.AU File Format

Introduction

The AU format was introduced by Sun Microsystems for its Unix workstations, and NeXT used the same layout with the extension .snd. It is one of the simplest audio files there is: a short header and then the samples. It was common on Unix systems and on the early web, and many files hold 8-bit µ-law audio at 8000 Hz, the encoding of telephone sound.

Structure

The file starts with the magic number .snd (2E 73 6E 64). Five 32-bit big-endian fields follow: the offset of the audio data, the data size in bytes (FFFFFFFF when unknown), the encoding, the sample rate and the channel count. Any bytes between the header and the data offset are a free-form annotation, often text. The samples follow, big-endian and interleaved by channel.

Common encodings are 1 (8-bit µ-law), 2, 3, 4 and 5 (8-, 16-, 24- and 32-bit linear PCM), 6 and 7 (32- and 64-bit floating point) and 27 (8-bit A-law).

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