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Offline phils2um  
#1 Posted : 01 February 2022 22:01:33(UTC)
phils2um

United States   
Joined: 12/01/2016(UTC)
Posts: 165
Location: Michigan, Ann Arbor
I must admit I did not understand how address zero was implemented. Address Zero was (is) a method of allowing operation of an analog DC locomotive on a DCC powered layout. I was under the impression that the DCC signal was superimposed on a DC signal. I was mistaken and that is not correct. As was pointed out by HO (Tom), address zero operates by "zero stretching". What this means is that zero bits of the digital waveform are no longer symmetric. The positive portion of the zero bit is made either longer or shorter relative to the negative portion. This asymmetry results in net positive or negative current flowing through anything connected across the rails depending on what part of the zero bit is "stretched".

However, I still maintain the DCC and mfx signals put across the rails are a form of alternating current (AC). The fact that the signal is "digital" does not obviate basic physics. This, https://dccwiki.com/DCC_...(Power)#Some_DCC_Details is somebody going to great pains to deny and then finally admit without admitting that DCC is a form of alternating current. I quote " In fact, the trace is not showing voltage, but which direction the current is flowing." Pardon me but since when is current flow that changes direction with time between two poles (rails) not alternating current? The fact that the waveform does not match the commonly thought of sinusoidal shape often associated with 50, 60 or even 16 2/3 Hz "AC" power does not change the reality that the DCC/mfx signal is AC - alternating current.
Phil S.
Offline Purellum  
#2 Posted : 01 February 2022 23:29:19(UTC)
Purellum

Denmark   
Joined: 08/11/2005(UTC)
Posts: 3,506
Location: Mullerup, 4200 Slagelse
Cool

Originally Posted by: phils2um Go to Quoted Post
I was under the impression that the DCC signal was superimposed on a DC signal. That was not correct. As was pointed out by Per, address zero operates by "zero stretching".


Actually it was Tom / H0 BigGrin

Originally Posted by: phils2um Go to Quoted Post


However, I still maintain the DCC and mfx signals put across the rails are a form of alternating current (AC). The fact that the signal is "digital" does not obviate basic physics. This, https://dccwiki.com/DCC_...(Power)#Some_DCC_Details is somebody going to great pains to deny and then finally admit without admitting that DCC is a form of alternating current. I quote " In fact, the trace is not showing voltage, but which direction the current is flowing." Pardon me but since when is current flow that changes direction with time between two poles (rails) not alternating current? The fact that the waveform does not match the commonly thought of sinusoidal shape often associated with 50, 60 or even 16 2/3 Hz "AC" power does not change the reality that the DCC/mfx signal is AC - alternating current.


It's all about definitions and what is the common terminology. Quoting from Wikipedia, as Dale did on his blog, you get this:

"Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC)"

So how do you define "periodically"? To me it means "at regular intervals of time", not like MM/MFX/DCC.

And how would you describe "changes its magnitude continuously" ??

Wikipedia also says:

"The usual waveform of alternating current in most electric power circuits is a sine wave, whose positive half-period corresponds with positive direction of the current and vice versa. In certain applications different waveforms are used, such as triangular waves or square waves. These currents typically alternate at higher frequencies than those used in power transmission."

But a square wave doesn't "change its magnitude continuously" as the sine wave and the triangular wave, so IMHO, even Wikipedia contradicts itself.

I fully agree that in a digital system the current is alternating, it can be between 0 and a positive value, or between a positive value and a negative value, and a lot of other possibilities, if it's not strictly a binary digital signal.

A morse signal sent by a battery driven flashlight is also a digital signal, the current to the light bulb alternates between 0 and "something", but I don't think anybody would call it AC, even if the message "eeeeeeeeeeeeeeeeeeeeee" was sent, in which the alternation of the current is also periodically / at regular intervals of time Cool

I don't mind people calling a digital signal "a form of alternating current", "digitial AC", "square wave AC" or whatever; I'm only opposing the use of only "AC" as a term to describe the digital signals we're using, since all over the world, "AC" is used for something else, IMHO Cool

Per.

P.S: Next interesting topic could be about WHY the digital signals for MM/MFX/DCC was designed using both
positive and negative currents instead of just one of the two; I think I know the answer BigGrin

Cool








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I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide.

