If you've paid any attention to the CD-ROM market in the last few months, you've noticed one thing: quad-speed (4x) CD-ROM drives are all the rage. Third-party, quad-speed drives for the Mac have been available from manufacturers such as NEC and Sony for some time, and Apple will be including quad-speed drives in desktop Macs. Double-speed CD-ROM drives are going the way of the 800K floppy drive, and quad-speed drives look to be the next step up the ladder. To make matters more complicated, there are also triple-speed (3x) and sextuple-speed (6x) CD-ROM mechanisms on the market.
Despite the enthusiasm for this technology, there's a lot of misapprehension. Do I need a quad-speed CD-ROM? I heard quad-speed drives can be slower than double-speed CD-ROMs?! What does it mean for a CD-ROM to be "engineered" for quad-speed drives? Although this article probably won't answer every question out there, hopefully it'll help clarify a few key issues. Most of the issues in this article apply to any high-speed CD-ROM drive, including 3x, 6x, and (yes) 15x drives, although I'll be using the term "quad-speed" generically.
What Quad-Speed Means -- "Quad-speed" means that a CD-ROM drive is capable of sending data at four times the speed of a "standard" single-speed CD-ROM drive. A single-speed CD-ROM delivers data to the computer at a speed of about 150K per second - and that's a best case figure. A double-speed CD-ROM - like the AppleCD 300 series common in the last few years' worth of desktop Macs - delivers information at about 300K per second (again, best case). A quad-speed drive can achieve in the neighborhood of 600K per second. So the rate at which a quad-speed CD drive can deliver information can be as much as four times faster than the original single-speed CD-ROM drives.
Please note the word "capable" in the first sentence of the paragraph above. All sorts of things can conspire to prevent quad-speed transfers from happening or helping you: some reasons are obvious and some are subtle. There are detailed examples below, but, succinctly, simply replacing a single-speed CD-ROM drive with a quad-speed CD-ROM drive is not guaranteed to give you a fourfold increase in actual performance.
Seek and Ye Shall Find -- The speed at which information is delivered to the computer isn't the only thing that's improved with time. Newer CD-ROM drives typically have much faster seek times, which is how long the drive typically spends looking for a particular spot on a CD-ROM disk. Single-speed drives have typical seek times of in the neighborhood of 300 to 500 milliseconds (ms), so it can take them as much as half a second to get to a particular location on a CD-ROM. Double-speed drives typically have seek times between 250 and 350 ms, and typical quad-speed drives have seek times around 150 to 200 ms. In contrast, today's hard drives usually have access times less than 10 ms, so even the best CD-ROM drive will take at least ten times longer to seek than your hard disk.
How does seek time affect you? To put it simply, the more often the CD-ROM drive has to go to a specific location on a disk (rather than just reading from the disk sequentially), the more often seek time is a factor in the perceived speed of the drive. The effects of seek times can often be seen when opening Finder windows with large number of files, particularly if those files have custom icons (like typical Photoshop files). In that case, for each file the CD drive has to get information from the desktop database, then scoot out to the file's physical location on disk to load the icon, then scoot back to the desktop database for the next file. All that jumping around can take some time and make the drive seem very slow. Similarly, reading a 2 MB text file that's fragmented on the CD-ROM and stored in twenty different pieces is going to take longer than reading the same file stored contiguously on the disk (one seek versus twenty seeks). So, it's important to note that (a) seek time can dramatically affect the perceived performance of the drive and, (b) because the physical layout of a given CD-ROM can be a major factor; when a drive seeks is largely beyond your control.
[If you're annoyed by waiting for all those custom icons to appear from a CD-ROM, check out Fabrizio Oddone's <firstname.lastname@example.org> version of Quinn and Peter Lewis's CDIconKiller, which suppresses display of custom icons on CD-ROM's, floppies, and network volumes. The performance increase is astounding!]
It's important to note that while most quad-speed drives have respectable seek times, the term "quad-speed" only applies to the rate at which the drive can deliver data to your computer, not the seek time. It's perfectly possible - and perfectly valid advertising - to have a quad-speed drive with a horrendously slow seek time.
