synth/audio experiments
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# UXN Audio Proposal
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## Problems
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Currently the UXN audio device doesn't work very well for playing
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complex music. There are a few reasons for this:
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* Note duration is conflated with envelope shape
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* Envelope resolution (67ms) limits tempos/subdivisions
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* Microtonal music is not possible (according to the spec)
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* Using audio callback requires scheduling pauses/silence
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## Proposal outline
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One way to improve the situation is to disentangle the envelope
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specification from the note duration, and more generally make it
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easier to specify things that a composer will frequently need to
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change (pitch, articulation, duration) without having to change the
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underlying voice (waveform/envelope settings).
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This proposal does four things:
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1. Add a two-byte `duration` port that configures a note's duration
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in milliseconds. The longest possible note is about 66 seconds.
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2. Double the size of the `adsr` port. This means replacing the
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existing two-byte port with four one-byte ports for `attack`,
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`decay`, `sustain`, and `release`. Since we have 4 extra bits per
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stage, we will reduce the resolution of each stage from 66ms to
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10ms (so 0x01 means 10ms). The longest envelope stage is now about
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26s (up from 1s previously).
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3. Add a one-byte `mode` port, which declares what kind of note or
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sound is being played. This provides an easy way to specify
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different behaviors such as:
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* staccato, legato, or standard playing styles
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* different sample rates (44.1, 22.05, 11.025)
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* looping or non-looping playback
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4. Move the `volume` port to `0x5` and add a one-byte `detune` port.
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A zero value (`0x00`) indicates a "normal" semitone pitch, and
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non-zero values indicate a fractional amount to add. The
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calculation is that the pitch is raised by `detune/256` cents. For
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example, a value of `0x80` will raise the pitch by a quarter-tone.
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The port is placed just before `pitch` so that microtonal music
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can write a "micro-pitch" using one `DEO2` instruction.
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## Microtonal music
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Here's how to encode the 17-tone equal temperment scale (17ET) as
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`detune/pitch` pairs starting from middle C (`0x3c`). Since each step
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of the scale consists of 70.588 cents, we can get accurate pitches and
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detunes by adding 70.588 for each step then dividing by 100 and using
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the quotient and remainder:
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```
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pitch 1: #003c (0 semitones + 0.00 cents)
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pitch 2: #b53c (0 semitones + 70.59 cents)
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pitch 3: #693d (1 semitones + 41.18 cents)
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pitch 4: #1e3e (2 semitones + 11.76 cents)
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pitch 5: #d33e (2 semitones + 82.35 cents)
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pitch 6: #883f (3 semitones + 52.94 cents)
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pitch 7: #3c40 (4 semitones + 23.53 cents)
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pitch 8: #f140 (4 semitones + 94.12 cents)
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pitch 9: #a641 (5 semitones + 64.70 cents)
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pitch 10: #5a42 (6 semitones + 35.29 cents)
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pitch 11: #0f43 (7 semitones + 5.88 cents)
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pitch 12: #c443 (7 semitones + 76.47 cents)
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pitch 13: #7844 (8 semitones + 47.06 cents)
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pitch 14: #2d45 (9 semitones + 17.64 cents)
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pitch 15: #e245 (9 semitones + 88.23 cents)
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pitch 16: #9746 (10 semitones + 58.82 cents)
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pitch 17: #4b47 (11 semitones + 29.41 cents)
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pitch 18: #0048 (12 semitones + 0.00 cents)
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```
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While it's somewhat cumbersome to calculate these detune values in
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advance, it only has be done for one octave and the resulting
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microtonal pitches can be compactly stored and used.
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## Note duration and tempo
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The `duration` and `vector` ports precisely specify the audio device
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behavior. The given note should be played for a number of milliseconds
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specified by `duration`, at which point the `vector` should be called
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to play the next note (or next silence). If the specified ADSR ports
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have a shorter duration, the *mode* defines how to extend the pitch
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(using the *note type* bits). If the ADSR ports have a longer
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duration, then the ADSR will be shortened to fit, starting with S/R
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but also truncating D and A if necessary.
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Composers can choose a duration for the smallest subdivision needed
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(e.g. 125ms per 16th note to achieve 120 bpm) and then compute precise
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durations for 8th notes, quarter notes, dotted-8th notes, whole notes,
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and so on. Similarly, composers can use the same envelope with
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stacatto and legato notes to easily achieve different articulations
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for different passages.
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## More flexible envelope and waveform settings
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The new envelope duration range (10ms to 25s) allows more more complex
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envelopes to be specified, from slow builds and fades to very fast
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attacks and releases. Similarly, allowing waveforms to be specified at
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lower sampling rates potentially allows more interesting percussion
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instruments to be specified without using too many bytes of the ROM.
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For comparison, the NES uses variable frequency samples to allow basic
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voices/sounds without using too much space. For NTSC devices the
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supported range is 4182-33144 Hz.
