Answers about – How DCC signal is superimposed on A.C.
Go to http://www.nmra.org/standards/consist.html#standards-DCC
Don Vollrath’s answer.
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 www.nmra.org.
DonV
Mark Gurries’s answer.
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
definition.
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.
It is not
the DCC signal....It is the microprocessor abilities that prevented this.
The
microprocessor in the engine decoder needs to decode the data from the DCC
signal. It must synchronize itself with
the DCC packets to know where to start, then assemble the all the individual
bits into a complete command, interpret the command and finally act on the
command if it was meant for this engine.
It needs to do all this and do it in a very small package and be very
low cost.
Microproccessors did not even exist in the 60's. They become known about in the early 70's,
finally powerful enough to be used as a personal computer in the late 70's
culminating in the PC revolution of the 80's.
The 90's brought down cost and size while improving speed and performance
at the same time. There is more to this
story...but you get the idea.
The first 1A
decoders used in DCC in the early 90's were all above $50, supported only one
headlight, only worked with short addressing and took up 3 times as much
space. Now you can get them under $15
with multiple features and functions!
>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 ago...like 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?
>
>Aros
>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 www.nmra.org.
>DonV
>
>-----Original
Message-----
>From:
>[mailto:WiringForDCC@yahoogroups.com]On
Behalf Of Aaron Lau
>Sent:
Monday, December 12, 2005 6:30 AM
>To:
>Subject:
[WiringForDCC] How DCC signal is superimposed onto AC
voltage
>
>
>Hi
>
>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.
>
>Aros