Sports Cars and Dirt Roads -- So, having a quad-speed drive means your CD-ROM multimedia titles and games will go faster and perform better, right? Not necessarily, and, actually, probably not. The common statement heard from CD-ROM vendors, software developers, and dealers is that the CD-ROMs must be engineered to take advantage of the quad-speed capabilities. What exactly does that mean?
One common confusion is that these specially-engineered disks are somehow physically different than other CD-ROMs. This is untrue. The physical format of the CD-ROM doesn't care whether the drive is single-speed, double-speed, quad-speed, or whatever. The real question is whether the software used to access that disk is capable of taking advantage of a quad-speed drive's improved data-delivery capability.
QuickTime video is the most commonly-used example when talking about these issues, so I'll use it here too. A quad-speed CD-ROM drive has the potential to smoothly play back larger and more complex QuickTime movies because it can deliver data to the computer faster. Does this mean a quad-speed drive will improve all QuickTime performance from CD-ROM? No. Here's why: a given QuickTime movie has a minimum data throughput requirement for smooth playback. This rate is determined by the capture and/or production of the QuickTime movie prior to being put on CD-ROM (using tools like Adobe Premiere). Once set, this minimum throughput requirement isn't changed by your computer or your CD-ROM drive.
For example: say a certain QuickTime movie requires a throughput of 220K per second for smooth playback. All else being equal, a double-speed CD-ROM drive will be able to play the movie smoothly (double-speed drives are rated around 300K per second, remember), whereas a single-speed CD-ROM drive would cause QuickTime to drop frames and maybe stutter in order to put the video on screen. Now, a more macho QuickTime movie - say 320 by 240 pixels (quarter-screen), 24-bit, at 12 frames per second - might require 550K per second for smooth playback. In this case (all other things being equal, remember) a quad-speed drive would be likely to play the movie back smoothly, whereas that double-speed drive would cause QuickTime to drop frames or give interrupted playback.
So what's the problem? The QuickTime video used in most multimedia titles - from games like Myst to reference works like Grolier's Encyclopedia or Cinemania - does not have throughput requirements in the range of quad-speed drives. In fact, most of these titles are only now beginning to include videos with throughput requirements greater than 120K per second. That's right: all this time and they're still including videos produced to play back on single-speed CD-ROMs! Why? Because a large enough subset of their customers have single-speed drives and would complain or demand refunds if the video was choppy. Since the CD-ROM industry has a very high return rate, these companies don't take many chances with ticking off their customer base. They don't like it either - they want to put cool videos on there - but it's just not that simple. By the same token, you can bet it's going to take a while - at least two years - before mainstream CD-ROM applications will assume a user has a quad-speed or better CD-ROM drive. And by then, CD-ROM as a media might well be going the way of the 800K floppy.
Game developers, on the other hand, often don't have qualms about requiring high-end hardware. In the DOS/Windows world, there are already games appearing which require quad-speed CD-ROMs for optimal play. You can expect the same thing in the Mac world if it's not happening already, especially with arcade-style and action games that make heavy use of QuickTime and graphics.
Want to see what other flies are in the ointment? OK: some disks developed with single-speed or double-speed CD-ROM drives in mind can be slower on quad-speed drives than on double-speed or even single-speed drives, especially on some mid-range and low-end machines. Why? Software, memory, and seek time. Say the computer asks for a 120K chunk of data at the beginning of a QuickTime movie. The CD-ROM drive seeks out to the file and starts pumping data in. While the computer is happily thinking about the data, the quad-speed drive runs off into the tumbleweeds, whereas the pokier double-speed and single-speed drives stay in the neighborhood. The result is that when the computer asks for the next 120K of the same file, the single-speed and double-speed drives can sometimes deliver it, whereas the quad-speed drive might have to re-seek to get back to the right location on disk, and that will be slower. This is an over-generalization - the specifics vary on a case by case basis - but you get the idea.
Cache or Charge? Purists will note the example above is technically inaccurate. "Wrong," they say, "that material is in the cache - the drive might not have to seek at all." Well, yes: but that's not true in all situations because - let's face it - caches aren't all that smart.
