Why do rhythm games need calibration?
Because the sound you hear and the image you see do not arrive at the same time, and neither arrives at the moment the game generated them — so a game that judges timing to within milliseconds must measure and correct the delay in your specific setup.
Where the delay comes from:
Display latency. A television or monitor processes the image before showing it — scaling, motion handling, colour processing — which can add anything from a couple of milliseconds to well over 100. This is why game mode exists and why using it is the single largest improvement.
Audio path. Analogue output through a wired connection is fastest. Bluetooth adds 100 to 250 milliseconds on standard codecs, which is catastrophic for rhythm games — audio over Bluetooth is essentially unusable for them. Soundbars and receivers add processing delay of their own.
The game's own buffering, since audio is prepared slightly ahead.
Frame pacing and input polling, which add a frame or two.
Why audio and video must be calibrated separately. They travel different paths with different delays. If the display lags more than the audio, the note appears after the beat; if audio lags more, the reverse. A game offering only one offset cannot fix both, which is why good rhythm games ask for two measurements.
How calibration works. The game plays a repeating beat or flashes a marker and asks you to tap along. It records the average difference between when it expected your input and when it arrived, then shifts its judgement window by that amount. It is measuring you plus your hardware, which is why the result differs between people on identical equipment.
Practical advice: enable game mode and disable all picture processing; use wired headphones or speakers; recalibrate after changing anything in the chain; recalibrate on a different television; and if timing feels consistently early or late despite calibration, adjust manually rather than repeating the automatic test.
The same latency affects fighting games and anything with precise timing — rhythm games simply expose it most clearly.