DCC: Polarity vs Phase terms

You may have heard the terms Phase and Polarity used interchangeably when talking about DCC.   However, Polarity and Phase technically they do not mean the same thing at all

The term "Phase" is only associated with AC voltages or audio.  In the AC power industry, you might hear the expression "3 phase power" in relation to big motors or power transmission.  You may also have heard the term "delta" vs "Y" power terms in relation to AC power transformers.   Anyway.....


POLARITY:  Polarity only refers to ONE SIGNAL.  It is the description of the relationship of the voltage state, positive or negative, between two wires that carry the same signal.  It doesn’t matter whether it is DC or an AC voltage.    With DC, the voltage polarity is static or unchanging which make is simple to understand.  One wire is the positive typically shown with a RED color indication while the other wire is the negative typically shown with a BLACK color indication.  Granted that with AC/DCC, the polarity is constantly changing over time.  However, if you could freeze time and measure the polarity, only one wire will be the positive making the other wire be the negative.  Changing the polarity is simply the reversal of the plus and minus voltage.  For DC, Turn a battery around in a flashlight and you have inverted or, more commonly stated, reversed the polarity of the voltage going to the light bulb.  For AC, interchange the two wires at the input terminals of a loudspeaker and you have reversed the polarity of the signal coming from that loudspeaker.

PHASE: Phase refers to TWO OR MORE SIGNALS.  The signals MUST be of the same frequency with the phase referring to their relationship in time shift between them.  If both signals arrive at the same point at the same time they are said to be in phase. (0 degrees or 360 degrees).  If they arrive at different times they are out of phase (Some degree value other than 0 or 360). The only question is how much are they out of phase, or stated another way, what is the phase shift between them?

KEY POINT: You can reverse the polarity of one signal and you can measure this change.  You need two signals to measure any phase shift.


There is only one command station allowed on a given layout.  Hence there is only ONE DCC SIGNAL that is sent to all the track.   There is no second DCC signal to look at that allows the term PHASE to be used correctly with DCC.