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Offline kiwiAlan  
#3 Posted : 02 February 2022 00:17:31(UTC)
kiwiAlan

United Kingdom   
Joined: 23/07/2014(UTC)
Posts: 8,121
Location: ENGLAND, Didcot
Originally Posted by: Purellum Go to Quoted Post

So how do you define "periodically"? To me it means "at regular intervals of time", not like MM/MFX/DCC.


a better definition of 'periodically' for this purpose is 'defined intervals'.

Originally Posted by: Purellum Go to Quoted Post

But a square wave doesn't "change its magnitude continuously" as the sine wave and the triangular wave, so IMHO, even Wikipedia contradicts itself.


You are now over complicating the scenario. If you wish to look at instantaneous values then you will tie yourself up in knots very quickly. A square wave also does not change its value instantaneously, there is always a switching time between the two extreme values, and in the case of a signal that is used to control model trains, this switching rate will be deliberately controlled to stop EMC problems affecting radio, TV and a possible host of other things we take for granted these days, without people realising the possible interactions. This is why things need to be tested to FCC or CE or UL or any of a number of other standards.

Originally Posted by: Purellum Go to Quoted Post

I fully agree that in a digital system the current is alternating, it can be between 0 and a positive value, or between a positive value and a negative value, and a lot of other possibilities, if it's not strictly a binary digital signal.

A morse signal sent by a battery driven flashlight is also a digital signal, the current to the light bulb alternates between 0 and "something", but I don't think anybody would call it AC, even if the message "eeeeeeeeeeeeeeeeeeeeee" was sent, in which the alternation of the current is also periodically / at regular intervals of time


Again, if you are not careful you will open another can of worms. The light bulb operates in two quite distinct states of 'lit' and 'not lit'. The brightness of the lit state doesn't matter providing it is bright enough for the receiving end to distinguish the lit state from any background radiation around. Attempting to define it otherwise is taking things out of context, and unfortunately a lot of the debate over the last few days has been bordering on this.

Originally Posted by: Purellum Go to Quoted Post

I don't mind people calling a digital signal "a form of alternating current", "digitial AC", "square wave AC" or whatever; I'm only opposing the use of only "AC" as a term to describe the digital signals we're using, since all over the world, "AC" is used for something else, IMHO


So what do you class audio signals passing through an amplifier as? They are most definitely AC. Any amplifier uses capacitors to couple the various stages of amplification, but the signal is anything but a constant amplitude.

Originally Posted by: Purellum Go to Quoted Post

P.S: Next interesting topic could be about WHY the digital signals for MM/MFX/DCC was designed using both
positive and negative currents instead of just one of the two; I think I know the answer BigGrin



That is easy, it is necessary to do that to ensure the decoder always has voltage available to supply the lighting and sound units. Otherwise you end up with the effect that you have when using LED lighting wired between a decoder output and chassis - it blinks in time with the digital signal because it is receiving only one half of the track signal as half the rectifier in the decoder is bypassed by connecting one side of the lights to chassis. It gets even worse with a lamp wired like that if used on a track with an mfx signal.


And for anyone wishing to doubt my authority to detail any of this, I started an electronics apprenticeship in 1969, serving 10 years with that company, through its factory and development laboratory, dealing with radio receivers and transmitters, from AM radio though marine and VHF transceivers. I then moved on to a computer company, for whom I worked for 17 years, installing everything from PCs and printers to mainframe peripherals. On moving to the UK I worked for a government laboratory assisting with the design, build, test, and inspection of scientific instruments to be fitted to space craft. I am now retired after 50 years across a very broad range of electronics.

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Offline Purellum  
#4 Posted : 02 February 2022 00:50:22(UTC)
Purellum

Denmark   
Joined: 08/11/2005(UTC)
Posts: 3,506
Location: Mullerup, 4200 Slagelse
Cool

Originally Posted by: kiwiAlan Go to Quoted Post
So what do you class audio signals passing through an amplifier as?


Audio signals follow the first definition given by Wikepedia:

"Alternating current (AC) is an electric current which periodically reverses direction and changes its magnitude continuously with time in contrast to direct current (DC)"

Thus, I would also call it AC. Edit: But if I had to describe it, I would probably call it modulated AC.

