DC versus DCC

DC Operation:

Before DCC, trains used a variable DC voltage to control the speed of a single train.   The train can have 1 or more engines.   Any DC voltage on the track is directly connected and applied to the motor.  For a given speed, you gave the engine a given voltage.  Vary the DC voltage up and down and you can speed up or slow down the engine respectively.  The voltage only changes when you need to change the speed but is otherwise DC.  When you want the train to stop, you turn the track voltage to zero.   The DC voltage is provided by a DC "power pack" or a DC throttle.   

If you wanted to control more than one train at a time, you had to purchase additional DC power pack(s) and implement what is commonly called "Block Wiring" to keep the two DC voltages apart from each other.  Independent DC power control means a given section of track cannot have more than one DC power pack powering it.  The complexity of the layout wiring goes way up for the sole purpose of keeping the trains running on separately powered track so they can remain independent.  This is NOT prototypical.  It is only a "necessary evil"  when using DC power.

DCC Control:

DCC's uses an indirect approach to control the Engine speed.  The motor is NOT connected to the track power but is instead is connect to a DCC "Decoder" which itself applies a form of DC voltage to the motor to control the speed of the engine.  Since each decoder has its own address (Engine Number) assigned to it, you can control each motor (Engine) speed independent of the track voltage.  This gives DCC the ability to control multiple Engines, and hence trains, on the same track.   This IS prototypical.  

So how does a given Decoder know what speed information to drive the engine at?

A Decoder uses an "Address" just like a given house's mailing address would have or a computer would have (IP address).  Typically the address is tied to or the actual engine number.   To change the speed of the motor controlled by the given decoder, all it needs is a "DCC speed command" that also contains the address of the decoder it is intended to control.    Dial up the engine number on the throttle, and away you go.  There are other DCC commands to control lights and sound too.

DCC Track Voltage:

The decoder needs a minimum voltage (power) at all times in order to function.  In order to keep all decoders on the layout alive, then DCC power must be global and reach every engine at all times.   The decoder electronics itself cannot run directly from DCC's AC voltage.   To convert the AC voltage to DC, the decoder contains what is called a "full bridge rectifier" (rectifier diodes).   Since the decoder drives the motor using this "rectified" DC voltage, the DC voltage must be equivalent to the value the motor would see see under full DC to get full speed if commanded to do so.   Since the DC voltage is directly tied to the DCC voltage, the DCC voltage must be just as high.   Full DCC voltage/Power is defined to be 14.25V nominal for HO scale.   Below is a comparison of DC, plain old AC and DCC waveforms that one would see using an oscilloscope.

DCC Motor Speed Control:

As discussed at the top, with DC the motor speed varies directly with the voltage on the track.   The higher the DC voltage, the faster the train went.  However under DCC, the track voltage is always at full voltage.  This is required to allow the decoder to always have power in order for it to process the DCC commands being sent to it.   Specifically when the decoder receives speed step commands with its address, it will then adjust the motor speed.  Variation of Motor Speed is now performed by the decode itself using a very efficient power technique called Pulse Width Modulation or PWM.   This is where the decoder uses pulses of full voltage that varies in terms of time duration to effectively trick the motor into thinking it is getting a variable DC voltage.   To learn more about PWM motor drive go here: PWM Motor Drive

DCC Wiring Polarity:

Like DC, DCC has a wiring polarity that must be followed when connecting to the track.   Failure to maintain the proper polarity in the wiring will create a short circuit in the wiring.

Technical note:  It would be incorrect to use the term "Phase" with DCC even though DCC is a form of AC.  To know why, go here:
DCC: Polarity vs Phase terms