Noddy's guide to CDs and DVDs

This document is intended to summarise the kinds of optical media available - in particular we will discuss CDs and DVDs.
More information is available here.

We are not attempting to provide you with a complete understanding, but a simplified overview. There are many other sources for information on this topic, however here we provide you with an introductory summary only. If you have any compliments about the information here, please tell your friends, link this page from your web site etc... If you have any criticism, please share it with us, we are always keen to improve our act!!

What are CDs and DVDs?

In essence they are just like those old vinyl records - simply discs that have the capacity to record information. Just like records, commercial Audio CDs and Movie DVDs they are pressed out. The manufacture of these optical discs is accomplished by a machine that places an impression into plastic. In practice these commercially pressed (called replicated) discs are produced at quite low cost when large quantities of a disc are required.

What are CD-Rs and DVD-Rs?

Unlike commercially pressed or replicated discs, many people require smaller quantities of discs, for example you may want to place family photos onto a disc, so you will simply copy them to a recordable disc.
These recordable discs are unlike their commercial brothers as they are burned rather than pressed.
A good way of perceiving these recordable discs is to think of them as records that have grooves already manufactured into them but initially there is nothing recorded onto them. They also include lots of information that tells the recording device what they are (brand, format etc) and what capabilities they have (e.g. burning speed).

To record a disc a laser simply burns holes through one layer of plastic to expose a reflective surface. (Yes this is an oversimplification, that will do for now!!) Once recorded in this manner they can not be erased or rewritten. This means that they can be RECORDED and so they are called CD R or DVD R. - Record Once! What are CD-RWs and DVD RWs? As mentioned above the recordable discs are burned and so can not be erased. Another variation of the optical disc allows for writing, erasing and re-writing. This disc is abbreviated to RW or ReWritable.

Unlike the Recordable disc these discs use material that can be changed from transparent to opaque by application of a laser heat beam. Essentially some heat makes the material change state and more heat returns it to its original state. This means that it can be zapped to record data and double zapped to erase it... really very similar to the way that video or audio tape is recorded, erased and re-recorded.
So now we have discussed CDs, CD-R, CD-RW we need to explain that there are some number of additional types of DVDs.

DVD-R and DVD+R DVD-RW and DVD+RW

We discussed the notion of R and RW but not the PLUS or MINUS varieties. At the risk of oversimplification there is very little difference between the PLUS and MINUS varieties of DVDs. You may not know but each and every CD made has required a license fee to be paid to the owners of the design of the CD. This means that if you own something like this ,AND you can,and lots of people need it, then you can make LOTS of money. For example Philips owned the design for the Audio Cassette.

Now if you are Sony or Philips you can not only make heaps of royalties but if you are also a user of this technology, you can effectively save yourself a packet in royalties that you pay your self..
So given a good technical reason or two you can go out and design and make a better (or at least different) disc and maybe make a packet out of the royalties..

What is better plus or minus?

Here is a table that provides some input:

	Aspect	MINUS R	PLUS R
	Capacity*	Same	Same
	Recording Speed*	Same	Same
	Compatibility in DVD Players	95%+ est.	98%+ est.
	Compatibility in DVD ROMs	95%+ est.	70%+ est.
	Cost of Discs**	lower	higher

Notes:
* From time to time one camp will have a faster or higher capacity disc available before the other. Presently DVD+R is the only one that offers dual layer 8.5GB discs, DVD-Rs is the fist format with a 16 speed burn capability.
** Typically at the factory door they are the same price BUT as so many more MINUS discs are sold, their street price is usually lower.

So what is DVD-RAM?

When the players got together to try to set a standard for DVDs, there was already one guy on the block (namely Panasonic) who had an optical disc that was known as DVD-RAM. This group called themselves the DVD forum and proceeded to set standards that got to be the specification for DVD-R and DVD-RW (note the minus, the other guys did the plus thing). While they were at it, they also wrote up the spec for the already existing DVD-RAM media. Whilst DVD-RAM is like a DVD-RW or DVD+RW it is designed as a more serious rewritable disc. It has a more professional disc director structure and is good for approx. 200,000 rewrites (as opposed to about 2,000 for a DVD..RW). DVD-RAM is used primarily by Panasonic and is seen in other non computer devices as a good, fairly high capacity removable storage device.

So what is Blu-Ray?

Blu-ray Disc (BD), is the name of a new generation optical disc format jointly developed by the Blu-ray Disc Association (BDA), a group entities including Apple, Dell, Hitachi, HP, JVC, LG, Mitsubishi, Panasonic, Pioneer, Philips, Samsung, Sharp, Sony, TDK and Thomson. The format was developed to enable recording, rewriting and playback of high-definition video (HD), as well as storing large amounts of data. The format offers more than five times the storage capacity of traditional DVDs and can hold up to 25GB on a single-layer disc and 50GB on a dual-layer disc. They generally look the same as CDs and DVDs being 120mm wide.