A dual-layer disc has two layers of data, one of them semitransparent so that the laser can focus through it and read the second layer. Because both layers are read from the same side, a dual-layer disc can hold almost twice as much as a single-layer disc, typically four hours of video.
Many discs use dual layers. Initially, only a few replication plants could make duallayer discs, but most plants now have the capability. The second layer can use either a parallel track path (PTP) layout where both tracks run in parallel (for independent data or special switching effects) or an opposite track path (OTP) layout where the second track runs in an opposite spiral.
That is, the pickup head reads out from the center on the first track and then in from the outside on the second track. The OTP layout is designed to provide continuous video across both layers. OTP is also called reverse-spiral dual layer (RSDL).
The layer change can occur anywhere in the video; it doesn’t have to be at a chapter point. No guarantee exists that the switch between layers will be seamless. The layer change is invisible on some players, but it can cause the video to freeze for a fraction of a second or up to four seconds on other players.
The seamlessness depends as much on the way the disc is prepared as on the design of the player. The advantage of two layers is that long movies can use higher data rates for better quality than a single layer.
Dual-layer discs can be recognized by three characteristics: 1) the gold color, 2) a menu on the disc for selecting the widescreen or letterbox version, and 3) two serial numbers on one side. The DVD specification requires that players and drives read dual-layer discs.
Few units have problems with dual-layer discs; this is a design flaw and should be corrected for free by the manufacturer. Some discs are designed with a seamless layer change that technically goes beyond what the DVD spec allows. This causes problems on a few older players.
All players and drives also play double-sided discs if you flip them over. No manufacturer has announced a model that can play both sides without someone manually flipping the disc, other than a few DVD jukeboxes.
The added cost of this capability is hard to justify because discs can hold over four hours of video on one side by using two layers. (Early discs used two sides because dual-layer production was not widely supported. This is no longer a problem.) Pioneer laserdisc/DVD players can play both sides of an laserdisc, but not a DVD..