What ATRAC compression is

ATRAC — Adaptive TRansform Acoustic Coding — is the compression scheme Sony built for MiniDisc, launched in 1992 to fit a little over an hour of digital audio on a disc a fraction of a compact disc’s physical size. ATRAC is lossy: it does not store the recording, it stores a version edited down by a model of what the human ear can and cannot hear, at roughly a fifth of the original data. That is the reason a MiniDisc recording, even at its best setting, has a characteristic softness — a slight rounding of transients, a stereo image that can read a touch narrower than the source — audible mostly in what stops being there rather than in anything added.

At its standard rate, ATRAC compresses 16-bit, 44.1-kilohertz stereo audio — about 1.4 megabits a second, uncompressed — down to roughly 292 kilobits a second, while remaining decodable in real time on a battery-powered deck built in the early 1990s. Near-CD duration and playback quality, at about a fifth of the data a CD spends.

The coder splits the audio spectrum into three frequency bands with a filter bank — roughly 0–5.5 kHz, 5.5–11 kHz, and 11–22 kHz — then applies a modified discrete cosine transform (MDCT) inside each band, turning short blocks of the waveform into frequency coefficients. The transform’s window length adapts to the material — long windows for steady, tonal passages, where fine frequency resolution matters; short windows around sharp attacks, where fine time resolution matters more, to limit the transform’s tendency to smear a hard transient across the whole block.

A psychoacoustic model then decides how many bits each block floating unit — a small group of adjacent frequency coefficients, not each coefficient singly — is worth. Loud content masks quieter content nearby in frequency and immediately after it in time — the ear cannot separate them — and the model spends its limited bit budget on what it predicts will remain audible, quantizing or discarding the rest. At ATRAC’s standard rate the model is usually right: the loss sits in cymbal decay, room tone, the trailing edge of a transient, the outer edge of the stereo field, rather than in anything a casual listener would flag as missing.

At the long-play modes added in 2000 — LP2 at 132 kilobits a second, LP4 at 66 — a later coder, ATRAC3, has to fit twice or four times as much material into the same space. At LP4 it runs out of room and codes the two channels jointly, folding the stereo image toward the centre: spatial information traded for bit budget once there is nothing else left to trade.

MiniDisc’s whole premise depended on this trade: an album on a disc a fraction of a compact disc’s physical size, played back inside a portable deck’s power and processing budget, using early-1990s digital signal processing. Uncompressed 16-bit audio was never going to fit at that size; a perceptual coder was the only way to make a recordable, pocketable disc format viable at all. ATRAC’s job was never bit-exact reproduction. It was fitting the perceptually important part of a recording into the rate the format could actually carry.

Yumeo built its MD-1 for exactly this generation of format — an ATRAC recorder sold as a nearly perfect digital medium at a moment when digital itself still felt like the promise. Orrery’s restoration is modeled from a specific surviving deck:

Yumeo MD-1 · ATRAC recorder · restored from unit no. M-2170534 · Yumeo, Japan, 1994

The restored unit — number 2,170,534 off the line — was, like most surviving MD decks, barely used: obsolete before it could wear out. The character is the codec, and we kept all of it.

The instrument: https://orrery.dk/yumeo/md-1/