Lighting and other non-DCC devices.

Another example. Take a DC power supply with a constant voltage regulator and a varying current load. Is the load current AC or DC?

Conversely take a constant current supply and connect a varying resistive load. Is the resulting voltage waveform AC or DC?

Think carefully before you answer.

MBQ

Any time varying waveform has AC *components* as shown by a fourier analysis. That doesn't mean that the original waveform, complete with DC bias is alternating.

What do you think the word "alternating" means?

MBQ

Ok, so from memory I rounded '55 to 61us' to 60us rather than 58us. '0' bits do not have a fixed duration, they must be in the range 95us to 9.9 seconds (start & stop bits >=100us), the duration is set by the need to keep the average of 1's an 0's at zero. So there is no nominal frequency for '0' bits.

NO. See above: 0's can lie anywhere in the range 95 to 9900us

No it is not FM, it would be frequency modulation if the baseband data had 1 & 0 of equal width, i.e. a constant data rate, and that data then caused the modulated data to shift between 2 different frequencies at the data rate. In the case of DCC the data rate is different between a 1 and a 0 due to the fact that the encoding is contained in the pulse width, i.e. PWM. If you also add to the mix the stretched 'bits' which can be up to 12 seconds and form form the start bit of a valid '0' in arbitrary positions, and the fact that 0 bits are variable length as well, it is NOT FM.

Jeff

If you read back you will see that I have always stated that anything other than a pure constant level is some form of AC. I think the problem is that some people seem to think that AC does not encompass a pulsed signal.

Unfortunalely is is no FM see the other posts.

Due to the differential encoding it is more like DSB.

Jeff

Switch your dc supply on and off once a day, is that DC or AC?

If you observe it over several days it is certainly AC!!!

Jeff

I did say in an earlier post that I was discounting stretched zeroes. These are only used for driving a single unchipped loco and are not even supported by all command stations. Use of an unchipped loco on DCC is deprecated by those who value their locos, due to the risk of destroying some types of motor.

PWM implies a fixed frequency with variable mark space ratio. Perhaps we'll just have to agree that it's neither.

Where do you get 12 seconds from?

As I said, stretched zeroes can be ignored.

MBQ

ote:

It's DC.

It's still DC. How often did the current or voltage reverse?

All time varying signals have an AC component. Not all resulting dignals are AC.

MBQ

.

It implies nothing of the sort. That may be the case for controlling a motor, but *not* when encoding data.

My apologies, on consulting the spec it should be 10 seconds with a total of 12s for the complete reversal.

No they cannot, as the period of the 0 bits is dependant on the length of any stretched bits, so as to keep the average at zero.

Jeff

The definition of an AC signal is that it has a time varying component, you seem to be regarding AC as some sub-set of that which is not correct.

Jeff

Every definition of PWM I have seen assumes fixed frequency. I would be inetrested to see a description of your version of PWM.

Oh FFS. Its 12000 us, or 12 milliseconds.

I am beginning to wonder how much practical, or even theoretical, knowledge of DCC you actually have.

Again, you clearly do not understand DCC!!!

Stretched zeroes are ONLY used to give a DC bias to allow unchipped locos to be driven. This feature should be disabled, and is not included in some system, due to the potential to damage some motors. When stretched zeroes are disabled EACH AND EVERY ZERO on the DCC bus is nominall 200us total (100us each half cycle).

MBQ

No, the definition of AC is that the current or voltage alternates, i.e. reverses periodically.

MBQ

Lets see what Cambridge University have to say on the subject

I'm struggling to find a definition that doesn't include reversal of current or voltage. Would you care to provide one, from an authoratative source?

MBQ

So you are trying to say that a pulsed signal is not AC, even though if it were a perfect square wave it would have components stretching up in frequency to infinity!!! You have to look more closely to understand where the periodic signals are, but they are there. A spectrum analyser would greatly assist you in seeing them.

Jeff

You said: "A pulse modulated signal is AC", which is simply false. You can have pulse modulated AC or DC.

And all waveforms can be described by a sum of sine waves, by that argument DC is AC. I have no opinion on what comes out of my Lenz DCC system because I still haven't got round to buying that second-hand oscilloscope I've been promising myself, and I don't care enough to look it up because it Just Works. Guy

And it's fairly common to use +x and -x rails in digital signals, this is still not described as AC for the simple reason that it's not, in any meaningful sense.

The question with DCC would be: what does the output waveform look like with no signal. I don't know the answer to that, I have no way of checking here at the moment. Guy

Not sure what you mean by that. There's never really a situation with "no signal". If a booster has no input from a command station then good design would be to shut down the track output.

If there are locos being controlled then the control packets are sent repeatedly to those locos.

If the command station really is quescent then it outputs a continuous preamble which is a string of '1' bits which results in a square wave at 8.6KHz.

MBQ

You can't have it both ways Guy!!! You say that "all waveforms can be described by a sum of sine waves" and then go on then say that you can have "pulse modulated DC"!! (DC cannot be described by sine waves unless you tale the case of zero frequency as a sine wave). If an FFT shows something other than *only* a DC component (zero frequency) the signal is AC!!!! All pulsed waveforms have a time varying component and are therefore AC.

Jeff

If it has a large enough DC offset it is not AC.

I know where they are.

The resultant signal is also period.

Being periodic is a neccessary but not sufficient condition for a signal to be AC.

MBQ

Please provide an authoratative reference for your definition of AC as anything that varies over time.

MBQ