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victrackerplugin

Plays the modland Vic-Tracker corpus: Commodore VIC-20 music in Daniel Kahlin's VIC-TRACKER (.vt) format.

How it works

A .vt file is a Commodore VIC-20 PRG — a 2-byte $3300 load address followed by Kahlin's T1 (or legacy T0) tune struct. The tracker's own 6502 replay routine interprets that struct in place and writes the VIC-20 sound registers $900A–$900E. We reproduce the VIC-20 IRQ loop exactly:

  1. Load the tune image at $3300 (vt_machine.cpp).
  2. Run pl_Init, then pl_Play once per interrupt tick (rate from the tune's pl_PlayMode byte) on the MyLittle6502 CPU core (vt_cpu.h).
  3. After each tick, latch $900A–$900E into the VIC-I sound core and render 44100 Hz samples.

Vendored components / licensing

Component Source License
victracker/ player sources (player.asm, playerdata.asm, includes) Daniel Kahlin, VIC-TRACKER 2.0 (http://www.kahlin.net/daniel/victracker/) BSD (see victracker/LICENSE.txt)
pd6502.h (6502, behind vt_cpu.h) MyLittle6502 public domain / CC0
vic_sound.c (VIC-I) written here —

Nothing is GPL, so the plugin ships in both build variants and all 11 .vt songs stay in the mas index.

Why these cores, and not the obvious ones

This plugin originally ran on the omarandlorraine fake6502 fork (GPLv2) and on a VIC-I core lifted from VICE's vic20/vic20sound.c (GPLv2). That made it unshippable on the Mac App Store, but — unlike uadeplugin, vicepluginbridge and goattrackerplugin — the GPL was internal rather than structural: the plugin is ours and only its two engine cores were the problem. Both were replaced in place, and the replacements were A/B'd by ear against the originals before the GPL ones were deleted, so there is no variant gate and nothing hidden from the index.

pd6502.h is the same code the GPL fork descends from — Mike Chambers' Fake6502 v1.1 (2011), released into the public domain — continued by gek169 as MyLittle6502 with decimal-mode, BIT and interrupt-masking fixes. Vendored unchanged; vt_cpu.c prefixes its non-static symbols (read6502, step6502, callexternal, …) before including it, since those names are far too generic for a link with ~60 other plugins.

vic_sound.c avoids the two things that made VICE's file a licensing problem, both of them data rather than logic:

  • VICE ships a 1024-byte dump of the noise generator's output. The generator is documented (Asger Alstrup Nielsen): a 23-bit shift register seeded 0x7ffff8 whose new bit 0 is bit 22 XOR bit 13. It is generated here, with the register geometry and the 128 - ((reg + 1) & 0x7f) divider mapping taken from MAME's src/devices/sound/mos6560.cpp (BSD-3-Clause, Peter Trauner).
  • VICE ships a ~350-entry table of measured output voltages — the authors' own measurement data. This uses a parametric saturating DAC instead, followed by a 2.5 kHz one-pole low-pass and a 20 Hz DC block.

The rest is the chip itself: three tone channels, each an 8-bit shift register clocked at clock/(divider << 4|3|2) with the inverted MSB fed back (so it emits a square wave at 1/16 of its shift rate), a noise channel on the same ladder, and a 4-bit master volume.

The one place the two cores measurably differed was the shape of that DAC curve. VICE's measured table rises steeply and then saturates hard; VIC_DAC_GAIN / VIC_DAC_LIMIT are a soft knee fitted to land in the same place rather than to trace the same path. Across the five tunes in chipmachine/testmus/victracker they agreed on RMS to within about 1 dB (0.89–0.97×), with peaks 8% under full scale, and were indistinguishable by ear. Retune those two constants if the output ever needs adjusting — do not reconstruct VICE's table.

Regenerating the player blob

vtplayer_bin.h is Kahlin's player pre-assembled (BSD) as a position-fixed 6502 blob loaded at $2000, so the build needs no assembler. To rebuild it after touching the vendored .asm sources, install dasm (brew install dasm) and run:

cd victracker && ./build_player.sh