Compressor

A simple audio compression programme, using techniques similar to MP3.

Currently targets Linux only.

This is a proof of concept. Do not use in production, do not expect support, do not submit PRs, issue tickets.

Installation

Requires Rust.

Usage

compressor only accepts wav files.

Encode a wav file:

# .xprs is the suggested files extension for compressed audio files
cargo run --release encode audio/mywavfile.wav audio/mycompressedfile.xprs

Decode and play a compressed file:

cargo run --release decode audio/mycompressedfile.xprs

Full usage:

Usage: compressor ACTION SRC [DEST] [opts...]

Encoding:
	compressor encode SRC DEST [--ratio RATIO] [--storage-bits BITS]

RATIO is the compression ratio. Default 8, must be >= 1.
BITS is the number of bits to use in storing float values. Accepted values are 16 (default), 32.

Decoding:
	compressor decode SRC

Details

Encoding

The input file is divided up into 'chunks', with a set number of samples in each.

The Fourier transform of each source chunk is taken, and the N 'loudest' frequencies are kept, the rest are discarded. These N frequencies and their Fourier transform values are written to a file, in groups of N samples ('encoded chunks').

Decoding

The inverse Fourier transform of each encoded chunk is taken, setting all frequencies not present in the file to have a value of 0. This gives, more or less, the original signal.

The file format

All numbers are 32-bit LE signed integers unless stated otherwise.

• 4 bytes: "XPRS" - the filetype magic number
• 4 bytes: the size of each source chunk
• 4 bytes: the number of chunks in the "DATA" section
• 4 bytes: the size of each encoded chunk
• 4 bytes: the (integer) sample frequency
• 4 bytes: the number of channels
• 2 bytes: the storage type. 1 = f16, 2 = f32.
• 4 bytes: "DATA" - the data header
• variable size: the compressed data chunks (interleaved), with each chunk in the format:
  • 2 bytes: the chunk discrete frequency (16-bit unsigned LE integer)
  • 2/4 bytes: the DFT real part (16/32-bit IEEE float, LE)
  • 2/4 bytes: the DFT imaginary part (16/32-bit IEEE float, LE)