Everything you need to know about CD-ROMs - and then some. Ian Waugh says read my pits...

Life's hectic in the fast lane, isn't it? Just when you start to get to grips with analogue synthesis along comes digital synthesis. Just as you're settling in with your new tape machine, along comes digital recording. Just when you've bought a removable hard drive for your computer or sampler, along come CDs.

Not your common or garden audio CDs, of course, we're talking about CDs holding samples, computer programs, games, graphic images and even photographs and movies. All these can be stored on a (fairly) normal CD but to access them you need a CD-ROM drive. The ROM in this case stands for Read Only Memory; CDs are a one-way street - you can only get data off them, you can't put it on. At least not yet (see the Orange Book section under Reservoir ROMs).

CD-ROM technology is virtually the same as audio CD technology - there's just a bit more of it, that's all - and it is fast becoming the storage medium of the future. Why? Because although cheap to produce, CDs can store large quantities of data. How large? Well a single CD can hold over 550Mb of data - that's about 300,000 pages of text or around 800 double-sided floppies.

Did I say it's cheap, too? Yes, you've heard it before and here it is again - CDs are unbelievably cheap to produce, typically 50p-£1. You can buy budget CDs for £2.99 and CDs on the front of magazines are now commonplace. Are publishers doing it at a loss? Certainly not: the high price of conventional audio CDs is totally down to the greed of the record companies - ask the artists!

Many CD-ROMs, however, still sell at a premium. This is partly because the medium is relatively new (and new things always cost more), and partly because new markets are generally small. But vendors also price their goods at levels they think the market will stand. It's a basic principle of commerce and as applicable to musical equipment as it is to soap powder.

There are, however, some distributors who are pricing more sensibly and so attracting more buyers into the market - which can only be a good thing. Many CD-ROMs cost no more than a game - £30-£40 - and there are even cheaper ones around, so don't be put off because some are expensive. There are moves, also, to reduce the price of sample CD-ROMs which typically cost two to three times more than their audio equivalents.

Some software distributors are actively encouraging the use of CD-ROMs - after all, it's far cheaper to produce a CD than half a dozen floppy disks. IBM, Apple and Microsoft distribute operating system updates on CD-ROM and several companies now release CD-ROM versions of their software including Microsoft's Works for Windows and Lotus 1-2-3. The CD-ROM version of Corel Draw contains an extra 10,000 clip art images and 200 fonts.

CDs have another big advantage for the seller, too: copying them is quite impractical. The cost of 800 floppies or a 600Mb hard disk required to copy a full-length CD is likely to be many times more than the cost of the CD itself! Of course, you can bet your life that sooner or later the yo ho ho squad will hole up somewhere with CD-ROM duplicating equipment and get to work. But the fact remains, CD-ROMs are not something you can copy and pass onto your mates. At least not without a CD recorder which currently costs around three grand.

Which brings us to the main drawback of CD-ROMs - the fact that they are a read-only medium. Some sources cite this as a reason why CD-ROMs didn't catch on more quickly - they have been with us for over five years - but I believe it was more to do with the price of CD-ROM drives. Even now a 'good' CD-ROM drive will set you back £400-£500 or more though there are several cheaper units currently on special offer for under £200. The difference? We'll get to that in a moment.

In order to know what you're getting for your money, you need to understand what makes a CD-ROM drive tick (or whir) and what the specs mean.

CD-ROMs are slow. Most modern hard disks have an access time (the time it takes to find and retrieve a particular block of data) of about 20ms. But even a fast CD-ROM drive is likely to have an access time of around 300ms.

Even more important than the access time is the data transfer rate. This is the speed at which the drive transfers the data to the host device. As CDs are read-only, data is written onto them in the optimum order, unlike a hard disk on which a single file can be split into many sections and stored all over the disk - a process known as fragmentation. This helps speed up CD reads.

Most CD-ROMs also have a RAM buffer or cache which acts as a halfway house between the drive and the host machine. As data can be read more quickly from RAM than from the CD, the buffer also helps speed up data transfer. Most CD-ROMs have a 64K buffer, although this does vary.

But even with both these features, single speed drives will only achieve a data transfer rate of about 150Kb/sec - which, incidentally, is the minimum MPC (Multimedia PC) requirement. By comparison, many PC hard disks have a transfer rate of 500Kb/sec, and some manage in excess of 1Mb/sec.

You may see a maximum data transfer rate or 'burst rate' quoted which will typically be 1.5 to 2.5Mb/sec. But this cannot be sustained for long periods so it's not a good figure to use in comparisons. In any event, if it's a SCSI drive, the transfer rate will be limited by the SCSI interface to around 1Mb/sec.

Many modern drives are multi-spin or double speeds and while this increases the data transfer rate it doesn't automatically double it in all situations. That said, the increase in speed is useful and certainly worth having if you can afford the extra.

The final area to consider is Kodak's Photo CD compatibility. In case you're not familiar with this, the idea is that you take a conventional film along to be developed in the normal way, but for an additional fee you have the photo images put onto a CD which can then be viewed on your TV or computer screen (with the correct playback equipment).

Given that a CD will typically hold about 100 piccies and that most of us don't run off anything like this number during our two weeks at Bognor, provision is made to record additional photos onto the CD in several stages - a process known as 'multi-session'.

So do you need a CD-ROM now? Well, if you're into multimedia, a CD-ROM is part of the MPC spec. And the quick and convenient access to large numbers of fonts, graphics and animation files does make it an essential addon. You can, of course, also play audio CDs on a CD-ROM drive, so if you don't already have a CD player you could kill two birds with one stone - although you'll have to boot up your computer to play the CDs.

