What innovation helped CD-ROM speeds increase exponentially?

Question

Early CD-ROM drives were considered both expensive and slow (in comparison to HDDs). This limited their adoption until around 1992, when both the prices fell and the speeds increased dramatically.

In the mid 90s (roughly 1993–1997), I recall CD-ROM speeds increasing very rapidly; probably doubling in speed every 6–12 months. I got my first PC, which included a 2x CD-ROM drive, in 1993. And it seems that 32x drives were the normal option by 1997–98, even for "budget" PC's.

For perspective, the original 1x CD-ROM's of the late 80s transferred data at 150 KiB/s, while a 40x drive from the late 90s would do 6000 KiB/s.

I know that, fundamentally, the speed up is down to higher rotational speeds. However, the pattern of doubling of performance, and at a high rate, suggests the innovation was probably in the digital circuity. Like better error correction or better peripheral interfaces to the CPU. Though, maybe it was just about tweaking the analog aspects (motors, optics) instead? It seems weird to me that small spinning motors would be an innovation area in the 1990s, especially after so much of that had already been perfected for hard drives; additionally, the apparent exponential rise in the speeds.

So where were the innovations happening during this interesting period: motors, laser/optics, digital circuitry? How is this rapid speed increase best understood?

Answer 1

While reading a CD, the main head assembly will simply move slowly from the inside of the disk outward, but unless the disk is absolutely perfectly centered--something that usually doesn't happen--the laser head itself will need to move in and out with each rotation in order to follow the track. Early CD mechanisms were designed to accommodate about five in/out wobbles per second, but spinning the disk faster will make it necessary for the head to move faster while following the tracks. Beyond the higher mechanical requirements, this poses another problem: at slower speeds, the head could move in response to indications that the beam was on one side or the other from where it "should" be, but as speeds increased it became necessary for the head positioning electronics to "predict" how the track would move, so as to need only small adjustments beyond that.

Another factor is that I don't think drive manufacturers really knew how fast drives could go without creating a risk that poorly manufactured CDs might explode catastrophically in such a manner as to destroy not just the disk, but also the drive as well. In many cases drives from more established brands wouldn't achieve speeds quite as high as those from lesser-known brands, which would compete on the basis of their higher speeds. If a lesser-known brand whose top spin rate was e.g. 4000RPM (compared with a normal "1x" top speed of about 500RPM) was on the market for a few months, and there were no apparent complaints despite the fact that consumers of budget drives might be more likely to use lower-quality media, then name brands could figure they could safely use comparable speeds.

From a practical standpoint, most drives ended up being limited to about 8000 RPM, which is 16 times the speed of a 1x drive when reading data near the center of a disk. Because one rotation's worth of the spiral track near the edge of the disk could hold about 2.5x as much data as one rotation near the center, and drives could maintain the 8000RPM speed even when reading data that was further out, such drives could read data near the edge of the disk 40 times as fast as a single-speed drive could, and were thus marketed as "40x drives".

Answer 2

'for perspective, the original 1x CD-ROM's of the late 80s transferred data at 150 KiB/s, while a 40x drive from the late 90s would do 6000 KiB/s.'

no. The original drives read at 150Kb/s, varying the speed of the disc to keep the data rate constant. 8x drives could read at 1200kb/s and some 12x drives at 1800 Kb/s. constant linear velocity, so same data rate anywhere on the disc. All drives rated faster than 12x used constant angular velocity with the stated speed being the rate readable at the outermost edge of the disc only, with the readable speeds at the inside of the disk being approximately 3x slower, and even increasing the stated speed because later CD-Rs are at the limit of the spec - 700Mb instead of 650, The increase in speeds was all marketing nonsense. In practice a 40x drive was no faster than a 12x drive from a few years earlier -the hardware was almost the same anyway.