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I agree that it is a confusing topic. I wish I had more physics and engineering, and a better memory than I have at present, but....

From the DCC Wiki:

...The voltage to the track is a bipolar alternating current (AC) signal. The DCC signal does not follow a sine wave. Instead, the command station quickly switches the direction of the DC voltage, resulting in a modulated pulse wave. The length of time the voltage is applied in each direction provides the method for encoding data.{ Me- 'zero stretching'} To represent a binary one, the time is short (nominally 58 µs for a half cycle), while a zero is represented by a longer period (nominally at least 100 µs for a half cycle).
Each locomotive is equipped with a mobile DCC decoder that takes the signals from the track and, after rectification, routes power to the electric motor as requested. Each decoder is given a unique running number (address) for the layout, and will not act on commands intended for a different decoder, thus providing independent control of locomotives anywhere on the layout, without special wiring requirements. Power can also be routed to lights, smoke generators, and sound generators. These extra functions can be operated remotely from the DCC controller. Stationary decoders can also receive commands from the controller in a similar way to allow control of turnouts, uncouplers, other operating accessories (such as station announcements) and lights...

A link to another site that says the same things:

 

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some decoders use BEMF to measure the motor speed to adjust the motor voltage to maintain a speed corresponding to the speed step communicated to it

i see no need for PID. PID is used to control a parameter that is not directly controllable such as motor position vs motor speed. i've used PID to control the position of a turntable and the phase of a clock signal
 

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i believe some form of pid control is used in many dcc engine/sound decoders. but then again, i don't actually know -- it's just what i read. you know, stick your hand in front of the loco to slow it down -- take it away, and it smoothly come back to the original speed, etc...
 

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OP --

Tell us:
Is your railroad dcc?
Or... is it dc?

Looking at the product page you gave in post #1, it looks like the Roco engine is dcc/sound equipped.

The product page description shows as having a decoder installed (listed as PluX16).

So... it should run as dcc when your railroad is set up for dcc, and...
... it should probably also run on a "straight dc" (i.e., non-dcc) railroad.

I'm not certain, but there may be SOME dcc/sound equipped locos that may not run out-of-the-box on straight dc railroads. Be aware that there's a CV setting in some decoders that will "turn off" this capability (although it usually can be "turned back on").

Also be aware that there are also dcc/sound equipped locos that will also run and provide limited access to sounds on straight dc railroads.
 

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From what I've read elsewhere DCC is run from AC, yet I wanted to get this clarified. The only reason I ask is because of a ROCO model I found which has sound, yet says that it's DC.

This is the train i'm looking at: Roco Modelleisenbahn Products LOCOMOTIVES Diesel locomotives 73733 Diesel locomotive 218 497-6, DB AG
DB Rails - Just to clarify, DCC decoders operate on track that's powered by DCC. Your DCC command station produces the DCC current, which is a unique form of current........DCC is not AC nor DC. Some command stations' power input require DC and then converts it to DCC; some other command stations require AC or either DC or AC. For example, the Massoth G Scale Command Station I use requires a DC Power Supply input. The Piko command Station that I also use for my Programming Track, originally allowed either DC or AC input, but their upgraded mode now only accepts DC power input. You can read about the unique nature of DCC current at:
 

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so many clarifications. thanks cid for correctly describing AC

i once had a supervisor who had a meeting to "clarify" a problem for everyone. He was the only person who was confused
 

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depends on where you put the ground, if it's on the end then you have current in one direction only , square wave direct current.. [in DCC]
 

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Aardvark, all residential 120VAC in the US has one side of the line (neutral) grounded. That does not turn it into DC.
Current still flows in opposite directions during the AC cycle. That's why it's called AC. The VOLTAGE on the hot side
alternates 85V above ground potential to 85V below ground potential, for an RMS 120VAC reading from line to line.
Give or take. The voltage regulation is less critical than the frequency.
 

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uh .. same as normal, one wire connected to power, one to ground.. ??
I do not understand your question ??
we are talking about what the DCC signal is and not ow to connect a DC power supply to the track?

You can easily demonstrate that DCC current can flow thru a path in both directions between the rails by connecting a pair of LEDs across the rails, one opposite polarity of the other.

if the DCC signal were Not AC, but simply an on or off pulse as you (and others) suggest, only one LED would be on, albeit half the time.

because DCC is alternating polarity, power is constantly available on the rails which can easily be rectified by the decoder to provide power to the motor.
 
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