So, take an AC signal and full-wave rectify it, connect a load resistor with no smoothing capacitor. Is the resulting current flow AC or DC?
Think carefully before answering.
MBQ
So, take an AC signal and full-wave rectify it, connect a load resistor with no smoothing capacitor. Is the resulting current flow AC or DC?
Think carefully before answering.
MBQ
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
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
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 are missing the point, a pulse wave form is AC, and can be broken down into its component parts that are sine waves and meet your definition of AC. Just because at a cursory glance there is no reversal of current does not make, say a 0-5V pulse train, DC.
View such a waveform on a spectrum analyser, or do an FFT, and you will not see just a DC component you will see a multitude of lines that make up the complete waveform, all of which meet you definition of AC.
Jeff
Sorry my brain was working in ms not us.
Please read the spec again, there is no requirement for the 0 bit to be a nominal 200us it may be anything to 12000us. So a nice 100/200us period fits your view of FM, but the spec allows
100/anything>200
Which is my point.
MBQ
But you point only works for one specific case where there happens to be a fixed 100/200us. If there are stretched bits, or for some other reason the bits periods are not constant, which is allowed by the spec, then you description fails. You cannot characterise DCC based on a restricted sub-set of what is allowed.
Jeff
n
I am basing it on what happens in practice. I was quite clear early in tne thread about ruling out the use of stretched zeros, as they are deprecated. I challenge you to find any DCC command station that does not use fixed duration equal timing when not using stretched zeros to drive an unchipped loco.
MBQ
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
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