Why Do Cds And Dvds Add Data From The Center Outwards

When burning a CD or DVD at home, you might find yourself wondering why data is always added from the center outwards? Would it really matter if the data was added in the opposite direction like old vinyl records were fixed? Is there more to it? Today’s SuperUser Q&A post has the answer to a curious reader’s question.

Today’s Question & Answer session comes to us courtesy of SuperUser—a subdivision of Stack Exchange, a community-driven grouping of Q&A web sites.

The Question

SuperUser reader Burgi wants to know why CDs and DVDs add data from the center outwards:

Why do CDs and DVDs add data from the center outwards?

Older rotating-disc media like vinyl records started from the outer edge and went in toward the center, so it could not have been for historical reasons.

I am looking for good sources that explain the reasoning for this data structure/setup.

The Answer

SuperUser contributor DavidPostill has the answer for us:

Have something to add to the explanation? Sound off in the comments. Want to read more answers from other tech-savvy Stack Exchange users? Check out the full discussion thread here.

Note that the assumption made in the question above is incorrect.

Summary

For historical reasons, writing (and reading) from the inside makes sense (different sized discs are possible as explained in other answers). For read performance reasons, modern discs may be written (and read) outside in or even in both directions (dual layer).

Notes

Most discs are a standard size. Commercially produced CDs and DVDs are not written at all, they are stamped on a press.

Source: The Difference Between Commercial and Home-Recorded DVDs

The rest of this answer concentrates on the performance aspect. The exceptions to writing (and reading) from the inside are Xbox games (and other game consoles’ games) and Dual Layer DVDs (movies).

Xbox Games

Xbox games have the data written from the outside in for performance reasons. Since the outside is spinning faster than the inside, the data can be read faster.

Source: Hardware Behind the Consoles – Part I: Microsoft’s Xbox

Dual Layer DVDs (Movies)

Dual Layer DVDs can be written in either direction, there are two write modes. Most movies are written using the opposite track path technique. A movie will be split across across the layers, so no seek back to the inner edge is needed at the layer change point.

Source: DVD-R DL

Image Credit: Freejpg (Flickr)