Recording a part at slow tempo then speeding up the tape hides timing errors and shifts the timbre

A production technique widespread in early Chicago house: record a keyboard or vocal part at a much slower tempo (Jefferson recorded at 40 or 60 BPM), then speed the sequencer/tape playback to the target tempo (around 120 BPM). This gives a non-virtuoso player more time between notes, so imperfect playing becomes cleaner once sped up, and the pitch/timbre shift makes the result sound beyond what the performer could play live. Speedup shortens note durations and can sound choppy, so a note-lengthening feature (on Jefferson’s Yamaha QX1) is used to restore a natural feel. Jefferson took the idea from Led Zeppelin, who recorded later albums slow and sped them up to sound like better musicians.

Examples

Jefferson recorded the Move Your Body piano at ~40 BPM with quantization off, applied the QX1’s note-lengthening, then sped it up to 120 BPM. He notes you can hear the same trick by comparing Led Zeppelin’s live The Song Remains the Same with the sped-up studio versions.

Assessment

Explain, in steps, how a non-musician could record a clean-sounding keyboard line using slow recording plus speedup. What artifact does the speedup introduce, and what compensates for it?