Andrew Kesterson
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Two complete games in `examples/breakout/`, both 100% BASIC: `characters/` draws its wall in the text grid with two `DATA` sprites for the ball and paddle, and `sprites/` loads CC0 artwork and captures its whole screen with `SSHAPE`/`SPRSAV`. They take opposite shapes for reasons that are entirely this interpreter's, which is what the chapters are for. `docs/17-tutorial-breakout.md` and `docs/18-tutorial-breakout-artwork.md` build each one a step at a time, and end in a checklist of the rules a real program runs into: create every name before the loop starts, write a text row whole, loop with `GOTO` rather than `DO`, put the float on the left. Every trap is a runnable block with its own output rather than a claim -- the value pool dying at four thousand names, the skipped `BEGIN` block that breaks its caller's `RETURN`, `SSHAPE` ignoring a subscript, `READ`'s single cursor. Five figures, generated from the listings beside them by `docs_screenshots`, and a `breakout_art` setup so the ones that load artwork load the example's own. The character game's wall cannot be photographed -- the screenshot host omits the text layer on purpose -- so it is shown as compared output instead. `docs/07-sound.md` never said `SOUND`'s frequency is a SID register value rather than hertz, which both games depend on. It says so now, with the conversion from `src/audio_tables.c:84`. `TODO.md` gains the thirteen defects the two games turned up -- §6 items 30 to 33 and all of §9 -- each with a reduction that fits on a screen, the file and line of the cause, and what a fix would touch. Verified: `docs_examples` passes in both build configurations, `docs_screenshots --check` re-renders all thirteen figures and byte-compares them, the full 109-test suite passes in both builds, every quoted fragment was checked to appear verbatim in the listing it came from, and every relative link and anchor resolves. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
106 lines
3.3 KiB
Markdown
106 lines
3.3 KiB
Markdown
# 7. Sound
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The sound verbs need the SDL build and an audio device. Without one they refuse by
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name, and a machine with no sound card still runs the interpreter — only `SOUND` and
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`PLAY` fail, and they say so.
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## SOUND
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```basic requires=akgl
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10 SOUND 1, 4000, 60
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```
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Voice, frequency, duration. Voices are numbered from 1. The duration is in *jiffies* —
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sixtieths of a second — as on a C128, so 60 is one second.
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**The frequency is a SID register value, not hertz.** As on a C128 the pitch is
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`frequency * 1022730 / 16777216` — the NTSC system clock over the oscillator's 24-bit
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accumulator — so the 4000 above is about 244 Hz, and the argument ranges from 0 to
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65535. Work the notes you need out once and write the arithmetic down beside them: 4298
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is C4, 8579 is C5, 17175 is C6. `PLAY` takes note names and does the conversion for you.
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Further arguments give a frequency sweep: a step, a direction and a range, which is
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what makes a siren or a laser.
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**`SOUND` does not block.** On a C128 the note starts and the program carries on; here
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the same is true, and it has to be — an embedded interpreter that stopped the host's
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frame loop for a second would freeze the game.
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## PLAY
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`PLAY` takes a string of notes:
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```basic requires=akgl
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10 PLAY "C D E F G"
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```
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Within the string:
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| `A` to `G` | a note |
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| `#` `$` | sharp, flat |
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| `O` *n* | octave |
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| `V` *n* | voice |
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| `T` *n* | envelope preset |
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| `U` *n* | volume |
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| `W`, `H`, `Q`, `I`, `S` | whole, half, quarter, eighth, sixteenth notes |
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| `R` | rest |
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| `.` | dotted |
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Notes are queued and released in time, paced by the host's clock. A program that
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`PLAY`s and then carries on works the way you expect; a program that `PLAY`s and then
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immediately `QUIT`s may not hear all of it.
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`M` — measure — is accepted and does nothing. There is no bar-line accounting to do.
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## TEMPO
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```basic requires=akgl
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10 TEMPO 8
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```
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How fast `PLAY` releases its queue. The number is the same range a C128 uses; the
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constant that turns it into milliseconds is a calibration choice rather than a
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transcription, so it may not match a real machine exactly.
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## ENVELOPE and VOL
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```basic requires=akgl
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10 ENVELOPE 0, 5, 9, 4, 6
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20 VOL 8
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```
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`ENVELOPE` defines one of the ten presets `PLAY`'s `T` selects: attack, decay, sustain
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and release. `VOL` sets the overall volume, 0 to 15.
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## FILTER
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`FILTER` parses and then **refuses at execution**:
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```basic
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10 FILTER 1000, 1, 0, 0, 8
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```
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```output
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? 10 : RUNTIME ERROR FILTER needs a filter stage this device does not have
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```
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It sets the SID's filter cutoff, band switches and resonance. The audio backend
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synthesises raw waveforms and mixes them; there is no filter stage to configure and
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SDL supplies no primitive to build one from. Refusing is deliberate — a program that
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asks for a low-pass and gets an unfiltered square wave has been lied to.
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This is the one verb still waiting on a capability from the graphics library. It is
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filed there rather than worked around here.
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## The clock
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Everything with a duration — `SOUND`, `PLAY`, `TEMPO`, and `SLEEP` from Chapter 2's
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neighbourhood — is paced by a clock the *host* provides. The standalone interpreter
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sets it from a monotonic timer every step, so it just works.
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An embedded host that never sets the clock gets durations that expire immediately:
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audible, but never a hang. That is the intended failure direction.
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