For a random AC square wave, at how low a frequency would you stop calling it AC and start calling it DC? BigGrin

Originally Posted by: kiwiAlan Go to Quoted Post
Originally Posted by: Purellum Go to Quoted Post
A morse signal sent by a battery driven flashlight is also a digital signal, the current to the light bulb alternates between 0 and "something", but I don't think anybody would call it AC, even if the message "eeeeeeeeeeeeeeeeeeeeee" was sent, in which the alternation of the current is also periodically / at regular intervals of time


Again, if you are not careful you will open another can of worms. The light bulb operates in two quite distinct states of 'lit' and 'not lit'. The brightness of the lit state doesn't matter providing it is bright enough for the receiving end to distinguish the lit state from any background radiation around.


I'm talking about the current going to the light bulb, not the brightness of the light seen by the receiver of the message Cool

Originally Posted by: kiwiAlan Go to Quoted Post
That is easy, it is necessary to do that to ensure the decoder always has voltage available to supply the lighting and sound units. Otherwise you end up with the effect that you have when using LED lighting wired between a decoder output and chassis - it blinks in time with the digital signal because it is receiving only one half of the track signal as half the rectifier in the decoder is bypassed by connecting one side of the lights to chassis. It gets even worse with a lamp wired like that if used on a track with an mfx signal.


Neither LED's or sound were installed in the trains when MM and DCC was developed, a small capacitor could keep the decoder alive Cool

I'm sure it's a matter of transmitting as much power as possible, without raising current or voltage to a dangerous level Cool

They could easily have designed a system having only positive ( or negative ) signals and zero; but that would not be smart, since you wouldn't get power transmitted while you were sending zeros ( If zeros were designed as 0V; but you know what I mean BigGrin )

By using both the positive and the negative side, they kept the peak voltage at the same level as in the analog systems, without raising the current Cool

Per.

Cool
If you can dream it, you can do it!

I, the copyright holder of this work, hereby release it into the public domain. This applies worldwide.

In case this is not legally possible:
I grant anyone the right to use this work for any purpose, without any conditions, unless such conditions are required by law.

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Offline phils2um  
#5 Posted : 02 February 2022 01:24:36(UTC)
phils2um

United States   
Joined: 12/01/2016(UTC)
Posts: 165
Location: Michigan, Ann Arbor
So we agree, the current is alternating if not alternating current!
Phil S.
Offline Bigdaddynz  
#6 Posted : 02 February 2022 05:07:16(UTC)
Bigdaddynz

New Zealand   
Joined: 17/09/2006(UTC)
Posts: 18,671
Location: New Zealand
Originally Posted by: phils2um Go to Quoted Post
https://dccwiki.com/DCC_Tutorial_(Power)#Some_DCC_Details


Well, whoever wrote the DCCWiki says DCC isn't AC in this blog

https://dccwiki.com/DCC_Power#

Firstly

"There are many myths surrounding the DCC signal. Most of those myths are the result of applying analog ideas to a digital concept. (Which I've realised might be part of my problem)

"This is not an AC signal, as the rail will have positive voltage on it, or none, at any point in time. What is happening is the current is changing direction as it moves from one rail to another. Measuring this with an oscilloscope will display a peak-to-peak signal, which many will claim supports their assertion that the signal is an AC waveform. Since there is no reference point, the trace only indicates which connection was more positive than the other at a point in time. "

And from the blog you posted

https://dccwiki.com/DCC_...(Power)#Some_DCC_Details

"There are a number of erroneous myths circulating about Digital Command Control that refuse to fade away. Most are the result of applying analog concepts to a digital technology.. "

" DCC is Alternating Current
DCC is a form of Alternating Current
DCC has Polarity.

Correct Answer: None of the Above. The DCC signal on the track is a binary signal, where it is either ON or OFF. One rail is always the inverse state of the other. "

"The DCC signal is not an Alternating Current sine wave, nor is it a special form of AC. The DCC signal switches quickly between states, and varies the time period the rail is High or Low to convey information to trains on the track. Contrary to popular belief, there is no negative voltage present on the rails.
There are many who claim there are both positive and negative voltages, citing the Peak-to-Peak amplitudes as seen in the 'scope trace. This is often a case of erroneous information being repeated so often that it's accepted as a fact. See DCC Power for more details which explain how this is not possible. It is not an analog waveform, where frequency, phase, polarity, or amplitude have meaning."

