diff --git a/audio.md b/audio.md new file mode 100644 index 0000000..aa033f8 --- /dev/null +++ b/audio.md @@ -0,0 +1,193 @@ +# UXN Audio Proposal + +## Problems + +Currently the UXN audio device doesn't work very well for playing +complex music. There are a few reasons for this: + + * Note duration is conflated with envelope shape + * Envelope resolution (67ms) limits tempos/subdivisions + * Microtonal music is not possible (according to the spec) + * Using audio callback requires scheduling pauses/silence + +## Proposal outline + +One way to improve the situation is to disentangle the envelope +specification from the note duration, and more generally make it +easier to specify things that a composer will frequently need to +change (pitch, articulation, duration) without having to change the +underlying voice (waveform/envelope settings). + +This proposal does four things: + + 1. Add a two-byte `duration` port that configures a note's duration + in milliseconds. The longest possible note is about 66 seconds. + + 2. Double the size of the `adsr` port. This means replacing the + existing two-byte port with four one-byte ports for `attack`, + `decay`, `sustain`, and `release`. Since we have 4 extra bits per + stage, we will reduce the resolution of each stage from 66ms to + 10ms (so 0x01 means 10ms). The longest envelope stage is now about + 26s (up from 1s previously). + + 3. Add a one-byte `mode` port, which declares what kind of note or + sound is being played. This provides an easy way to specify + different behaviors such as: + * staccato, legato, or standard playing styles + * different sample rates (44.1, 22.05, 11.025) + * looping or non-looping playback + + 4. Move the `volume` port to `0x5` and add a one-byte `detune` port. + A zero value (`0x00`) indicates a "normal" semitone pitch, and + non-zero values indicate a fractional amount to add. The + calculation is that the pitch is raised by `detune/256` cents. For + example, a value of `0x80` will raise the pitch by a quarter-tone. + The port is placed just before `pitch` so that microtonal music + can write a "micro-pitch" using one `DEO2` instruction. + +## Microtonal music + +Here's how to encode the 17-tone equal temperment scale (17ET) as +`detune/pitch` pairs starting from middle C (`0x3c`). Since each step +of the scale consists of 70.588 cents, we can get accurate pitches and +detunes by adding 70.588 for each step then dividing by 100 and using +the quotient and remainder: + +``` + pitch 1: #003c (0 semitones + 0.00 cents) + pitch 2: #b53c (0 semitones + 70.59 cents) + pitch 3: #693d (1 semitones + 41.18 cents) + pitch 4: #1e3e (2 semitones + 11.76 cents) + pitch 5: #d33e (2 semitones + 82.35 cents) + pitch 6: #883f (3 semitones + 52.94 cents) + pitch 7: #3c40 (4 semitones + 23.53 cents) + pitch 8: #f140 (4 semitones + 94.12 cents) + pitch 9: #a641 (5 semitones + 64.70 cents) + pitch 10: #5a42 (6 semitones + 35.29 cents) + pitch 11: #0f43 (7 semitones + 5.88 cents) + pitch 12: #c443 (7 semitones + 76.47 cents) + pitch 13: #7844 (8 semitones + 47.06 cents) + pitch 14: #2d45 (9 semitones + 17.64 cents) + pitch 15: #e245 (9 semitones + 88.23 cents) + pitch 16: #9746 (10 semitones + 58.82 cents) + pitch 17: #4b47 (11 semitones + 29.41 cents) + pitch 18: #0048 (12 semitones + 0.00 cents) +``` + +While it's somewhat cumbersome to calculate these detune values in +advance, it only has be done for one octave and the resulting +microtonal pitches can be compactly stored and used. + +## Note duration and tempo + +The `duration` and `vector` ports precisely specify the audio device +behavior. The given note should be played for a number of milliseconds +specified by `duration`, at which point the `vector` should be called +to play the next note (or next silence). If the specified ADSR ports +have a shorter duration, the *mode* defines how to extend the pitch +(using the *note type* bits). If the ADSR ports have a longer +duration, then the ADSR will be shortened to fit, starting with S/R +but also truncating D and A if necessary. + +Composers can choose a duration for the smallest subdivision needed +(e.g. 125ms per 16th note to achieve 120 bpm) and then compute precise +durations for 8th notes, quarter notes, dotted-8th notes, whole notes, +and so on. Similarly, composers can use the same envelope with +stacatto and legato notes to easily achieve different articulations +for different passages. + +## More flexible envelope and waveform settings + +The new envelope duration range (10ms to 25s) allows more more complex +envelopes to be specified, from slow builds and fades to very fast +attacks and releases. Similarly, allowing waveforms to be specified at +lower sampling rates