LastspikemikeThe DCC signal varies in frequency. That's how it works.
Moreover, the DCC signal uses a square waveform, with the necessary allowance from 'ideal' compensated for (admittedly this is hidden in the engineer-speak in the standards definition to correct for any normal deviation from 'ideal' due to real-world risetime and decay and some of the 'ringing' artifacts, and of no practical interest to actual users other than it makes the assumption of 'square wave' real-world practical.)
FM is frequency modulation.
The other option would be to vary the amplitude, voltage modulation. That would be an AM solution to the problem.
When you read a good introduction to the development of the DCC standard you will find that various forms of digital-via-AM were considered but rejected.
Just because FM and AM refer to specific radio signals doesn't make those specific terms inapplicable generally.
Often the expert cannot see the simple stuff.
It is not clear to me that the two locos are same make and model?
Sheldon
rrebell ATLANTIC CENTRAL rrebell ATLANTIC CENTRAL rrebell Kinda wondered as I just hooked up my DCC system and it runs better on DCC at very slow speeds than on DC, ran engines as compatable as I could for comparison and no dual decoder for DC mode. What kind of DC throttle/power pack? Sheldon DC is a tech 2 #2500 by MRC. DCC isba Digitrax DCS 51. Honestly, I am confused by your original post. Are we talking about one specific locomotive? With sound? Without? What are we comparing here? How a loco runs on DC with no decoder, and how the same loco runs on DCC with a decoder, and how that same loco runs on DC with a dual mode decoder are going to be three different results depending on a number of factors. A tech2 2500 is niether the best or the worst DC throttle. But decoders use full voltage pulse width modulated speed control when in DCC, so that is generally better slow speed control than "average" DC powerpacks. I honestly don't know exactly what circuitry is used in a dual mode decoder running on DC. And I really don't care. I have no plans use them. But I know this, all my DC locos (no decoders) run very well, with good slow speed, using my Aristo Train Engineer throttles which also use full voltage pulse width modulated speed control. My absolute slow speed is not always as slow as the best DCC decoders, but it is very close, and very good, and very "stall free" when starting trains. Based on my own experiances with dual mode decoders, and my extensive use of DCC on other layouts, I don't get this being on the fence thing. Go DCC or don't. But good DC throttles can and do provide slow speed control similar to DCC, without some decoder in the way............ Sheldon People need to read carefully and not interpit. One layout with pigtails so I can switchn out power supplys (layout only half finished wiring so these are my test leads). Two different power sources, one DC one DCC, two engines, one DC only, one DCC and sound only. No dual decoders, never mentioned except to say this (unless I screwed up on my typing, using a less than user freindly laptop).
ATLANTIC CENTRAL rrebell ATLANTIC CENTRAL rrebell Kinda wondered as I just hooked up my DCC system and it runs better on DCC at very slow speeds than on DC, ran engines as compatable as I could for comparison and no dual decoder for DC mode. What kind of DC throttle/power pack? Sheldon DC is a tech 2 #2500 by MRC. DCC isba Digitrax DCS 51. Honestly, I am confused by your original post. Are we talking about one specific locomotive? With sound? Without? What are we comparing here? How a loco runs on DC with no decoder, and how the same loco runs on DCC with a decoder, and how that same loco runs on DC with a dual mode decoder are going to be three different results depending on a number of factors. A tech2 2500 is niether the best or the worst DC throttle. But decoders use full voltage pulse width modulated speed control when in DCC, so that is generally better slow speed control than "average" DC powerpacks. I honestly don't know exactly what circuitry is used in a dual mode decoder running on DC. And I really don't care. I have no plans use them. But I know this, all my DC locos (no decoders) run very well, with good slow speed, using my Aristo Train Engineer throttles which also use full voltage pulse width modulated speed control. My absolute slow speed is not always as slow as the best DCC decoders, but it is very close, and very good, and very "stall free" when starting trains. Based on my own experiances with dual mode decoders, and my extensive use of DCC on other layouts, I don't get this being on the fence thing. Go DCC or don't. But good DC throttles can and do provide slow speed control similar to DCC, without some decoder in the way............ Sheldon
rrebell ATLANTIC CENTRAL rrebell Kinda wondered as I just hooked up my DCC system and it runs better on DCC at very slow speeds than on DC, ran engines as compatable as I could for comparison and no dual decoder for DC mode. What kind of DC throttle/power pack? Sheldon DC is a tech 2 #2500 by MRC. DCC isba Digitrax DCS 51.
