pyjch

A standalone, pure-Python reader and player for JCH NewPlayer C64 SID songs (the JCH / Jens-Christian Huus tracker player used across the demoscene). It parses a JCH tune into a typed song model and runs the playroutine to produce byte-exact per-frame SID register output, plus a register-log surface for downstream tooling.

Everything (read/play/register-log) is pure stdlib — no dependencies.

pip install pyjch

Quick start

import pyjch

song = pyjch.read("tune.sid")            # PSID/RSID/.sid or bare .prg

# Per-frame SID register writes (changed registers only, after frame 0).
for writes in pyjch.iter_frames(song, max_frames=50 * 60):
    ...                                  # writes: list[(register, value)]

# Forward-filled 25-register-per-frame snapshot grid (the oracle form).
grid = pyjch.render_grid(song, nframes=400)

# Register log (clock reg val triples).
pyjch.write_reglog(
    pyjch.iter_register_writes(song, max_frames=2500), "tune.reglog"
)

CLI:

pyjch info   tune.sid
pyjch reglog tune.sid tune.reglog --seconds 30

Public API

• read(src) -> Song / parse(bytes) -> Song — read a .sid/.prg.
• Player(song) — .play_frame() -> list[(reg, val)], .regs.
• iter_frames(song, max_frames) — per-frame writes.
• render_grid(song, nframes) -> list[list[int]] — forward-filled grid.
• iter_register_writes / read_reglog / write_reglog / RegWrite.
• Model: Song (with orderlist_ptr / subpattern_ptr resolution).
• Errors: JCHError, SidParseError.

The JCH NewPlayer format

Three independent per-voice opcode streams. The player binary is identical across tunes; only its DATA and the addresses that data lives at differ, so the reader discovers each per-tune immediate and table base from the player code's instruction operands (relocation-safe — no fixed-address assumption):

• AD/SR defaults — image[load+0xAA] / image[load+0xB5].
• Gate-on / gate-off (rest) CTRL — image[load+0x1E1] / image[load+0x199].
• Subpattern pointer-table bases — the 16-bit operands at load+0x12C (lo) / load+0x131 (hi).
• Orderlist pointers — three per-voice pairs at $1010, indexed by subtune * 8; the 7th byte seeds the tempo.
• Frequency tables — $121F (lo) / $1220 (hi), 0x80 entries, indexed by note with a per-voice transpose.

Init (FUN_1060) copies the orderlist pointer pairs into per-voice cur/base pointers, loads the tempo, and seeds the SID registers. Play (FUN_10E8) walks each voice's opcode stream via the zero-page pointer $fb/$fc: transpose-set opcodes ($80–$9F), subpattern references (<$80 in the orderlist push a pointer and jump via the subpattern tables), gate/tempo counters ($80–$8F inside a subpattern), and note bytes that index the frequency table.

Byte-exact verdict

Validated against the preframr-sidtrace register oracle (PAL, 19656 cycles/frame, forward-filled, one leading silent play-call aligned over, PW-hi registers nibble-masked):

Tests

Test tunes are HVSC copyright works and are never committed. They are resolved from a local HVSC tree ($JCH_LOCAL_HVSC) or fetched on demand (python scripts/fetch_tunes.py) into a gitignored cache; the byte-exact tests validate against the live preframr-sidtrace binary when $SIDTRACE_BIN is set, else against the committed frozen oracle grids in tests/fixtures/, so CI passes with neither the binary nor the tunes.

pip install -e ".[dev]"
./run_tests.sh
SIDTRACE_BIN=/path/to/sidtrace ./run_tests.sh   # validate live, too

License

Apache 2.0 — see LICENSE.