Kicks sound doubled at slowed speeds #728

What goes wrong

Below 1x speed, kicks and other sharp hits sound doubled, like a flam. At 0.75x it is clearly audible on kick drums.

It affects anyone practising at a slowed speed with the pitch stage available, which is the normal case on desktop and in the browser.

Why

The time-stretch stage builds its output from 82 ms grains. At 0.75x each grain plays about 70 ms of input but only moves forward about 52 ms, so consecutive grains overlap. Sustained sound hides the overlap. A sharp attack that lands in it is played twice, roughly 50 ms apart.

Constraints

• Everything goes through one shared tempo stage, including the click, so music, drums and click share one clock. That has to stay true.
• The pitch stages run their own stretch and are aligned with the unpitched drums sample for sample. A change there would pull pitched lanes away from the drums.
• Any timing borrowed to avoid a repeat has to be paid back, or the audio drifts away from the playhead, the lyrics and the count-in.
• An earlier attempt split attacks into a separate path and put a click at every onset, because the two halves could not sum back to the input (see the header of static/vendor/soundtouch-processor.js).