The RDL ST-VP2 is a compact, stand-alone audio processing module designed to automatically lower (or "duck") the volume of a background audio source in the presence of a primary voice or announcement signal. This function is critical in commercial audio systems for applications such as retail stores, airports, and public buildings, where background music must be temporarily reduced in level to allow important paging announcements to be heard clearly. The module operates by comparing the level of a control input (typically the paging microphone or announcement feed) to a user-set threshold. When the control signal exceeds this threshold, the module automatically attenuates the level of the program input (the background music) by a predetermined amount, and then smoothly restores it to its original level after the announcement ends.

The ST-VP2 is part of RDL's "Stick-On" series, characterized by its small, metal enclosure that can be easily mounted virtually anywhere using adhesive backing or screws, requiring no rack space. It is designed for simplicity and reliability, with minimal user adjustments. Key controls typically include a threshold adjustment for the control input, a depth control to set how much the background audio is reduced (the "ducking" depth), and attack and release time controls to determine how quickly the ducking engages and how smoothly the music returns. The module operates on a single low-voltage DC power supply, commonly 24 volts, which is often available from the same centralized supply powering other system components like amplifiers.

Integration into an audio system is straightforward: the background music source is connected to the program input, the paging microphone or announcement feed is connected to the control input, and the ducked output is sent to the amplifier and speakers. The ST-VP2 performs its function transparently and automatically, requiring no manual intervention during operation. For system integrators, this module provides an inexpensive and effective solution to a common design requirement, ensuring that announcements always take priority over background audio without the need for complex, manually operated mixing. Its compact size and simple wiring make it an ideal choice for retrofitting into existing systems or incorporating into new, space-conscious designs.