ATLANTIC CENTRAL rrebell Kinda wondered as I just hooked up my DCC system and it runs better on DCC at very slow speeds than on DC, ran engines as compatable as I could for comparison and no dual decoder for DC mode. What kind of DC throttle/power pack? Sheldon
rrebell Kinda wondered as I just hooked up my DCC system and it runs better on DCC at very slow speeds than on DC, ran engines as compatable as I could for comparison and no dual decoder for DC mode.
Kinda wondered as I just hooked up my DCC system and it runs better on DCC at very slow speeds than on DC, ran engines as compatable as I could for comparison and no dual decoder for DC mode.
What kind of DC throttle/power pack?
DC is a tech 2 #2500 by MRC. DCC isba Digitrax DCS 51.
Honestly, I am confused by your original post.
Are we talking about one specific locomotive?
With sound? Without? What are we comparing here?
How a loco runs on DC with no decoder, and how the same loco runs on DCC with a decoder, and how that same loco runs on DC with a dual mode decoder are going to be three different results depending on a number of factors.
A tech2 2500 is niether the best or the worst DC throttle. But decoders use full voltage pulse width modulated speed control when in DCC, so that is generally better slow speed control than "average" DC powerpacks.
I honestly don't know exactly what circuitry is used in a dual mode decoder running on DC. And I really don't care. I have no plans use them.
But I know this, all my DC locos (no decoders) run very well, with good slow speed, using my Aristo Train Engineer throttles which also use full voltage pulse width modulated speed control.
My absolute slow speed is not always as slow as the best DCC decoders, but it is very close, and very good, and very "stall free" when starting trains.
Based on my own experiances with dual mode decoders, and my extensive use of DCC on other layouts, I don't get this being on the fence thing. Go DCC or don't.
But good DC throttles can and do provide slow speed control similar to DCC, without some decoder in the way............
People need to read carefully and not interpit. One layout with pigtails so I can switchn out power supplys (layout only half finished wiring so these are my test leads). Two different power sources, one DC one DCC, two engines, one DC only, one DCC and sound only. No dual decoders, never mentioned except to say this (unless I screwed up on my typing, using a less than user freindly laptop).
OK, no matter what you think is comparable quality, if they are different brands and different models, your comparison of their performance is meaningless, it is apples and oranges.
There is no baseline of comparison because you do not know if the DC loco would run slower/better on DCC? It might not. You don't know how good or bad the DCC loco might run on DC - with or without a decoder.
You don't have enough data to conclude anything.
I have a large and varied fleet of good quality DC locos, I have pulse width modulated throttles, and I have some locos that run better than others. But ALL OF THEM run better than they run on some mid grade power pack - because of the superior PWM control of my Aristo throttles.
And all my friends with DCC have similar varied fleets of locos, and some run better than others.
-Kevin
Living the dream.
Here's the problem. "Layman's terms" to explain a technology too often lead to oversimplifactions and misunderstanding of what the technology is. You can make non-technical explanations of the basic operation of something like DCC< but to actually understand what goes on under the hood - that requires talking in technical terms or you do not get an accurate picture. Words actually mean things to engineers, too. Usually more literally than for non-technical people. Yes, simplification happens - when matching reverse loops with DCC< you are matching phase, no polarity, but many people, myuself included, often use phase and polarity nearly interchangeably. That isn't necessarily being lazy or ignorant - the vast majority of non-technical people may actually know what you mean when you say "polarity" but draw a blank when you say "phase".
As for the OP's actual question:
Same loco, with and without a DCC decoder. Without the decoder, running on a good DC power source that uses PWM, it will behave at least as well as it would on a DCC system using DCC. Using a pure filtered DC, it will run smoothly, but starts and stops will be more abrupt than either a pulsed DC power pack or a PWM DC power pack. I believe it was Linn Westcott that coined the term "stiction" for the combine electromagnetic and mechanical resistence in the loco - the motor and drivetrain. Pulsed power allow breaking the initial friction without a full motor rotation, so it starts slower. Using PWM does the same, but usually even better because the pulses are always at full amplitude.
Fit the exact same loco with a decent DCC decoder, and run it on a DCC system - it will run equally as well. Not likely to run better, unless you tested the DC operation with a vastly inferior power source. DCC isn't a magical fix for a poor running loco - if it runs poorly on DC, it will run poorly with DCC. Highly unlikely for the reverse - that it runs well on DC but poorly on DCC. The difference with DCC is that since the 'throttle' portion is onboard the actual loco, you can adjust the loco's response to the physical throttle you hold in your hand in ways you just can't with DC. At least not with commonly available DC power packs - back in the day there were some pretty fancy ones that had start voltage, max voltage, pulse width, pulse strength, and pulse duration settings, and other things. So you COULD tweak the control to the loco. The downside to that is, you have to readjust it for each loco.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
gregc Lastspikemike It is a digital rendition of FM radio, basically.
