Raspberry Casket

A fast and small open source Pretracker replayer

Raspberry Casket Player V1.0 (26-Dec-2022)

Provided by Chris 'platon42' Hodges chrisly@platon42.de

Rewritten by platon42/Desire based on a resourced, binary identical version of the original Pretracker V1.0 replayer binary provided by hitchhikr (thanks!), originally written in C by Pink/Abyss.

This version is the hard work of reverse engineering all the offsets, removing all the C compiler crud, removing dead and surplus code (maybe artefacts from earlier ideas that did nothing), optimizing the code where possible. This resulted in both reduced size of the replayer, faster sample calculation and speeding the tick routine up significantly.

I also added a few optional features that come in handy, such as song-end detection and precalc progress support.

It took me more than a month and it was not fun.

Also: Open source. It's 2022, keeping the code closed is just not part of the demoscene spirit (anymore?), at least for a replayer.

Also note that this is not the final state of the source code. I could go over many places still and try to rework them. But I wanted the code to be out in public.

Verification

The replayer has been verified on about 60 Pretracker tunes to create an identical internal state for each tick and identical samples (if certain optimizations switches are disabled).

I might have introduced bugs though. If you find some problems, please let me know under chrisly@platon42.de. Thank you.

Usage

The new replayer comes as a drop-in binary replacement if you wish. In this case you will get faster sample generation (about 12% faster on 68000) and about 45% less CPU time spent. However, you won't get stuff as song-end detection and precalc progress this way. This mode uses the old CPU DMA wait that takes away 8 raster lines.

If you want to get rid of the unnecessary waiting, you can switch to a copper driven audio control. If you want to use the top portion of the copperlist for this, you probably need to double buffer it. Otherwise, you could also position the copperlist at the end of the display and use single buffering if you call the tick routine during the vertical blank.

Please use the documented sizes for the MySong and MyPlayer data structures, which are the symbols sv_SIZEOF and pv_SIZEOF respectively (about 2K and 12K with volume table).

The source needs two common include files to compile (custom.i and dmabits.i). You should leave assembler optimizations enabled.

1. (If you're using copper list mode, call pre_PrepareCopperlist.)
2. Call pre_SongInit with
   • a pointer to MySong (mv_SIZEOF) in a1 and
   • the music data in a2. It will return the amount of sample memory needed in d0.
3. Then call pre_PlayerInit with
   • a pointer to MyPlayer (pv_SIZEOF) in a0
   • a pointer to chip memory sample buffer in a1
   • the pointer to MySong in a2
   • a pointer to a longword for progress information or null in a3 This will create the samples, too.
4. After that, regularly call pre_PlayerTick with MyPlayer in a0 and optionally the copperlist in a1 if you're using that mode).

Size

The original C compiled code was... just bad. The new binary is about 1/3 of the original one.

The code has been also optimized in a way that it compresses better. The original code compressed with Blueberry's Shrinkler goes from 18052 bytes down to 9023 bytes.

Raspberry Casket, depending on the features compiled in, is about 6374 bytes and goes down to ~4410 bytes (in isolation).

So this means that the optimization is not just "on the outside".

Timing

Unfortunately, the replayer is still pretty slow and has high jitter compared to other standard music replayers.

This means it may take up to 33 raster lines (14-19 on average) which is significant more than a standard Protracker replayer (the original one could take about 60 raster lines worst case and about 34 on average!).

Watch out for Presto, the LightSpeedPlayer variant that should solve this problem.

Changelog

V1.1 (unreleased)

• Optimized base displacement by reordering variables
• Further optimized ADSR code
• Optimized wave loop code
• Bake in this strange vibrato speed multiplication to precalculated vibrato value (where possible)

V1.0 (26-Dec-22)

• Initial release