Did you know that CDs wobble differently depending on your geographical location? Or that they play from the inside out?

You probably own a CD player, and have a decent CD collection at home. But how much do you actually know about CDs, about what the CD format will and will not allow, about what else besides audio data you might find on a CD, or about how much they cost to manufacture? Here, then, is a quick list of 20 Things You Probably Never Knew About CD...

1. Not all of the data storage capacity of a CD is devoted to music; the CD format sets aside about 3% of a normal music CD for graphics, song titles, and lyrics, and even Teletext-style pictures, though this facility may not actually be used.

2. CDs wobble differently depending on your geographical location. At the equator and the poles you will get the least wobble, whilst in between it varies depending on whether you live in the Northern or Southern hemisphere. The reason for the difference is the Coriolis Effect, caused by the earth's rotation; if you think about it, at one pole the CD is rotating the same way as the earth, whilst at the other it is going round the opposite way.

3. One reason why CDs don't like being moved around whilst playing is that they act like a gyroscope, and so prefer to point in the same direction all the time. If you try to change the direction, the CD will 'precess', ie. twist to try and counteract the movement.

4. CDs play from the inside out, towards the outer edge, the opposite way to LPs.

5. One of the major limiting factors on the length of a CD is the amount of wobble near the edge. Too much wobble = shorter playing time. Early CDs, made when manufacturing techniques were relatively unrefined, were not sufficiently flat to allow playing times much beyond 60 minutes. Playing times of 72 minutes and beyond are now commonplace.

6. The top surface of a CD is the delicate one. The underside may be the side that faces the laser, but the light travels all the way through the clear plastic of the disc and reflects back off the aluminium-coated top surface. The top is protected by a thin layer of lacquer and the silk screened printing on the disc. Scratch away part of the top surface and you will permanently damage your CD.

7. Putting sticky labels on a CD can upset the balance so badly that it can become unplayable. Worse, the solvents in the glue may affect the metal surface which contains the digital information, and thereby cause errors in playback.

8. CDs have their own equivalent of the clicks and surface noise of LPs. Normally, a CD player can compensate for occasional errors by working out what the missing or erroneous bits. When the number of errors received by the playback circuitry rises above a preset threshold, then the CD player does a combination of calculation and 'guessing' to estimate what the missing information is, and often gets this interpolation pretty close to the original. When errors come thick and fast, the CD player will mute the audio, initially for short periods of time, but if the errors become catastrophic, then the audio may disappear completely. Very few domestic CD players have displays of error detection...

9. If you hold a CD up to the light, you may be able to see holes in the silvery metal coating.

10. You can put MIDI information on a CD, so that a piece of music can be played via the audio output and also via your MIDI equipment. You need a special CD-MIDI player and compatible discs — it does not work with normal CDs.

11. Although CDs are digital, CDV (CD Video) discs have analogue TV pictures stored on them.

12. A CDV disc made for the American NTSC TV system will not play on a European PAL TV player, nor vice-versa.

13. The laser used to access the information on the CD works in the infra-red part of the spectrum. It is thus invisible to the human eye. The green light that is often used to illuminate the top of the CD is for show only.

14. CDs can store audio in many other formats than the 44.1kHz sample rate and 16-bit resolution that is often referred to as 'CD quality'. Eight bits at 44.1kHz is typical of the many ST/Amiga add-on samplers and sounds acceptable in some circumstances; it gives you more than two hours capacity. Reducing the sample rate to 22kHz can will give four hours playing time, whilst coding similar to that used in MiniDiscs and DCC could extend the time well beyond eight hours. But how many pieces of music (or albums) last this long?

15. The boxes that CDs come in are called 'jewel cases'. But don't try putting the family heirlooms in them.

16. CDs cost less than one pound to manufacture in units of 1000 or more, yet retail prices are typically £11.99 or more. The wholesale price of CDs in the UK is typically higher than the retail price in the US (the UK retail price is 40% higher than the US), and WH Smith (the UK's biggest record retailer, and also owners of Our Price) are on record as saying that they believe CD prices are too high. Ever feel like you're being ripped off?

17. Quite a lot of early CDs were manufactured on equipment originally designed to make 12" videodiscs.

18. The clean-room conditions needed to make CDs are comparable to those used by semiconductor manufacturers when making microprocessors.

19. The plastic used to make CDs is the same as that used to make the Perspex Liberty Bodices, as modelled by Jane Fonda in the film Barbarella.

20. There are about 600 million bytes of information stored on a CD. Reading all of these manually, at the rate of one per second, would take just over 19 years.