One sheet · Signal path + action · Rev A
Eighty-eight weighted keys, a modelled concert grand, a speaker the size of a lunchbox. Nothing on the left half of this sheet makes a sound. Between finger and air there are seven translations, and one cable performs two of them at once.
Your foot shorts two contacts. A switch pedal is binary: down or up, no half-pedal, no pressure curve.
The keybed times each hammer between two sensors. That interval becomes a velocity number, 1 to 127. Nothing here vibrates.
The numbers clock out of the DIN socket as a current loop: 31250 bits a second, three bytes per note, about 1 ms.
The interface rewraps the same bytes as USB-MIDI and hands them to the iPad. It changes the envelope, not the message.
Pianoteq plays no recording. It solves a string, a soundboard and a room in real time and renders the result as digital audio.
Audio returns down the same cable. The converter turns those numbers into a voltage on the RCA jacks at line level.
The speaker amplifies that voltage and pushes a cone. Air moves. This is the first actual sound in the chain.
Steps 04 and 06 run through the same USB lead in opposite directions at the same time. That is the whole trick.
There is no fallback sound if the return leg breaks. Debug backwards from the OB-4: speaker, 3.5 mm cable, U-44 output level, app, then MIDI.
Pianoteq standalone or as an AUv3 inside GarageBand, never both. Two apps fighting for the interface is the usual cause of keys that light up but stay quiet.
An iPad is a stingy USB host. If the U-44 drops out, run it on 2×AA or its 5 V adapter, or put a powered hub between the two.
RCA out is line level and the OB-4's input expects exactly that. Set the U-44's knob first and leave headroom rather than pushing the app's master.