Alesis CD24
Not a machine at all, but a file format - Alesis's bid to put 24-bit/96kHz masters on a plain CD-R.
Overview
The Alesis CD24 is not a piece of hardware. It is a recording format, a way of writing high-resolution audio to ordinary CD-R media, and it shipped as a feature of the Alesis MasterLink ML-9600 hard disk mastering recorder. Collectors hunting a "CD24" on the used market are really hunting an ML-9600, because that is the machine that writes and reads the format. The naming is confusing, so get it straight before spending money on the wrong thing.
A standard Red Book CD is locked to 16-bit/44.1kHz, and that ceiling is baked into the CD specification. CD24 gets around it by storing audio as AIFF files rather than as Red Book audio. That lets the ML-9600 write sample rates of 44.1, 48, 88.2, or 96kHz at word lengths of 16, 20, or 24 bits. According to the ML-9600's documentation, the resulting disc is recognized by many DVD-Audio mastering facilities, which was the whole point. A mixing engineer could skip laying a mix down to analog tape, DAT, or the awkward bit-split schemes of the era and go straight to a high-resolution disc. Mix (2000, p.63) described the MasterLink as potentially becoming a staple of both the recording and mastering studio because of this ability to create and play back a high-resolution CD.
CD24 does not require exotic media. It writes to standard CD-R blanks, and the AIFF files it produces are readable by most computer-based audio editing programs, with the proprietary information arranged so the discs remain compatible with CD and CD-ROM drives. That compatibility is why the format found a niche rather than dying on the vine. A mastering house could hand off a CD24 disc and know the receiving facility had a fighting chance of reading it.
The trade-offs are real. CD24 files eat capacity fast. On a 650MB CD-R, the ML-9600 can fit 29.6 minutes of 16-bit/96kHz audio, but only 19.7 minutes once you step up to 20- or 24-bit at 96kHz. Burning takes time, too. A full 650MB Red Book disc writes in roughly 19 minutes, while a full CD24 disc takes about 36 minutes, at a rate between 2× and 4.5×. The documentation notes there is little incentive to use 20-bit word lengths for CD24, since the AIFF format pads 20-bit samples out to 24 bits anyway. You get the same record time as true 24-bit without any capacity benefit.
CD24 is not a DVD-Audio format. The ML-9600 does not record to any DVD-Audio format, and the CD24 AIFF wrapper is simply a high-definition container that most DVD mastering facilities can ingest. Anyone expecting true DVD-Audio authoring from this workflow will be disappointed.
Because the format is inseparable from the ML-9600, questions about cost, failures and what to look for are really questions about that recorder. The ML-9600 carried a list price of $1000 and shipped in a 19″ × 3.5″ × 11″ rack-mountable steel chassis weighing 16.55 lb, with a one-year parts-and-labor warranty. Mix (July 1999, p.108) covered it as a 24-bit/96kHz hard disk and CD-R mastering recorder, positioning it alongside Alesis's own ADAT as another attempt at a breakthrough technology. Alesis built its reputation on giving project studios professional capability at prices the established players could not match, and the ML-9600 with its CD24 format was squarely in that tradition.
The ML-9600 stores 20-bit data on its internal hard drive in true 20-bit fashion, so 20-bit files give more hard disk recording time than 24-bit ones. That is handy to know when planning a session. The unit also supports 700MB CD-R blanks, though the documentation warns not all CD players will read them. MP3 is not an option.
CD24 is a footnote format, but an interesting one. It dates from a moment when the industry was trying to push high-resolution audio through the existing CD infrastructure rather than waiting for a new physical medium to take hold. DVD-Audio and SACD arrived, and neither truly conquered the market. The ML-9600 sold in enough numbers, and the CD24 workflow was practical enough, that the format left a real trace. If you find an ML-9600 with a working optical drive and a healthy hard disk, you can still burn a CD24 disc today. Whether anyone downstream can read it is a different question.
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