Andrew Kesterson 34a076b851 Add akgl_audio_sweep for a note that changes pitch ...

BASIC's SOUND ramps a tone's pitch toward a limit, and nothing here could
do it. Faking it from the caller means re-issuing tones from its own step
loop, which ties audible pitch to how often that loop runs -- a siren that
changes shape with the frame rate. The step has to be taken on the mixer's
frame counter, which only this library can see.

akgl_audio_sweep(voice, from_hz, to_hz, step_hz, ms) steps once every
1/60 second, the rate the machine this vocabulary comes from used, which
divides 44100 exactly so a step always lands on a whole frame. Direction
comes from the two frequencies rather than the sign of the step, the last
step clamps to the target, and equal frequencies are a held tone rather
than an error. akgl_audio_tone() is the same start path with a step of 0,
so a voice reused for a plain tone stops sweeping.

A swept voice accumulates its phase instead of deriving it from the frame
counter: deriving assumes a constant frequency and would jump the waveform
at every step. tests/audio.c drives the mixer by hand and asserts the exact
frame the pitch moves on.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

2026-07-31 12:52:21 -04:00

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