What are caches? Since CD-ROM drives are notoriously slow, most if not all drives have a bit of onboard memory they use to stash material for quicker access. The amount of memory varies, but is generally between 64K (older, single-speed drives) and 256K (typical quad-speed drives). On-board cache implementations vary, but generally they're sector-based: when the computer asks for information at a particular sector of the CD-ROM, the disk reads and caches material around that location, figuring it's pretty likely the computer's next request will be for some of that adjacent material. If the caching mechanism is right, the next set of requested data is rapidly transferred out of the cache without the drive having to seek for it (or, more likely, covering for the drive while it's seeking and reading yet more data). On the other hand, if the next request is for some file off in the CD's unexplored linen closet, the material in the cache is discarded and the drive has to seek and read as it would without the cache. Bottom line: these caches can help, sometimes.
There are also third-party software products like Casa Blanca's DriveCD and FWB's CD-ROM ToolKit that implement smarter caching schemes using your hard disk and RAM. Since the Mac's RAM and hard disk are significantly faster than the CD-ROM, these products transfer material from the CD-ROM and stash it in RAM and/or on your hard disk. When your Mac asks the CD drive for some of that material, the caching system politely intercepts the call and says "Why, I have that information right here." These systems typically want about 1 MB of RAM and between 2 and 5 MB of hard disk space. They generally deliver measurable performance improvements, particularly with typical multimedia titles and games. Also, the intelligence built into these products can be helpful with some CD-ROMs. For instance, if a particular reference CD-ROM always asks for the same file over and over again, the caching system might make sure that file is always on your hard disk so the program never had to get it from the CD-ROM. This can be far more helpful than the simple, blind sector caching that CD-ROM drives do on their own.
On the other hand, these products do have a price. For one thing, on lower-end Macs (like LCs and LC II's), the overhead of managing and sustaining a caching system on your Macintosh often takes enough time away from your processor to significantly hinder the performance of a typical CD-ROM title. For another, losing 1 MB or more of RAM and a few megabytes of your hard disk isn't an option for some Macintosh users: sure, these caching products might help, but they can be resource-intensive.
Also, there are certain CD-ROM applications where these caching products won't help much, for all their intelligence. The classic example is a set of CD-ROMs with compressed U.S. Census data I have to work with every once in a while. With those disks, I rarely seek: I just read about 100 megabytes of straight, contiguous data, then pop in the next disk. A caching product would actually slow me down as it tries to adjust to all the disk-swapping and analyze what I'm doing. In this case (which is admittedly pretty specialized), I don't want my Mac spending time analyzing my CD drive's behavior: I want it reading and decompressing that information as fast as possible.
In summary: third-party CD-ROM caching products can often boost the performance of typical CD-ROM titles on mid-range and high-end Macs, if you're willing to sacrifice some RAM and hard disk space.
Filling the Gap -- Let's review the points above one by one:
Quad-speed CD-ROM drives are not going to be a panacea for slow CD-ROM titles, and you shouldn't necessarily expect a CD-ROM title to perform better just because it's in a faster drive.
There's no physical difference between CD-ROMs that take advantage of quad-speed drives and those that do not: it's all in the content of the CDs and the software which accesses it.
Seek time is an important factor in the perceived performance of a CD-ROM. Shoddy preparation of a disk can humble even the fastest drives.
Most consumer CD-ROMs will not explicitly take advantage of quad-speed drives for some time to come, although games will lead the way.
CD-ROM caching products can significantly improve the perceived performance of typical CD-ROM applications, although they have a cost and don't help in all cases.
Should you spring for a quad-speed CD-ROM drive now, or take advantage of the reduced prices on double-speed drives? As always, the answer is "it depends." For some groups of users, the performance difference between a double-speed and quad-speed CD-ROM drive will be negligible; for people working with high-quality video, games, and other bandwidth-intensive projects, the quad-speed drives are an absolute godsend.
However, here's one thing to bear in mind when purchasing: the industry is gravitating away from the double-speed CD-ROM standard, and that trend will accelerate with time. If you're looking to buy a drive to use for the next few years, it makes sense to go for a technology that will be less obsolete as time passes. If money is a big factor in your decision, a double-speed drive will be adequate for some time to come.