What is the difference between a Pressed CD and CD-R?

Answer 1

Although the discs may look very similar, there is a world of difference between a Recordable disc that has been burned, and a "Pressed" (or "Glass Mastered") disc that has been manufactured. Before going into the specifics of RECORDABLE versus MOLDED DISCS, it would help to explain the very fundamentals of how a disc plays music or video. Let's use the AUDIO CD as an example.

How an Audio CD plays "music"?

First, a standard Audio CD is said to have 16 bits and 44.1 khz/s. These specifications actually refer to a digital alphabet. But, unlike the English alphabet with 26 letters, this digital alphabet only has 2 letters - which are 0 (off) and 1 (on) ! The term 16 bits refers to the number of letters in each digital word (Example: 0101010101010101 would be a 16-bit digital letter! A 16 bit digital alphabet would actually provide 65,536 possible combinations (the digital "vocabulary"). The 44.1khz refers to how many of these "digital letters" would be read in a second off of a compact disc (44,100 per second). In effect, the laser reading a CD is actually reading 44,100 words made up of 16 digital letters per word ("1's" and "0's") each and every second! That data forms the information that the computer translates to music from an Audio CD.

Now back to our question… how is a recordable CD-R or DVD-R different from a Pressed CD or DVD? For a recordable disc, the digital letters ON (1) and OFF (0) are represented by holes that are shot through a layer of DYE on the CDR by a recording laser. The laser shoots this hole to represent an OFF (0) letter and leaves the DYE intact to represent an ON letter (1). This is why this process is refered to as "burning" a disc... i.e. the disc is actually being burned with holes by a laser in the photo-sensitive dye layer of the disc!

On a pressed disc, however, the disc is manufactured with microscopic bumps (ON) and valleys (OFF) to represent the two digital letters of the digital alphabet. A Glass master is used to produce a piece of nickel that acts as the stamper for the CD (same applies to DVDs and even Bluray). Each manufactured CD would have a unique custom nickel stamper. The stamper has all of the hills and valleys of the Master CD on it, and is used to transfer the information to the finished disc in a molding process that molds plastic pellets (polycarbonate) to the finished product. To watch a video of the CD/DVD/Bluray molding process, visit this link: http://www.microforum.ca/video/ .

Although these discs are all the same size and similar colour and shape, the underlying technology and manufacturing process differs tremendously. So now you know…. CD-R (recordables) and CD-ROMs (read-only memory) are not all the same!

To recap: A "pressed" disc with music or video on it would be manufactured with the data imprinted right on the copies. A Recordable CD, DVD or Bluray disc is actually manufactured with a DYE that can be written to with a laser device. Before there were recordable discs, the original CD and DVD players were only able to PLAY discs and not WRITE to them. These players used lasers to read bumps and pits on the surface of the disc and convert the data stream to binary for a computer or a chip to translate into sounds, videos, images, etc.

Recordable CD , DVD and Blu-ray Drives have the added ability to burn holes in the DYE of a recordable disc, thereby allowing small scale production of CDs and DVDs.