One of our readers (in our suggestion box) asked for a walkthrough on the different types of CD and DVD.
Unfortunately, a full description of the differences between all of the CD and DVD formats would fill an entire book. There are many, many different types of physical media, methods for storing the data, methods for writing the data and media formats (MPEG-2, MP3, AVI, MP4, etc.)
But… since most people use just a few of the CD/DVD formats, we’ll cover just those. The most common formats are generically known as CD-ROM, DVD-ROM, CD-R, CD-RW, DVD-R, DVD+R, DVD-RW, CD-Audio, DVD-Video, and the newer HD DVD and Blu-ray discs.
First off we need to define a few expressions and words.
- Burning – This is the process used to store data on the media. Which means we burn “holes” in the writable layer inside the disc.
- Session – “Period of time in which something occurs”. Which translated means a file or files added in the same operation.
- Single Session – All files on disc are added in one operation.
- Multi Session – Several sessions on one disc.
What is burned on disc, stays on disc
As you’ve read in one of my earlier articles, ROM stands for Read Only Memory. This means that any information stored on the disc is there to stay. This is commonly used on the Music CD’s or Movie DVD’s you buy in the store. These discs are not Burned but pressed, much like the good old LP Records. An inverted original (or master) is pressed onto a heated sheet of polycarbonate which is then coated with aluminum powder to create a reflective coating.
What is the difference -R +R and RW ??
Basically the minus, plus and R’s tells you what kind of burnable media you are using and how the data is stored upon them. The different markings also show you how long lifespan you can expect. The different kinds of media, use different types of degradable materials. The ROM media can easily (with proper care and storage) be expected to last many many years, and the cheapest RW media can be expected to be unreadable after as little as 6 months.
- R : The ‘R’ it self stands for Recordable. Hence a disc without the R cannot be recorded onto.
- -R : The Minus marked media (it being a CD or DVD) is a Single Session Media. Meaning that you cannot add more data to the disc once the burning has locked it (even if there is room for more). Some systems are able to do a mulitsession on the -R discs (like DVD Recorders) but not all systems are designed to read these extra sessions. Usually only the first (or none) will be readable on other media players.
- + R : The Plus marked media is intended for multi-sessions. Meaning that you can add data to the disc in Sessions. You don’t have to fill it up all at once but can use it as a backup media to add files over a period of time. Each Session can be Added to an existing session or be created as a separate one. As an added bonus you can tell a session to “delete” an existing file. Delete in the sense that a code is inserted telling the media player to disregard it.
- RW : Means, Rewritable. Basically the burnable layer inside the disc is made of a material that can be erased and overwritten. There are limitations as to how many times it will work and still be readable. The RW discs have more or less been replaced by the USB Flash Chips – which of course are much more versatile.
Are there other differences ?
Yes there are. They use different kind of technology which controls how the data is stored, packed and retrieved, read/write-speed, error handling and reliability. As with the old VHS vs Beta, and the last HD-disc vs BlueRay – The DVD-R and DVD+R were developed by different manufacturers, Phillips and Sony as the main contributors. And they are both convinced theirs is the best solution.
How is a disc identified ?
If you look closely to an empty disc you will see a small circle at the very inner part of the reflective area. This is where the disc information is stored. We can compare this to the BIOS or ROM on your computer. This information tells your writer or reader how the disc will work.
Why can’t some media players read all discs ?
That depends on the manufacturer. Each kind of media has its own way of storing data. And these have to be programmed into the device. Much like an ordinary DVD player can’t read DivX formatted movies unless it has all necessary codecs installed in its firmware. Each Session on a disc have Start and End Entries telling the device where the data is stored, and where there are none. Therefore if you force multi sessions on a disc that is not coded for it, devices that read the discs will not know what to do with the extra information stored.
So there you have it. Keep those suggestions coming into our suggestion box.
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