Lastspikemike It is a digital rendition of FM radio, basically.
rrebellAlso of note I tried to use engines of comparable quality.
I am really interested in the experiment that you performed, and I would love to read about it in more depth.
I wish this thread could stay on an even keel.
A very simple question, not answered as such to my satisfaction in previous posts:
As I understand it, you are comparing the slow-speed performance of an engine running on DCC, controlled digitally via the motor control from the decoder, with the slow-speed performance of the same make and model of locomotive, without any decoder at all, being operated entirely by DC track-voltage control.
No DC compatibility-mode issues at all. And no issues of anything but DC power being applied in the second case... no sound chip, for example, that needs a particular minimum average voltage to start and run.
After this is answered, please repeat exactly what kind of straight-DC equipment you are using for the 'comparison'.
If I was right about the original question, you will be comparing slow-speed performance at the motor between a reasonably 'intelligent' decoder's PWM output and the probably less intelligent but still well-engineered PWM from a good DC speed-control powerpack. If that is the discussion some of the reasons why you see a bit 'finer' control of slow-speed performance from DCC can be taken up in correct context.
Removed as not relevant to discussion.
Lastspikemike I cross examine them in court routinely, read (and understand) their expert reports and so on. It is a digital rendition of FM radio, basically.
It is a digital rendition of FM radio, basically.
huh?
greg - Philadelphia & Reading / Reading
Lastspikemike I am not trying to describe DCC as an engineer would. I've read a bunch of that stuff before I made my inquiries on this board, including reading everything I could find on this board. Most remarkable is the complete absence of an intelligible description of DCC for a lay person, I.e. not an electrical engineer.
I am not trying to describe DCC as an engineer would. I've read a bunch of that stuff before I made my inquiries on this board, including reading everything I could find on this board.
Most remarkable is the complete absence of an intelligible description of DCC for a lay person, I.e. not an electrical engineer.
Alton Junction
Also of note I tried to use engines of comparable quality.
Overmod I'm largely staying out of this, but: 1) PWM for control of permanent magnet motors is done for voltage control, for example as in gregc's diagram immediately preceding. 2) in DCC, this is done entirely by the decoder, following digital commands it receives. The motor is isolated from 'track power' in that mode. 3) The DCC track power is also PWM, for digital logic purposes. (As the LocoFi material alludes, this is a modulation like that of audible Morse, where the duration of 'short' and 'long' pulses chosen to be much longer than usual noise sources are chosen to represent binary states) 4) If it is not obvious by now, the actual PWM in (3) bears no relationship to any PWM in (2). 5) It should therefore be no particular surprise that a power-level PWM signal modulated for motor control, imposed in place of DCC track voltage, will be wrong for anything expecting logic signals. How wrong, might be difficult to predict, but sure as hell into the world of monkeys on typewriters inadvertently sending commands computers might recognize -- and not balancing DC charge transfer across the logic connection. We can get to a discussion of how 'DC compatibility modes' are arranged on decoders; I would be particularly interested in reading actual details. One logical but naive approach would be to bypass the motor leads to track power while arranging voltage-to-voltage conversion and at least keep-alive power that works with whatever other decoder-based functionality is provided in DC operation. If this is done expecting a potentiometer-controlled or minimal-ripple DC voltage, perhaps assuming some superposed signal or AC modulation for 'compatible-with-DC' device control ... there may be problems if interrupted PWM DC is encountered instead. (Not with respect to the motor, which would have to be of a type that would run on the bypassed PWM voltage... oh wait, wasn't there something about coreless motors not doing well on it sometimes...) Now I'll grant you that PWM fine motor control has been a mainstream hobby technology for so long that it would be surprising if commercial manufacturers designed things ignorant of its existence. But it may be easier just to design what is essentially a crude compatibility mode for a 'least common denominator' kind of DC control, and just forbid the wrong kind of fancy DC that causes issues.
I'm largely staying out of this, but:
1) PWM for control of permanent magnet motors is done for voltage control, for example as in gregc's diagram immediately preceding.
2) in DCC, this is done entirely by the decoder, following digital commands it receives. The motor is isolated from 'track power' in that mode.
3) The DCC track power is also PWM, for digital logic purposes. (As the LocoFi material alludes, this is a modulation like that of audible Morse, where the duration of 'short' and 'long' pulses chosen to be much longer than usual noise sources are chosen to represent binary states)
4) If it is not obvious by now, the actual PWM in (3) bears no relationship to any PWM in (2).