If you're into sampling, you can use the drive to load sample CD data into your machine. You might think that as data on a CD is stored digitally and sample data is also digital, you can transfer the data digitally. Not necessarily so. Data is transferred between a CD-ROM and a computer (for which they were primarily designed) digitally, but it has to be via a SCSI link or a similar bus. A sampler connected directly to a CD-ROM will not recognise a CD as it does not have the same format as a hard disk.

You need to transfer the data via a digital interface - but these are not always included as standard on samplers or CD players. The more up-market audio CD players have digital outs, but I've yet to see a CD-ROM similarly equipped (if you know of one, drop us a line). Also, the data on the CD needs to be recorded in a special digital format in the first place.

Of course, the vast majority of sample CDs are audio format and this can't be transferred digitally. There are still not that many sample CDs which include digital data and of those which do, not all have been entirely successful.

The moral of this story is that if you want to transfer digitally you'll probably need a separate audio CD player - and make sure that there are enough CDs with digital data to make it worthwhile. It's perhaps also worth pointing out that the transfer of data from a CD to a sampler via an audio cable produces a negligible loss of quality. The main advantage of digital transfer is a saving in time, not a preservation of quality.

And speaking of quality, if you want to play audio on a CD-ROM, it might be worth checking the audio specs. These are not normally included in adverts; playing audio CDs, it seems, is regarded as a bonus. You may find a signal-to-noise ratio of only around 80dB. Some manuals (such as the one for the Apple CD300) don't even quote audio specs. In any event, it's best to opt for a machine with proper audio outs rather than just a headphone socket - although this may not be an option on internal drives.

Given the vast array of CD-ROM standards and specs, which type should you go for? Well, as usual, that depends on what you want to do. If you're sure you won't ever want to put the family snaps on a CD there's no need to bother with a Photo CD compatible drive. And if you don't want to run QuickTime movies or if you don't mind waiting a few seconds longer for data transfer then you needn't go for a dual speed drive either.

The greatest choice of CD-ROM drives exists for the PC market. But that's not the problem for Mac users it may at first seem. The Apple CD300, you see, is a double-speed Photo CD, multi-session compatible drive available at a very reasonable £323. And you should be able to pick one up for under £300 if you shop around - less if you want the internal version and have a slot in your Mac in which to fit it!

It has an access time of 295ms and a transfer rate of 342Kb/sec and comes with CD Remote for controlling audio CDs and Apple Photo Access for reading Photo CDs. Incidentally, rumour has it that Apple is subsidising the CD300 to encourage the use of CD-ROMs. As of writing, it's probably the best buy for Mac users.

There's also the relatively new portable Power CD (£405 RRP) which you can carry around with you like a Walkman and annoy fellow passengers (although it's too bulky to be comfortable and it eats batteries). It's Photo CD and multi-session compatible with an access time of 550ms and a transfer rate of 175Kb/sec.

Both these machines make Apple's older CD150 look somewhat obsolete, but if you don't need all the bells and whistles, there should be some CD150 bargains to be had as dealers make way for the new models. Incidentally, many third-party PC drives can also be used with a Mac. This usually requires a special Mac interface kit. If you decide to buy one, make sure you get the correct drivers and software for it.

As for the PC itself, the choice isn't so clear cut. New drives arrive on the market quite regularly so it's really a question of seeing what's around and comparing prices and specs. One of the current bargains seems to be the Mitsumi LU005S at £175. It's multi-session, Photo CD compatible with a 350ms access speed and a 175Kb/sec data transfer rate. However, it's an internal model (you'll need a spare 5.25" drive bay in your PC) and you just lie the CDs in the machine rather than using a caddie.

Panasonic's CR-562B (£311 for the internal version, £464 for the external version) is available in AT and SCSI formats (an AT interface kit is another £81). It's a dualspeed drive, Photo CD and multi-session compatible.

The new Toshiba XM-3401 (from £493 for the internal version) is rather nice. It's Photo CD and multi-session compatible and has a fast 200ms access time with a transfer rate of 330Kb/sec (perhaps now you can see why Apple's CD300 is such good value!).

It also has a SCSI 2 interface which is rather interesting as SCSI 2 hasn't really caught on yet. Indeed, the question that has to be asked is how much faster will the drive work via SCSI 2? In most cases it will only make a small difference to the performance of current drives and it means having to budget for a SCSI 2 interface, too. However, manufacturers are developing CD-ROMs with greater capacity and faster transfer rates. To make the most of a faster transfer protocol the drives themselves need to be faster.

If you're wondering why there is such a disparity in the price of drives, check the performance and the extras. The cheaper ones are probably not dual speed (the Mitsumi is single speed). Some may only have a small buffer (the Mitsumi has 32K) and there are various dust seals, double dust protection mechanisms and automatic lens-cleaning mechanisms to consider.

PC users also need to check the drive interface. SCSI drives are probably the most common (SCSI lets you daisy chain several devices from a single SCSI card) but some drives use a local bus which, though often cheaper than a SCSI card, will take over one of your card slots. Swings and roundabouts. Incidentally, some people may claim that local bus is faster than SCSI. It may well be, but current drives usually can't yet take advantage of the extra speed.

To sum up, if you want a CD-ROM for multimedia purposes, check that it is XA compatible. If you want to read Photo CDs make sure the drive is multi-session. Although all XA drives can read Photo CDs not all can read multi-session discs. Because the directory is written to the CD in several stages, some discs may be able to read the first set of pictures, but not subsequent sets. Another thing the adverts sometimes don't make clear...