Optical Storage - Upgrading and Repairing PCs - page 552

error-correction-bit calculations are repeated and they're checked to see whether they are the same as
before. To see how this works, let's change one of the data bits (due to a read error) and recalculate
the error-correcting bits as follows:
Click here to view code image
Data bits |
1 2 3 4 5 6 7 8 | PI
----------------------------- |---------
Byte 1 0 1 0 0 1 1 1 0 | 1
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
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 DVDs 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 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 (17.1GB double-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