95 lines
2.8 KiB
Markdown
95 lines
2.8 KiB
Markdown
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# 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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```
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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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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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```
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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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|---|---|
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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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```
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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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```
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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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```
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? 10 : RUNTIME ERROR FILTER needs an audio device that can filter, and this one cannot
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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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