How a DualDisc works
DualDiscs were based on double-sided DVD technology such as DVD-10, DVD-14 and DVD-18 except that DualDisc technology replaced one of the DVD sides with a CD. The discs were made by fusing together a standard 0.6 mm-thick DVD layer (4.7-gigabyte storage capacity) to a 0.9 mm-thick CD layer (60-minute or 525-megabyte storage capacity), resulting in a 1.5 mm-thick double-sided hybrid disc which contained CD content on one side and DVD content on the other.
The challenge for the designers of DualDisc was to produce a dual-sided disc which was not too thick to play reliably in slot-loading drives, while the CD side was not too thin to be tracked easily by the laser. DVDplus, though conceptually similar, used a thicker CD layer and thus is more likely to get stuck in a slot-loading player (although this appears to be almost unknown); DualDisc took the other course by thinning the CD layer.
Because the 0.9 mm thickness of the DualDisc CD layer did not conform to Red Book CD Specifications, which called for a layer no less than 1.1 mm thick, some CD players could not play the CD side of a DualDisc due to a phenomenon called spherical aberration. As a result, the laser reading the CD side might get a "blurry" picture of the data on the disc — the equivalent of a human reading a book with glasses of the wrong strength. Engineers tried to get around this by making the pits in the CD layer larger than on a conventional CD. This makes the CD side easier for the laser to read — equivalent to a book using bigger print to make it easier to see, even if the person's glasses are of the wrong strength. The downside to this, however, is that the playing time for the CD layer of some early DualDiscs decreased from the standard 74 minutes of a conventional CD to around 60 minutes, although this early limitation was later overcome.
Because the DualDisc CD layer did not conform to Red Book specifications, Philips and Sony refused to allow DualDisc titles to carry the CD logo and most DualDiscs contain one of two warnings:
- "This disc is intended to play on standard DVD and CD players.
May not play on certain car, slot load players and
- "The audio side of this disc does not conform to CD specifications and therefore not all DVD and CD players will play the audio side of this disc."
The DVD side of a DualDisc completely conformed to the specifications set forth by the DVD Forum and DualDiscs have been cleared to use the DVD logo.