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>
A mutation smoke run scored src/audio.c at 50% and src/draw.c at 70% and named
three real holes:
- Nothing asserted that an unconfigured voice is audible, which is the entire
reason the voice table defaults to a square wave at full level instead of a
zeroed struct. Deleting the defaults survived.
- No envelope test used a non-zero attack and decay together, so the decay
measuring from the start of the note rather than from the end of the attack
survived.
- The circle was only checked at its four axis points, which a mis-signed
octant reflection survives. It now checks that every plotted pixel has a
mirror in the other three quadrants.
Also adds a backend that exists but was never given an SDL_Renderer, which is
the state a host is in between allocating one and initializing it; every draw
entry point has to report it rather than dereference it.
audio 50% -> 75%, draw 70% -> 90%. The survivors are recorded in TODO.md and
are honestly untestable from here.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
There was no audio API at all: SDL3_mixer was vendored and AKGL_REGISTRY_MUSIC
was declared, but nothing opened a mixer, and a mixer plays recordings anyway.
BASIC 7.0's SOUND, PLAY, ENVELOPE and VOL describe notes, so this adds a tone
generator -- three voices, five waveforms, an ADSR envelope each, mixed to one
float stream and fed to an SDL_AudioStream.
The voice table works whether or not a device is open. akgl_audio_init connects
it to one; a host that owns its own audio pipeline can call akgl_audio_mix
instead. That is also what makes the tests deterministic: a device pulls samples
on SDL's audio thread whenever it likes, so tests/audio.c mixes by hand and only
opens a device in its last test.
Phase is computed from the frame counter rather than accumulated, because a
float increment of hz/44100 added 44100 times a second walks a held note off
pitch. A voice that has never been configured defaults to a square wave at full
level, since a zeroed voice would have a sustain of 0.0 and make no sound with
no error to explain why. Voices summing past full scale are clamped rather than
scaled, so one voice plays at the level it was asked for.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