5) It should therefore be no particular surprise that a power-level PWM signal modulated for motor control, imposed in place of DCC track voltage, will be wrong for anything expecting logic signals. How wrong, might be difficult to predict, but sure as hell into the world of monkeys on typewriters inadvertently sending commands computers might recognize -- and not balancing DC charge transfer across the logic connection.
We can get to a discussion of how 'DC compatibility modes' are arranged on decoders; I would be particularly interested in reading actual details. One logical but naive approach would be to bypass the motor leads to track power while arranging voltage-to-voltage conversion and at least keep-alive power that works with whatever other decoder-based functionality is provided in DC operation. If this is done expecting a potentiometer-controlled or minimal-ripple DC voltage, perhaps assuming some superposed signal or AC modulation for 'compatible-with-DC' device control ... there may be problems if interrupted PWM DC is encountered instead. (Not with respect to the motor, which would have to be of a type that would run on the bypassed PWM voltage... oh wait, wasn't there something about coreless motors not doing well on it sometimes...)
Now I'll grant you that PWM fine motor control has been a mainstream hobby technology for so long that it would be surprising if commercial manufacturers designed things ignorant of its existence. But it may be easier just to design what is essentially a crude compatibility mode for a 'least common denominator' kind of DC control, and just forbid the wrong kind of fancy DC that causes issues.
I have only tried a few dozen different ones, but I have yet to find a dual mode decoder that will run on my ARISTO CRAFT TRAIN ENGINEER wireless radio throttles which use full voltage PWM control.
As posted above, my non decoder locos all run at performance levels similar to those same models decoder equiped on DCC.
Those cheap Bachmann decoders sold pretty well on Ebay years ago......
Being an 'expert' in symantics doesn't also qualify one to describe what Digital Command Control is, how it works, its limitations, its benefits over DC or other systems currently available, or even be intelligible to those attempting to square themselves with the topic. Secondly, it's a system conceived of, designed by, and constructed by...wait for it... engineers. Thirdly, it is the most successful new method of controlling scale model trains, and is at least as popular as the system it replaced still is, that being DC. The manuals imparting operating instructions and information were written by non-symantically expert engineers, God bless their wrinkled little hearts, and are widely understood and shared by users everywhere who are no more symantically expert than their authors.
I wonder what went wrong....
richhotrainIt is really a shame that several of the recent threads in this Electronics and DCC forum have been corrupted by replies that provide nothing but misinformation about DCC.
+1
Layout Design GalleryLayout Design Special Interest Group
richhotrainWell, isn't that the problem here in the first place? The Electronics and DCC forum has always been a site where model railroaders can come with questions and get knowledgeable and factual answers.
Yes, that is the problem.
I have been on these forums for over three years, and I always read the DCC threads because I find the conversations fascinating.
The conversations have become much less fascinating because now they are dominated by explanations of why information posted is incorrect. This makes it difficult to get answers or good information.
1) I do not use DCC on my own layout.
2) I almost never post in these forums because I know very little about the subject.
3) I learn A LOT in the DCC forums by opening my mind and shutting my mouth.
4) If you do not know what you are talking about, please be like me, and keep your mouth shut and learn from those who are sharing their knowledge and experience.
Over.
Overmod I took the reply out, and I think you should too. He's not advancing these topics, but discussing that doesn't help those who need objective better answers. Let's focus on those.
I took the reply out, and I think you should too. He's not advancing these topics, but discussing that doesn't help those who need objective better answers. Let's focus on those.
When we get to the point that we don't what to believe, a thread becomes worthless. And, if enough of these threads get corrupted by such postings, then the Electronics and DCC forum itself becomes worthless.
Rich
And he certainly has given good advice in other modeling threads.
Overmod Lastspikemike I make no claim to being an engineer although I have reason to believe my grasp of engineering is quite good. The issue is that you may believe that, but no one here who actually knows engineering seems to even begin to agree with you. That does not bode well for an objective confirmation of your 'belief', let alone your 'reason'. I drew no analogy between google and DCC. We don't care. The point was that your "engineering" description of what Google search does was not even remotely right.
Lastspikemike I make no claim to being an engineer although I have reason to believe my grasp of engineering is quite good.
The issue is that you may believe that, but no one here who actually knows engineering seems to even begin to agree with you. That does not bode well for an objective confirmation of your 'belief', let alone your 'reason'.
I drew no analogy between google and DCC.
We don't care. The point was that your "engineering" description of what Google search does was not even remotely right.
