DCC Waveform
Summary: The DCC signal is an alternating DC waveform, which is considered AC. Confused?
 |
This is a highly debated topic. This article attempts to settle the argument if DCC Waveform is AC or DC. Leave the debate in the Forum, but changes to this page are welcome. |
Is it AC or DC?
In short, the DCC waveform can be considered "bipolar DC". The NMRA defines it as a Differential Signal without a ground, meaning it is floating. Floating refers to the absence of a common reference point.
A common belief is that power is considered AC if it is associated with sinusoidal waves, but this is not the case with DCC. Many will point to the typical graphical representation of waveform as proof it is an AC signal, ignoring the fact that representation does not have a zero volt reference point (it is floating).
The DCC signal on the rails does not have polarity, therefore it is not DC. It does have a phase relationship, but it is not AC either.
What DCC Waveform is
The DCC signal, or waveform, has enough energy to run your layout, while at the same time being able to transmit commands.
FSK vs PWM
The DCC waveform is created by imposing a series of pulses on each rail. It does not use Frequency Shift Keying (FSK) nor Pulse Width Modulation (PWM).
- FSK: No modulator or carrier used. The pulses are created by an amplifier switching the state of the rails from High to Low.
- With PWM, the pulse repetition rate (period) remains constant, the duty cycle varies.
- There are no high frequency carriers.
The DCC Booster is an amplifier. It amplifies the logic level data stream provided by the command station. The data is represented by the period between the leading edges. A value of "1" is represented by a short period of ≈106µSeconds, and a zero by ≈200µS. The periodic nature of the signal means measurement with a DC Voltmeter will indicate ≈50% of the actual value when measured from either rail to a common point.
Zero Stetching
- Main article: Zero Stretching
Zero stretching is a technique where the pulse on one rail is maintained in the High state for much longer than the nominal Zero state. By doing so, current flow in the motor is unbalanced, allowing the motor to develop torque. The direction of travel is determined by which rail the zero stretching is applied to, and speed by the length of the pulse. Under normal circumstances the current flow between the rails is in balance, preventing the motor from developing torque.
Zero stretching doesn't make the DCC Waveform DC, instead, it stretches the time the waveform is held high to provide a pseudo DC voltage. This allows analog trains to work. This comes at the expense of overheating the locomotive and consuming lots of electricity.