198 lines
8.7 KiB
Markdown
198 lines
8.7 KiB
Markdown
# 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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2.6s (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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* articulation (e.g. staccato, legato, etc.)
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* different sample rates (44.1k, 22.05k, 11.025k, 5512)
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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`, i.e. `#3c`).
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Since each step of the scale consists of 70.588 cents, we can get
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accurate pitches and detunes by adding 70.588 for each step then
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dividing by 100 and using the quotient and remainder:
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```
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pitch 1: #003c (0 semitones + 0.00 cents) -- root note is C (#3c)
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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) -- almost perfect 5th (#43)
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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) -- octave is C (#48)
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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. If duration is zero the
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duration will be calculated dynamically from ADSR, as it is now.
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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 2.6s) allows more more
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complex envelopes to be specified, from slower builds and fades to
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very fast attacks and releases. Similarly, allowing waveforms to be
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specified at lower sampling rates potentially allows more interesting
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percussion instruments to be specified without using too many bytes of
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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 `xxWW xAAA`.
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The `A` bits correspond to articulation, which determines how to fill
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extra space when the duration exceeds the envelope length, and also
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which parts of the envelope to exclude (if any) which is denoted with
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an underscore:
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```
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0x00 (xxxx x000) regular (ADSR, pads with silence)
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0x01 (xxxx x001) short (AD_R, pads with silence)
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0x02 (xxxx x010) staccato (_D_R, pads with silence)
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0x03 (xxxx x011) staccatissimo (_D__, pads with silence)
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0x04 (xxxx x100) legato (ADSR, extends S)
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0x05 (xxxx x101) begin slur (ADS_, extends S)
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0x06 (xxxx x110) slur (__S_, extends S)
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0x07 (xxxx x111) end slur (__SR, extends S)
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```
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For shorter durations, the envelope is truncated starting from the
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end. For example, with `short` articulation the release will be
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truncated first, then the decay, and finally the attack.
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The `W` bits correspond to waveform type:
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```
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0x00 (xx00 xxxx) waveform sampled at 44100 Hz (44.1 kHz)
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0x40 (xx01 xxxx) waveform sampled at 22050 Hz
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0x80 (xx10 xxxx) waveform sampled at 11025 Hz
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0xc0 (xx11 xxxx) 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. For sounds which are not samples
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at a lower frequency using one of the lower frequencies will have the
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effect of "pitch shifting" the sample down one or more octaves.
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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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* Looping notes
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If we want to support even slower builds/fades we could use existing
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mode bits to change the units used by the envelope. That would allow
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us to specify larger resolutions.
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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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