Description
This article is from the DVD Formats
FAQ, by jtfrog@usa.net (Jim Taylor) with numerous
contributions by others.
1.18 What's a dual-layer disc? Will it work in all players?
A dual-layer disc has two layers of data, one of them semi-transparent so
that the laser can focus through it and read the second layer. Since both
layers are read from the same side, a dual-layer disc can hold almost twice
as much as a single-layer disc, for over 4 hours of video (see 3.3 for more
details). Many discs use dual layers. Initially only a few replication
plants could make dual-layer discs, but most plants now have the
capability. The second layer can use either a PTP (parallel track path)
layout where both tracks run in parallel (for independent data or special
switching effects), or an OTP (opposite track path) layout where the second
track runs in an opposite spiral; that is, the pickup head reads out from
the center on the first track then in from the outside on the second track.
The OTP layout is designed to provide continuous video across both layers.
The layer change can occur anywhere in the video; it doesn't have to be at
a chapter point. There's no guarantee 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 4 seconds on
other players. The "seamlessness" depends as much on the way the disc is
prepared as on the design of the player. OTP is also called RSDL
(reverse-spiral dual layer). The advantage of OTP/RSDL is that long movies
can use higher data rates for better quality than with a single layer. See
1.27 for layer change details.
There are various ways to recognize dual-layer discs: 1) the gold color, 2)
a menu on the disc for selecting the widescreen or letterbox version, 3)
two serial numbers on one side.
All DVD players and drives can read dual-layer discs -- it's required by
the spec. All players and drives also play double-sided discs if you flip
them over. No manufacturer has announced a model that will play both sides.
The added cost is probably not justifiable since discs can hold over 4
hours of video on one side by using two layers. (Early discs used two sides
because dual-layer production was not widely supported. This should no
longer be a problem.) Pioneer LD/DVD players can play both sides of an LD,
but not a DVD. (See 2.12 for note on reading both sides simultaneously.)
Continue to:
Share and Enjoy
Bookmark this story so others can enjoy it:
Tags
pc, dvd, dvd-rom, dvd-video, DVD-R, DVD-RAM, DVD-RW, DVD+RW, recording, playing
