Overview
ISO/IEC 15200:1996 specifies the Adaptive Lossless Data Compression (ALDC) algorithm - a standardized, lossless compression method intended to reduce the number of bytes required to represent data. ALDC is an implementation of the LZ1 (Lempel–Ziv I) class of compression algorithms and was registered (ECMA/ISO) for use on high-density storage media. The standard defines encoding formats, data stream structure, and conformance rules for ALDC implementations.
Key topics and technical requirements
- Algorithm class: LZ1-style sliding-window compression using a History Buffer to find repeated sequences (matching strings).
- History Buffer sizes (registered variants):
- ALDC (512-byte History Buffer) - identifier 3
- ALDC (1024-byte History Buffer) - identifier 4
- ALDC (2048-byte History Buffer) - identifier 5
- Encoding primitives:
- Literals (single bytes not matched in history) are emitted preceded by a bit set to 0.
- Copy Pointers (references to matching strings in the History Buffer) are emitted preceded by a bit set to 1 and contain a Match Count Field and Displacement Field.
- End Marker: a sequence of twelve 1 bits marks the end of the compressed data stream; Pad Bits (zeros) are added to maintain byte alignment.
- Field sizes and limits:
- Displacement Field length: 9, 10, or 11 bits for 512, 1024, or 2048-byte buffers respectively.
- Match Count Field length: variable (2, 4, 6, 8 or 12 bits) to represent match counts up to 271 (271 chosen for implementation reasons).
- Operational rules:
- History Buffer initialized to all zeros and updated identically by encoder/decoder (circular addressing).
- If multiple matches have equal length, the lowest displacement (nearest or oldest as defined) is chosen.
- Conformance is based on the produced compressed data stream meeting the standard’s format and rules.
Applications and users
- Manufacturers of magnetic tape, optical media, and other high-density storage devices seeking standardized lossless compression.
- Firmware and systems engineers implementing compression/decompression for archival, backup, or storage subsystems.
- Software developers integrating ALDC as an option for data interchange where standardized compression identifiers are required.
- System integrators and standards compliance teams validating encoded streams for interoperability.
Related standards
Keywords: ISO/IEC 15200:1996, ALDC, Adaptive Lossless Data Compression, lossless compression algorithm, history buffer, copy pointer, match count, displacement field, LZ1, data compression standard.