potentially allows more interesting percussion +instruments to be specified without using too many bytes of the ROM. + +For comparison, the NES uses variable frequency samples to allow basic +voices/sounds without using too much space. For NTSC devices the +supported range is 4182-33144 Hz. + +## Appending A: proposed specification: + +ADDR SIZE NAME DESCRIPTION +0x30 2 bytes vector callback address to use when note finishes playing +0x32 2 bytes duration (new) duration to play sound in fractional seconds (1ms resolution) +0x34 1 byte mode (new) configures how to interpret addr/adsr/pitch (see below) +0x35 1 byte volume (moved) 4-bit volumes for left/right channels (6.7% resolution) +0x36 1 byte attack (new) envelope: attack duration (vol 0-100%, 10ms resolution) +0x37 1 byte decay (new) envelope: decay duration (vol 100-50%, 10ms resolution) +0x38 1 byte sustain (new) envelope: sustain duration (vol 50%, 10ms resolution) +0x39 1 byte release (new) envelope: release duration (vol 50-0%, 10ms resolution) +0x3a 2 bytes length length of waveform data to read (in bytes) +0x3c 2 bytes addr address to read waveform data from +0x3e 1 byte detune (new) fraction of semitone to raise (0x80 gives a quarter tone) +0x3f 1 byte pitch 1-bit loop and 7-bit MIDI note (0x00 gives silence) + +MODES + +Mode consists of the bits `Lxxx WWNN`. + +The `N` bits correspond to note type: + +``` +0x00 (xxxx xx00) standard note (uses ADSR, extends with silence) +0x01 (xxxx xx01) staccato note (uses ADR, ignores S, extends with silence) +0x02 (xxxx xx10) legato note (uses ADSR, extends S as needed) +0x03 (xxxx xx11) slurred note (uses S, ignores ADR, extends S as needed) +``` + +Since we are no longer computing note duration from the ADSR +durations, the note type specifies what to do when the duration is +different than the envelope. For shorter durations, the sustain and/or +release are truncated; for longer durations it varies by type. + +The `W` bits correspond to waveform type: + +``` +0x00 (xxxx 00xx) waveform sampled at 44100 Hz (44.1 kHz) +0x40 (xxxx 01xx) waveform sampled at 22050 Hz +0x80 (xxxx 10xx) waveform sampled at 11025 Hz +0xc0 (xxxx 11xx) waveform sampled at 5512 Hz +``` + +Upsampling will be performed by repeating sample values as many times +as needed (2x, 4x, or 8x). The underlying sound engine is still +expected to play sounds at 44.1 kHz. + +The `L` bit corresponds to whether to loop or not: + +``` +0x00 (0xxx xxxx) play once (do not loop) +0x80 (1xxx xxxx) repeat note indefinitely +``` + +Looping will continue until a new `pitch` is written (at which point +that note's looping behavior will be used). + +## Appendix B: not currently supported + +There are some features which would be nice to add but which are not +strictly necessary and would require more significant changes. They +could potentially be supported in the future using additional bits +from the `mode` port, by new devices, or by a larger change. + + * Vibrato/Tremelo + * Portamento/Glissando/Glide + * Effects (reverb, equalization, overdrive, etc.) + * Frequency generators/software synths + +## Appendix C: existing specification + +ADDR SIZE NAME DESCRIPTION +0x30 2 bytes vector callback address to use when note finishes playing +0x32 2 bytes position read current position in sample +0x34 1 byte output read envelope loudness at this moment (0x000 to 0x888) +0x35 (unused) +0x36 (unused) +0x37 (unused) +0x38 2 bytes adsr four 4-bit envelope values (attack/decay/sustain/release) +0x3a 2 bytes length length of waveform data to read in bytes +0x3c 2 bytes addr address to read waveform data from +0x3e 1 byte volume 4-bit volumes for left/right channels +0x3f 1 byte pitch 1-bit loop and 7-bit MIDI note diff --git a/microtone.py b/microtone.py new file mode 100644 index 0000000..8fbbc95 --- /dev/null +++ b/microtone.py @@ -0,0 +1,10 @@ +# print 17-tone equal temperment + +base = 0x3c # start on middle c + +for n in range(0, 18): + pitch = n * 70.588 + whole = int(pitch // 100) + cents = pitch % 100 + frac = round((256 * cents) / 100) + print(' pitch %2d: #%02x%02x (%2d semitones, %5.2f cents)' % (n + 1, frac, base + whole, whole, cents)) diff --git a/synth.tal b/synth.tal index 0a7442b..8211837 100644 --- a/synth.tal +++ b/synth.tal @@ -1,3 +1,5 @@ +( synth demo using old device ) + |30 @Audio0 [ &vec $2 &pos $2 &out $1 &pad $3 &adsr $2 &len $2 &addr $2 &vol $1 &pitch $1 ] |0000 @@ -6,7 +8,8 @@ |0100 #ff .Audio0/vol DEO ;square .Audio0/addr DEO2 - #0002 .Audio0/len DEO2 +( #0002 .Audio0/len DEO2 ) + #0008 .Audio0/len DEO2 ;play .Audio0/vec DEO2 ( fall-through ) @@ -30,4 +33,5 @@ 1b 1b 1b 27 1b 27 1d 1d 1d 29 1d 29 -@square ff 00 +( @square ff 00 ) +@square ff ff ff ff 00 00 00 00