Summary: Uninterruptible Signal Processing (USP) is Lenz's name for the ability of a Multifunction Decoder to pick up Digital Command Control signals from the track even when the electrical connection is interrupted by dirt, bad contacts, or corrosion.
What is USP?
Lenz's USP is a signal processing technique which allows it to decode a weak or distorted DCC waveform, thus maintaining a connection to the DCC system and uninterrupted operation. It should not be confused with an Energy Storage accessory for a multifunction decoder, although for USP to be effective an energy storage module is required to supply power to the decoder and the motor while the supply from the rails is interrupted.
If the rail (or wheel) is coated with foreign matter such as dirt or oxide, this creates an insulator, or a dielectric. When two conductors are placed against each other, with a layer of a dielectric between them, it forms a capacitor. The current cannot travel from one conductor to another, but it can hold a charge between the two conductors or plates.
Capacitors have a phenomenon known as reactance. As the frequency applied to the capacitor increases, the reactance decreases, and current can flow between the two plates. At a high enough frequency, the capacitor is effectively a short. As the frequency approaches DC (zero Hertz), the capacitor's reactance approaches infinity.
USP exploits this property. At DCC frequencies, the insulating layer between the wheel and the rail forms a capacitor. Some current can flow to the decoder, and the USP circuitry can detect and decode the DCC waveform. The currents that flow will be very small, and a harmonic of the original frequency. Since these interruptions will be very brief, control is maintained.
The Lenz USP modules and their Gold series decoders fitted with the technology include a small Energy Storage circuit to maintain power to the decoder and keep the motor turning. It is possible to add additional energy storage.
With multi-axle locomotives with multiple or all wheel pickups, the issue is resolved by the multiple contacts with the rails. A small, short wheelbase locomotive, or those with only two axles may have issues with frogs or dirty track, and this technology will help.
Trackwork and Wiring
This technology is not meant to be a cure for poor trackwork and inadequate wiring. Track laid to a high standard goes a long way to eliminating DCC issues (and mechanical problems), as does proper wiring. Distorted DCC signals caused by inadequate wiring can still cause a runaway.