Re: How DCC signal is superimposed onto AC voltage

Mark Gurries

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 that
concept of what AC means and stick to it. That is not the persons fault
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 current
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 a
fixed 120V current that changes polarity 60 times a second.

Many AC toy transformers (old Lionel) also put out AC power. The only
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 definition.

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

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

Hope this helps.

Quite simple in theory. There are many means to modulate one signal
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 shift
keying. For DCC the carrier is
a flat-topped rectangular AC rather than sinusoidal.
The detection means simply looks for the zero crossings and measures
the time between them to
determine if a one or zero is being transmitted.
The DCC communication protocol (the particular language of ones and
zeroes) is spelled out in the
details found at

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


Can someone explain to me how signal can be superimposed onto a AC
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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