That all seems to conflict with what Dale Schultz says in his new blog

"there is no other logical conclusion other than the fact that DCC is a type of alternating current. "

Who to believe?

There's also a blog on measuring track voltage, although I have a feeling the volt meter method may not be appropriate in a Marklin environment given there's no separate booster ground reference point to use on a Marklin booster. - https://dccwiki.com/Measuring_Track_Voltage

If you can't use a True RMS volt meter to properly measure the track voltage, how do the track voltage measuring devices work, such as the Rrampmeter and Bachmann / Proses devices?

https://www.hobbysmith.c...ii-amp-volt-meter-detail

https://www.conrad.com/p...ter-2-rail-track-1233499

The suggestion to use the AC+DC setting on my Fluke to measure the CS3 output came from forum member bph - https://www.marklin-user...t47702-CS3-Track-voltage

In that thread he references a Fluke article about True RMS

https://www.fluke.com/en...ctrical/what-is-true-rms

In that article Fluke advise that a good True RMS meter can correctly measure a square wave signal.

Both the Fluke 289 bph has and my Fluke 287 (essentially the same meter but the 289 has some additional features) have 100 kHz AC bandwidth.

As a result of the suggestion to use AC+DC mode I copped derision from another forum member, which led me to suggest the particular forum member might be inhabiting an alternate universe. I probably should not have done that but was somewhat frustrated by all the conflicting opinions being fired around. As I said, I'm not an expert by any means so rely on folks more knowledgeable than me. When they can't agree.....

Then there is the method of rectifying the signal and measuring the DC voltage from that, a method I had seen referenced before on the net (though I can't recall where).

In all of these discussions all I was wanting to know was how to properly measure the CS3 track output. I've discovered that the definitive answer to that question is that there is no definitive answer.
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Offline Hopobcn  
#7 Posted : 02 February 2022 21:16:43(UTC)
Hopobcn

Spain   
Joined: 18/06/2020(UTC)
Posts: 2
Location: Barcelona
Quote:
"This is not an AC signal, as the rail will have positive voltage on it, or none, at any point in time. What is happening is the current is changing direction as it moves from one rail to another. Measuring this with an oscilloscope will display a peak-to-peak signal, which many will claim supports their assertion that the signal is an AC waveform. Since there is no reference point, the trace only indicates which connection was more positive than the other at a point in time. "


Yup you’re right, the signal is not AC, nor DC, is modulated digital signal:

85CC94EA-0671-4D4E-8152-31726AC45A6B.jpeg958E49D8-7DF7-4F91-84E8-802B017B9D04.jpegF67C6328-2879-4E0F-9B28-619CF7CB8AFE.jpeg949CA833-8102-484C-93C9-CF514AF0477D.jpeg

The more debatable part is if it goes from -17V to +17V as my oscilloscope says.

Fun fact: bought the oscilloscope to end the same discussion with my father :-)

PD: maybe I should have choosen another topci for my first post :-D
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Offline Bigdaddynz  
#8 Posted : 03 February 2022 02:08:35(UTC)
Bigdaddynz

New Zealand   
Joined: 17/09/2006(UTC)
Posts: 18,671
Location: New Zealand
Originally Posted by: Hopobcn Go to Quoted Post
...maybe I should have chosen another topic for my first post :-D


Not at all Hopobcn, welcome to the forum and thanks for your post showing the scope traces.

The traces are interesting but I'm also interested in the Hankek scope you have as I've been considering this particular scope to purchase for myself. Aside from the obvious MRR uses, I have a couple of open reel recorders that I'd like to replace all the capacitors in and re-calibrate after completion for which I would need a scope. These particular scopes seem to be ideal for hobbyists.

The only thing that drives me nuts about this scope is that they only give you one probe despite it being a dual channel scope!

Offline Hopobcn  
#9 Posted : 03 February 2022 17:26:21(UTC)
Hopobcn

Spain   
Joined: 18/06/2020(UTC)
Posts: 2
Location: Barcelona
Yes the single probe thing is a bit meh from their part. I had to buy another one myself. Regarding the scope, cannot say much, for my needs (fiddle with arduino to make dcc accessory decoders) is more than enough.
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