CHAPTER 13

726 Chapter 13 Optical Storage
before. To see how it works, let’s change one of the data bits (due to a read error) and recalculate the
error correcting bits as follows:
Data bits |
1 2 3 4 5 6 7 8 | PI
---------------------------------------|---------
Byte 1 0 1 0 0 1 0 1 0 | 0
Byte 2 0 1 0 0 1 1 1 1 | 0
---------------------------------------|---------
PO 1 1 1 1 1 0 1 0 | 1
Now, when you compare the PI and PO bits you calculated after reading the data to what was originally
stored, you see a change in the PI bit for byte (row) 1 and in the PO bit for bit (column) 6. This
identifies the precise row and column where the error was, which is at byte 1 (row 1), bit 6 (column
6). That bit was read as a 0, and you now know it is wrong, so it must have been a 1. The error correction
circuitry then simply changes it back to a 1 before passing it back to the system. As you can see,
with some extra information added to each row and column, error correction codes can indeed detect
and correct errors on-the-fly.
Besides the ECC frames, DVDs also scramble the data in the frames using a bit-shift technique and
also interleave parts of the ECC frames when they are actually recorded on the disc. These schemes
serve to store the data somewhat out of sequence, preventing a scratch from corrupting consecutive
pieces of data.
DVD Capacity (Sides and Layers)
Four main types of DVD discs are available, categorized by whether they are single- or double-sided,
and single- or dual-layered. They are designated as follows:
■ DVD-5 - 4.7GB Single-Side, Single-Layer. A DVD-5 is constructed from two substrates bonded
together with adhesive. One is stamped with a recorded layer (called Layer 0), and the other is
blank. An aluminum coating typically is applied to the single recorded layer.
■ DVD-9 - 8.5GB Single-Side, Dual-Layer. A DVD-9 is constructed of two stamped substrates bonded
together to form two recorded layers for one side of the disc, along with a blank substrate for
the other side. The outer stamped layer (0) is coated with a semitransparent gold coating to
both reflect light if the laser is focused on it and pass light if the laser is focused on the layer
below. A single laser is used to read both layers; only the focus of the laser is changed.
■ DVD-10 - 9.4GB Double-Side, Single-Layer. A DVD-10 is constructed of two stamped substrates
bonded together back to back. The recorded layer (Layer 0 on each side) usually is coated with
aluminum. Note that these discs are double-sided; however, drives have a read laser only on the
bottom, which means the disc must be removed and flipped to read the other side.
■ DVD-18 - 17.1GBDouble-Side, Dual-Layer. A DVD-18 combines both double layers and double
sides. Two stamped layers form each side, and the substrate pairs are bonded back to back. The
outer layers (Layer 0 on each side) are coated with semitransparent gold, whereas the inner layers
(Layer 1 on each side) are coated with aluminum. The reflectivity of a single-layer disc is
45%–85%, and for a dual-layer disc the reflectivity is 18%–30%. The automatic gain control
(AGC) circuitry in the drive compensates for the different reflective properties.
Figure 13.9 shows the construction of each of the DVD disc types.
Note that although Figure 13.9 shows two lasers reading the bottom of the dual-layer discs, in actual
practice only one laser is used. Only the focus is changed to read the different layers.