Re: How DCC signal is superimposed onto AC voltage

Vollrath, Don <dvollrath@...>

Well...Yes & no. The concepts of modulating one signal (the carrier) with another (the signal) have been around for a long time. Think Radio & TV. Then add the cpmplications of color TV, then hundreds of channels multiplexed on a single cable. They didn't have microcomputers/microcontrollers when those products were invented and sold by the millions. But if you will pardon the pun, you are on the right track. Model RR products are physically small. One COULD use discrete components (Resistors, capacitors, transistors, etc.) to detect and properly decode the composite signal. But without using any integrated circuits you would be talking about thousands of components, in a really big box...And a nightmare of reliability problems. First came Integrated circuits (available in 1960's), then the programmable Microccomputers (1970's), then Microcontrollers (1980's) and other small surface mount components. The basic concepts of DCC were invented in the 1980's but used by only by one or two competitive companies. However, it is the combination of availabity of compact low cost microcontrollers, and the standardization of DCC through efforts of the NMRA, and the mass of interested modelers with money that makes it all available today. The microcontroller does make it possible to fit that much flexible logic in that small of a box at a reasonable price.

So, Yeah. Without the Microcontroller we wouldn't have DCC.


-----Original Message-----
[]On Behalf Of Aaron Lau
Sent: Monday, December 12, 2005 7:08 PM
Subject: [WiringForDCC] Re: How DCC signal is superimposed onto AC

Thank you very much. I have a clearer picture on how signal is
superimposed on AC.

Just for curiousity sake, since this is so simple, why wasn't this
being introduced long time in the 60's or 70's? What make
it so 'acceptable' in the 90's? The only reason I can of (not even
sure if i am right)is that the signal can only be decoded by a
microcontroller. Am i right to say that?


--- In, Mark Gurries <gurriesm@c...>

I would also add this thought.

Most people only think of "AC" to be 60Hz sinewave power as found
in the
AC power outlet. Thus when the term AC is spoken, people jump to
concept of what AC means and stick to it. That is not the persons
for that is often all they ever been exposed to (known) as the

In reality the meaning of AC goes way beyond that specific
implementation and is more generic and basic at the same time. The
terms AC is short for "Alternating Current". AC simply means
that is flowing back and forth changing voltage polarity as it
goes at a
rate described in terms of a frequency.

The definition does NOT include or automatically imply a numerical

Voltage Level
Voltage Frequency
Voltage Waveshape

The everyday 120V 60Hz (Sinewave) AC power is but only ONE possible
implementation of AC current flow that we all know so well. It is
fixed 120V current that changes polarity 60 times a second.

Many AC toy transformers (old Lionel) also put out AC power. The
difference is that they step down the voltage to something safer
such as
16V. But it is still a 60Hz Sinewave.

So DCC introduces yet another signal the ALSO meets the AC

Low Voltage, Dual Frequency, SquareWave as Don Described below.
the voltage is 14.25V, 12V for N scale.

Thus to understand DCC, one must free one's mind from fixed
definitions of AC power.

Hope this helps.

Quite simple in theory. There are many means to modulate one
with another, but for DCC the
frequency (or period) of each cycle of AC is changed between two
different frequencies to represent a
binary one or zero. This is a form of integral cycle frequency
keying. For DCC the carrier is
a flat-topped rectangular AC rather than sinusoidal.
The detection means simply looks for the zero crossings and
the time between them to
determine if a one or zero is being transmitted.
The DCC communication protocol (the particular language of ones
zeroes) is spelled out in the
details found at

-----Original Message-----
[]On Behalf Of Aaron Lau
Sent: Monday, December 12, 2005 6:30 AM
Subject: [WiringForDCC] How DCC signal is superimposed onto AC


Can someone explain to me how signal can be superimposed onto a
voltage? I tried finding websites to explain it in technical
term, but
to no avail.

Generally, I can't picture how this can happen, needless to say
how to
decode it.

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Best Regards,

Mark Gurries
Linear Technology
Power Supply & Battery Charger Applications Engineer/Manager
Model Railroad Club and NMRA DCC presentations are at:
Audio Enthusiast (Love SAE equipment)
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