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130 lines
4.0 KiB
Plaintext
130 lines
4.0 KiB
Plaintext
The PT3_player
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~~~~~~~~~~~~~~
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by Vince "Deater" Weaver
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17 May 2019
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http://www.deater.net/weave/vmwprod/pt3_player/
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Plays Vortex Tracker II .pt3 files on the Apple II
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Background
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~~~~~~~~~~
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Vortex Tracker is commonly used to create AY-3-8910 music for the
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ZX Spectrum and Atari ST systems. A large number of great pt3 files
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can be found on the internet.
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There are many benefits to using .pt3 format, but until now there
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was no player available for Apple II/6502. (Though some people
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had been playing the files using a z80-card in their system).
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Part of the challenge was the documentation for the format was
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in Russian, and the only source-code available for .pt3
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players were in uncommented z80 assembly or else Russian-commented
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Pascal.
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Why .pt3? Why not play YM5 files?
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There are already many projects (including one of my own) for
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playing the .YM file format, which is much simpler. It is just a
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series of AY-3-8910 raw-register dumps which can quickly be sent
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to the Mockingboard.
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The issue is uncompressed YM5 files are really huge. So you have to
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do some sort of decompression on the fly, and even then you are looking
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at 32k+ or more of RAM. This might be fine on an Apple IIe with
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128k of RAM, but I am targetting a II+ where I don't have that much
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available.
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A PT3 file is a tracker format, meaning a list of notes, patterns, and
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ornaments and can be played in place. Once you load the music file and
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the player, no additional RAM is needed at all. It does take more CPU
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than playing a YM5 file.
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Using the PT3_PLAYER
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~~~~~~~~~~~~~~~~~~~~
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The pt3_player.dsk should boot automatically.
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Controls:
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Right/Left arrow Switch songs
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Spacebar Pause playback.
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M Switch from 1MHz to 1.77MHz mode
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FUTURE FEATURES:
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Key to disable visualization
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Key to skip ahead/back in pattern list
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L to enable looping in the song
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ZX Compatability:
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~~~~~~~~~~~~~~~~~
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ZX Spectrum songs expect a 1.77MHz AY-3-8910, but the Apple II
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is 1MHz so by default songs would sound lower
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pitched. By default now we multiply by 9/16 to scale
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things properly, but that does eat up extra CPU cycles.
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You can toggle this on/off by pressing "M" when playing.
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In theory we can create songs that expect 1MHz, but the pt3 format
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has no way of indicating this.
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Adding other files
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~~~~~~~~~~~~~~~~~~
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Unlike YM5 players which require complicated preparation of files
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(with often complex, or hard-to find tools) the PT3_PLAYER can in
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theory play plain .pt3 files.
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To play your own files, just copy the file you want over an
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existing file (using the same filename).
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TODO: It would be great if the player was smart enough to catalog
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the disk and just play all the files it finds, but that's a level
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of disk manipulation I don't have time to mess with right now.
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Other Future Plans
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~~~~~~~~~~~~~~~~~~
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The PT3 format can handle 6-channel songs (which a Mockingboard
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can play). I'd like to add support for this, but it will
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need some low-level changes to the code.
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Code Optimization
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~~~~~~~~~~~~~~~~~
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The original working code is about 4k (not counting the pt3 file)
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and has an overhead of roughly 20% when playing a song interrupt-driven
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at 50Hz.
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I'm keeping some stats here as I try to optimize the size and speed.
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Song: "Summer of Rain"
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~~~~~~~~~~~~~~~~~~~~~~
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lz4 compressed
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pt3 size: raw size: ym5 size: pt3.lz4:
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3871 137015 7637 1793
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Decoder Type size ZP use decode total CPU overhead
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-------------------------------------------------------------
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Original 4k(?) 22B 28.16s 171s 16%
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Random Programming Reminders
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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ASR = CMP #$80 / ROR
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signed 8-bit comparison
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see http://6502.org/tutorials/compare_beyond.html#2.2
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SEC ; prepare carry for SBC
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SBC NUM ; A-NUM
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BVC LABEL ; if V is 0, N eor V = N, otherwise N eor V = N eor 1
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EOR #$80 ; A = A eor $80, and N = N eor 1
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LABEL
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If the N flag is 1, then A (signed) < NUM (signed) and BMI will branch
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If the N flag is 0, then A (signed) >= NUM (signed) and BPL will branch
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