Why a CD skips and stutters

A scratched compact disc does not fail all at once. It fails in stages, and each stage sounds different: first nothing at all, because the disc’s error correction absorbs damage that would ruin a simpler scheme; then a brief, almost undetectable smoothing where a sample or two goes missing; then a short, flat silence; and only at the end, the stutter — a repeated fragment or a jump — that most people mean when they say “the CD is skipping.” All four are real, distinct behaviours, and they escalate in that order as the damage gets worse: the first three belong to the disc format’s error-correction and concealment system, and the last is the transport giving up mechanically once that system has nothing left to offer.

The system doing the work is CIRC — Cross-Interleaved Reed-Solomon Code — the error correction built into the compact disc format from the start, part of the Red Book standard Philips and Sony published in 1980. CIRC does not try to protect any single stretch of the disc from harm. Instead it spreads each block of audio across a wide span of the disc surface before writing it, using two layered Reed-Solomon error-correcting codes with interleaving between them. A scratch that physically wipes out a short run of track — a burst error, severe by the standards of any single codeword — gets un-spread on playback into many small, separated errors scattered across different codewords, each recoverable from its own code’s redundancy provided the burst runs no longer than about 4,000 bits of track — roughly 2.5 millimetres. Interleaving is the whole trick: it turns one large, unrecoverable error into many small, recoverable ones.

When the damage is a deep gouge, a chip, a disc handled badly for years — enough to exceed roughly 4,000 bits of burst error — CIRC’s correction runs out, and the player falls back to concealment rather than correction. The first fallback is interpolation: estimate the missing samples from the genuine ones on either side, which for a short gap is inaudible, because most musical material changes little between adjacent samples at 44.1 kilohertz. Interpolation holds up to a much larger gap, roughly 12,000 bits — about 7.5 millimetres of track — before there is too little left to estimate from with any confidence; past that, the player mutes instead — a brief, deliberate silence, chosen because playing back whatever data actually survived would sound worse than playing nothing at all. And if the physical damage is severe enough to throw the laser off the track entirely, the transport’s own tracking servo can lose the track outright — holding the last valid samples in its buffer and repeating them, or jumping the pickup ahead to reacquire the track. That is the stutter and the skip a scratched disc is remembered for — a mechanical failure, not an algorithmic one: correction and concealment are choices about damaged data; a lost track is the pickup losing the disc, not the code losing it.

None of this was optional. A pressed compact disc’s ordinary handling — fingerprints, dust, the fine scratches of years in a sleeve — guarantees bit errors on nearly every play; a format with no correction scheme would have been unlistenable almost immediately. CIRC’s job was to make an imperfect physical medium behave like a reliable digital one, and its layered response — correct, then interpolate, then mute, then let the mechanism do what it still can — is the honest hierarchy of a system built to fail gracefully rather than never fail at all.

Yumeo’s CD-9 is a compact disc player from exactly the moment that hierarchy becomes audible — a disc worn enough that correction alone is no longer doing all the work. Orrery’s restoration is modeled from a specific surviving unit:

Yumeo CD-9 · compact disc player · restored from unit no. C-3090622 · Yumeo, Japan, 1988

A failing disc, held at the moment it gives up — the interpolation, the brief digital silences, the stutter. The concealment logic is the real subject; a random glitcher only imitates it.

The instrument: https://orrery.dk/yumeo/cd-9/