/lzss-sap

LZSS compressor for Atari SAP music files

Primary LanguageCMIT LicenseMIT

Atari SAP Type R File Compressor and Player

This is a compressor for Atari POKEY music in SAP Type R format with a player for the Atari 8-bit computers.

The SAP Type R file format is simply a dump of the POKEY registers to write in each frame, and can be produced by many emulators. By compressing each register write with the LZSS format, a smaller file can be produced, and can be played by decompressing in real time in the Atari.

The LZSS compression is an LZ77 variant optimized for small memory and file sizes, and it is very fast to compress and decompress in old hardware.

LZSS compressor: bin/lzss

Program to compress a type R SAP (SAP-R) file into the lzss format.

Usage: bin/lzss [options] <input_file> <output_file>

If output_file is omitted, write to standard output, and if input_file is also omitted, read from standard input.

Options:

  • -8 Sets default 8 bit match size, this is the same as -b 8 -o 4 -m 2.
  • -2 Sets default 12 bit match size, this is the same as -b 12 -o 7 -m 2.
  • -6 Sets default 16 bit match size, this is the same as -b 16 -o 8 -m 1.
  • -o BITS Sets match offset bits (default is 4 bits).
  • -l BITS Sets match length bits (default is 4 bits).
  • -b BITS Sets match total bits (=offset+length) (default is 8 bits).
  • -m NUM Sets minimum match length (default is 2 bits).
  • -e Don't force a literal at end of stream. Normally, the encoder enforces that the last encoded byte of at least one channel is a literal, this allows the decoder to find the end of the song. With this option, the compressed file will be smaller but the decoder won't detect the end correctly.
  • -t Trim the SAP-R data before compressing, removes silences at start and the end and detects looping at the end of the song.
  • -x Reverts to old format version, use for compatibility with old players.
  • -v Shows match length/offset statistics.
  • -q Don't show per stream compression.
  • -h Shows command line help.

The compressed files can be played with the included assembly player sources, there are three sources included:

  • asm/playlzs.asm : This player support the -8 compression option, it uses one byte for each match, with 16 bytes of buffer and a maximum of 17 bytes for each match.

    It uses the smallest amount of memory (16 * 9 bytes of RAM are needed) but does not compresses well.

  • asm/playlzs12.asm : This player support the -2 compression option, it uses 12 bits for each match, with 128 bytes of buffer and a maximum of 33 bytes for each match.

    This uses a larger buffer (128 * 9 bytes of RAM), so it compress almost as good as the 16 bit variant.

  • asm/playlzs16.asm : This player support the -6 compression option, it uses 16 bits (two bytes) for each match, with 256 bytes of buffer and a maximum of 256 bytes for each match.

    This player is faster than the above, as each match is two full bytes, but uses the largest amount of buffer (256 * 9 bytes of RAM). It tends to compress better than all the other, but your mileage may vary depending on the specific SAP file.

Other tools included

There are two other tools included in the repository:

  • bin/lz4s

    This is a compressor for a modified LZ4 compression format. It uses more RAM and compresses worst than the LZSS compressor in most files, it is only included as a reference.

    Tee LZ4 format performs better with larger buffer sizes (more than 1kB), that would imply using more than 8kB of RAM in the player.

  • bin/split

    This simple program just splits a SAP Type R file into one file for the data of each POKEY register, allowing to try external compressors on each stream.