.
Never mind.
rrinkerI'm not so sure the analogy to google searches is correct.
While perusing S9.1, which really repeatedly punts on the issue of rail-to-rail supply voltage, think about going to the power-station standard immediately following in the list (from 2012) which answers a number of additional timeless topics. Note that at one time it was possible to extract one-half the DCC power waveform (the pulses to +Vdcc being as modulated, but the other half-waveform being clamped essentially at zero crossing). If you were to modulate this with voltage-determining 'average' PWM (which, note, at the DCC modulation clock rate could easily match the effect of any fast PWM power control even with significant DCC logic modulation for 'piggybacking' included) you would generate something that a DC motor of suitable construction (including suitable impedance or 'ripple filtering') could happily run on. I don't think there are any practical 'hybrid' systems now made that would actually run this way, but for a controller that could produce either DCC control or straight DC motor control in 'unmodified' locomotives with as good PWM control as the best DC powerpacks, the idea is intriguing...
From what I gather from all this is that I might be right in concept but not nessisarily in practical use.
tstageThat's a problem when you choose to use many words then make statements that are misleading.
This ^
I'm not so sure the analogy to google searches is correct. And if you look at the waveform shown in Greg's post, you can see just what is going on.
A DC loco seeing power like that (the part BEFORE the waveform marked as a stretched zero) is a power going positive and negative in equal amounts, a net zero DC voltage. But the poor armature of the motor is trying to reverse direction at the speed of the DCC signal. Power is being dissipated in the windings, but the motor isn't turning, it's buzzing at the frequency of the applied waveform. And getting hot. That's why you can't leave a DC loco on DCC powered track.
Now, move over to the part that shows a "stretched zero". That's how some systems can run a DC loco on DCC track. It's not particularly complicated - the DCC standard sets a specific length limit for a 1 bit, but the zero bit is anything longer than a specified minimum up to a rather large maximum. By stretching either the positive half or the negative half of the zero bit, a net DC voltage of one polarity or the other can be produced. It works, but not very well, since there is still plenty of negative going pulses as part of the DCC communications stream to the DCC locos that are running. The strecthed zeros do not affect DCC decoders, other than to reduce the overall number of bits per second, but unless you have dozens of locos running, this isn;t a problem. The problem is, some DC motors can handle this better than others and work, others run at a fraction of their DC top speed and make nasty sounding noises doing it.
This feature was more important in the early days of DCC, when even cheap decoders were $70 or more, and many locos still were the old "all space inside the shell is solid metal" to make them pull better design so fitting a decoder often required milling weights, not a skill all modelers possess. This was particularly the case in N scale.
At least one major manufacturer that is still around originally supported this but has removed support. Only one of the major manufacturers even still supports it, Digitrax.
A decoder on the track is constantly reading the data stream. After the preamble of each trnamission comes the address - if the address does not match what the decoder is configured to, it simply ignores the rest of the message and waits for the next preamble.
(obviously I was typing my reply at the same time Greg was typing his - now here's a feature this forum desperately needs, it's on the one EE forum I participate in - if someone else has snuck in a post before you submit yours, it warns you and lets you see this new post so you can either edit yours or just forget about it)
sorry for the repetition
LastspikemikeAnd as I understand this, the sets of bits are also identified for the specific locomotive by an address encoded in a similar fashion
the above illustrates how data bits (1s and 0s) are transmitted on the track as DCC
the image below (x-axis is not time) shows how the demodulated bits form a DCC packet composed of a preamble, address, instructions and error correction.
LastspikemikeElectrically, all locomotives on track continuously receive the entire signal and must actually block the control signal intended for all but the one specific address intended for that locomotive.
the decoder doesn't "block the control signal"
like ethernet, IP, ..., the decoder constructs a packet from the demodulated data bits and simply ignores packets not with its' or a broadcast (e.g. emergency stop) address.
Lastspikemike(I suspect all DC locomotives that may be on track get and react to the same generic signal)
DC locomotives have no decoder. the motor in a DC locomotive reacts to the average voltage on the track which while not 0 VAC is normally 0 VDC.
when a DCC command station attemtps to control a DC locomotive, it generates the streched zero pulse with appropriate polarity resulting in an average track voltage that is not zero which will drive any/all DC locomotives on the track.
you don't need to hypothesize how DCC works, the S-9 standard describes all this in detail
bi-polar DCC track signal.
In a "1" bit, the first and last part of a bit shall have the same duration, and that duration shall nominally be 58 microseconds
In a "0" bit, the duration of the first and last parts of each transition shall nominally be greater than or equal to 100 microseconds