Format Overview

BIOPAC AcqKnowledge .acq files are binary files used to store multi-channel physiological data recorded with BIOPAC hardware. The format has evolved through more than 50 revisions since the early 1990s, spanning big-endian Classic Mac 68k builds through modern Windows/macOS 64-bit applications.

Format versions

All version identifiers are stored in the lVersion field of the graph header as a signed 32-bit integer. Negative values indicate big-endian byte order (legacy Mac builds).

File structure (top-level sections)

Every .acq file is a flat sequence of typed sections:

[GraphHeader]
[ChannelHeader × channel_count]
[CompressionHeader × channel_count]  — only in revision ≥ 68
[DataSection]                         — interleaved sample data
[MarkerSection]                       — event markers (optional)
[JournalSection]                      — free-text journal (optional)
[ForeignDataSection …]                — unknown/vendor-specific sections

Interleave pattern

Sample data is written in interleaved blocks. Each channel writes frequency_divider worth of samples per "tick" of the base rate. A full interleave block is:

ch0_samples[0..n0] | ch1_samples[0..n1] | … | chN_samples[0..nN]

where nI = base_samples_per_block / channel[I].frequency_divider.

Biodream's streaming reader consumes exactly one interleave block at a time, so memory usage is O(block_size) not O(file_size).

Further reading

The field-by-field binary layout (graph header, channel header, compression header, all section types) is documented in the internal ADR and in the parser source at src/parser/.