Volume control can be upstream (in digital) or downstream (analog), depending on how you want to configure things. In practice a single sound card can handle both inputs and outputs. The crossover/eq are performed (in real time) in software on the PC, with output (4- or 6-channel, digital or analog) to a multi-channel sound card. The input can be an analog (line-level) or digital (S/PDIF) feed to a sound card, or else a digital audio file stored on disk and decoded by the PC. I’ve written a long article detailing my development of the LX521 implementation my Pluto implementation is similar. For those interested, I’ll post specific configuration files in the Owners’ area of the Orion/Pluto/LX521 Forum: together with this how-to, those files can be used to build a DSP version of the Analog Signal Processor (ASP) for those loudspeakers. One could use this system to implement the xover/eq for the Linkwitz Labs Pluto 2.1 or LX521. It’s silent, always on, and requires no user interaction. My design goal was a small, self-contained unit that can live on a shelf beside an amplifier: it runs without need of a display, keyboard, or mouse, and it looks like this: It runs on a linux PC and uses only free, open-source software. The following setup can be used to implement a 2- or 3-way digital crossover (with equalization) for active loudspeakers.