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## Appending A: proposed specification:
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ADDR SIZE NAME DESCRIPTION
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0x30 2 bytes vector callback address to use when note finishes playing
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0x32 2 bytes duration (new) duration to play sound in fractional seconds (1ms resolution)
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0x34 1 byte mode (new) configures how to interpret addr/adsr/pitch (see below)
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0x35 1 byte volume (moved) 4-bit volumes for left/right channels (6.7% resolution)
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0x36 1 byte attack (new) envelope: attack duration (vol 0-100%, 10ms resolution)
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0x37 1 byte decay (new) envelope: decay duration (vol 100-50%, 10ms resolution)
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0x38 1 byte sustain (new) envelope: sustain duration (vol 50%, 10ms resolution)
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0x39 1 byte release (new) envelope: release duration (vol 50-0%, 10ms resolution)
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0x3a 2 bytes length length of waveform data to read (in bytes)
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0x3c 2 bytes addr address to read waveform data from
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0x3e 1 byte detune (new) fraction of semitone to raise (0x80 gives a quarter tone)
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0x3f 1 byte pitch 1-bit loop and 7-bit MIDI note (0x00 gives silence)
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MODES
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Mode consists of the bits `Lxxx WWNN`.
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The `N` bits correspond to note type:
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```
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0x00 (xxxx xx00) standard note (uses ADSR, extends with silence)
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0x01 (xxxx xx01) staccato note (uses ADR, ignores S, extends with silence)
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0x02 (xxxx xx10) legato note (uses ADSR, extends S as needed)
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0x03 (xxxx xx11) slurred note (uses S, ignores ADR, extends S as needed)
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```
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Since we are no longer computing note duration from the ADSR
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durations, the note type specifies what to do when the duration is
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different than the envelope. For shorter durations, the sustain and/or
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release are truncated; for longer durations it varies by type.
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The `W` bits correspond to waveform type:
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```
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0x00 (xxxx 00xx) waveform sampled at 44100 Hz (44.1 kHz)
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0x40 (xxxx 01xx) waveform sampled at 22050 Hz
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0x80 (xxxx 10xx) waveform sampled at 11025 Hz
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0xc0 (xxxx 11xx) waveform sampled at 5512 Hz
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```
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Upsampling will be performed by repeating sample values as many times
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as needed (2x, 4x, or 8x). The underlying sound engine is still
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expected to play sounds at 44.1 kHz.
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The `L` bit corresponds to whether to loop or not:
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```
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0x00 (0xxx xxxx) play once (do not loop)
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0x80 (1xxx xxxx) repeat note indefinitely
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```
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Looping will continue until a new `pitch` is written (at which point
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that note's looping behavior will be used).
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## Appendix B: not currently supported
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There are some features which would be nice to add but which are not
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strictly necessary and would require more significant changes. They
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could potentially be supported in the future using additional bits
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from the `mode` port, by new devices, or by a larger change.
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* Vibrato/Tremelo
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* Portamento/Glissando/Glide
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* Effects (reverb, equalization, overdrive, etc.)
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* Frequency generators/software synths
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## Appendix C: existing specification
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ADDR SIZE NAME DESCRIPTION
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0x30 2 bytes vector callback address to use when note finishes playing
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0x32 2 bytes position read current position in sample
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0x34 1 byte output read envelope loudness at this moment (0x000 to 0x888)
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0x35 (unused)
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0x36 (unused)
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0x37 (unused)
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0x38 2 bytes adsr four 4-bit envelope values (attack/decay/sustain/release)
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0x3a 2 bytes length length of waveform data to read in bytes
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0x3c 2 bytes addr address to read waveform data from
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0x3e 1 byte volume 4-bit volumes for left/right channels
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0x3f 1 byte pitch 1-bit loop and 7-bit MIDI note
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@ -0,0 +1,10 @@
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# print 17-tone equal temperment
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base = 0x3c # start on middle c
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for n in range(0, 18):
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pitch = n * 70.588
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whole = int(pitch // 100)
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cents = pitch % 100
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frac = round((256 * cents) / 100)
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print(' pitch %2d: #%02x%02x (%2d semitones, %5.2f cents)' % (n + 1, frac, base + whole, whole, cents))
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@ -1,3 +1,5 @@
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( synth demo using old device )
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|30 @Audio0 [ &vec $2 &pos $2 &out $1 &pad $3 &adsr $2 &len $2 &addr $2 &vol $1 &pitch $1 ]
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|0000
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@ -6,7 +8,8 @@
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|0100
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#ff .Audio0/vol DEO
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;square .Audio0/addr DEO2
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#0002 .Audio0/len DEO2
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( #0002 .Audio0/len DEO2 )
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#0008 .Audio0/len DEO2
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;play .Audio0/vec DEO2
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( fall-through )
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@ -30,4 +33,5 @@
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1b 1b 1b 27 1b 27
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1d 1d 1d 29 1d 29
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@square ff 00
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( @square ff 00 )
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@square ff ff ff ff 00 00 00 00
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