value of the AC waveform.  This helps with signal to noise ratio and is the big reason the "signal" can be run all over the place with unshielded wires which includes the track, which are also electrically wires.

There are three common misunderstandings, or at least misuse of technical terms, in the DCC world.  One is the aforementioned PWM.

Another is "it is not AC".  There are some who call it "bipolar DC" which to an electronics engineer is really just another term for AC.  In the electronics world, this only implies the means of how the AC was created which is by switching DC voltages around and therefore simply creating a square wave.  The result is still AC.  And AC does not have to be a sine wave.  Some are adamant that it is impossible to be AC because there is "no negative voltage exists in the system".  But this is a false conclusion.  Most systems alternately power the rails between a circuit ground and a positive voltage.  So, in that sense, there is no negative voltage RELATIVE TO THE CIRCUIT GROUND.  But relative to the load (whatever is across the tracks) the polarity is changing.  (Otherwise, why would locomotives need rectifiers to convert the AC to DC for the motor?)  You could produce the same DCC waveform using two voltages that were negative referenced to a circuit ground.  Or produce the AC voltage directly with an absence of a circuit ground.  What matters is the voltage that is across the load.  Some say it isn't an AC voltage because it is a "signal".  The NMRA uses the term "DCC signal" themselves, but it is a voltage that is modulated to contain a signal no different than an RF frequency shift keyed radio system.

The third is "it has polarity".  Since AC by definition does not have polarity, this is clearly the wrong term.  I suspect it is used because it is more easily comprehended by many.  After all, they understand the polarity of a battery just fine.  And the DC model railroad history certainly used the term polarity in relation to reverse loops and such (and DCC has the same problems) so people are used to "Polarity".  Even most manufacturers of things like auto reversers use "polarity" because it is what people now expect.  But the correct term is "phase" (since it is AC.)  If you have the "phase" reversed between two track sections, you get a short.  This is no different than using multiple AC toy train transformers and getting their "phase" connected backwards.  Lionel certainly discusses this in some of their literature and correctly uses the term "phase".

I have attached a document that probably has WAY more than you want to know.  I made it awhile back for the Digitrax groups,io group.  It mainly focuses on ways to measure DCC voltage since many have problems with that - especially not understanding why their multimeter might not work well.  But Appendix A discusses the DCC waveform in detail.

@Rich Melvin posted:

In reality, DCC  is a DC voltage with a digital signal superimposed on it. I suppose one can argue that the square wave digital signal makes it "AC", but it really is a DC voltage carrying a square wave digital signal.

Note that this statement only applies to NMRA Standard DCC (Digital Command Control) and NOT to DCS or Legacy. Those systems use a different method of communicating with locomotives.

It is not a DC voltage because current travels through the load in both directions.  All AC voltages have what is called a "DC" component which is merely the average voltage overall.  For example, your house AC power has an average DC component of approximately zero volts (hopefully).  A real DC voltage with a signal superimposed would be like the Digitrax 16 scheme described in a Model Railroader article and made by at least a couple of manufacturers.  In this, there was an actual 11 volts DC and on top of that a 4 volt AC signal.  Technically, this would be called "pulsating DC".  Or "DC with an AC component."  Although Digitrax 16 had an AC signal riding on top of the DC, at no time does the voltage go negative and at no time does the current in the load reverse direction.  The DCC voltage is clearly AC and it is frequency shift modulated.  There is a reason why the decoders require bridge rectifiers - to convert this AC to DC for the DC motors and for providing DC power supply for the electronics.

I love discussions like this makes me think of things I usually don't. In a sense, then, what DCC is sort of doing if I understood this correctly is it is using a DC offset on what is basically AC (bipolar DC as far as I know, as others have said, simply differentiates how it is created from DC and in fact is A/C). In that sense, then, it isn't all that different than Lionel superimposing DC on the AC to the track to trip the whistle relay.