Simply Unread, Or Just Never Heard?

[Vox Humana]

Nachthorn: many thanks for that excellent and interesting explanation.

[Guest Patrick Coleman]

The big difference between the two is how the data is physically stored. On a pressed ('traditional') CD, tiny holes are pressed into the reflective layer, so a hole represents a digital '0' bit, and the absence of a hole represents a digital '1' bit. When the reading laser passes over the disc (in one long spiral line, like an LP) the holes don't reflect the light back, but the areas where there is no hole DO reflect the light, so the pattern of reflections and non-reflections are used to construct a stream of data, i.e. zeros and ones.

 

On a CD-R, in front of the reflective layer, there is a layer of coloured dye. The writing laser 'burns' this dye where a digital '0' is required, and leaves it untouched where a digital '1' is needed. The end result looks the same to the reading laser, i.e. the burned area on a CD-R looks the same as a hole on a pressed CD. The problem is that light reflected from a CD-R with the coloured dye layer is less strong than light reflected from a normal CD. Some players, particularly older players, have difficulty reading CD-Rs because of the lower level of light reflecting back off them. This effect will vary with the type of CD-R and the model of player used. Modern players should be able to cope with all types of CD-R, but as you say, not all do.

 

If a player can't play various pressed CDs, either the CDs are faulty in some way, or more likely, the player drive needs cleaning (I'm assuming you've checked the surface for scratches.)

 

(To explain the other formats I mentioned, you can only fit so many zeros and ones onto a standard CD. DVD and SACD discs allow 5-7 times the amount of data to be stored, and Blueray and HD-DVD allow another magnitude again. The larger the space is to store data, the more data can be stored, which can either be used to store more minutes of recording, more data to reproduce the recording at higher quality, or some combination of the two. Of course, as we know, this has so far not translated into many recordings using these benefits, as not many people own players capable of playing them. Herein lies one problem for the recording industry. The other is how to join up these new high quality standards with the demand for low quality downloadable tracks, as mentioned by Cynic at the start of this tangent.)

 

Now I understand! Thanks!

[nachthorn]

Just before we get back on topic, how does a CD-R differ from a CD-RW?

JC

Sorry - been away all week.

 

A CD-RW works on a similar principle to a CD-R, in that a laser 'burns' parts of a surface to simulate pressed holes on a CD. The only difference is that a different type of dye layer is used, so that areas already 'burned' can be 'unburned' (i.e. made transparent, allowing the reflective layer to reflect once more) by setting the laser to a different power setting. Once again this dye layer reduces the strength of the reflected light, leading to playback problems with some (usually older) players.