Hardware Reference
In-Depth Information
Now the code is complete, and the extra bits are stored along with the data. So, instead of
just the 2 bytes being stored, 11 additional bits are stored for error correction. When the
data is read back, the 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:
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.Thisidentifiesthepreciserowandcolumnwheretheerrorwas,whichisat
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
toeachrowandcolumn,error-correctioncodescanindeeddetectandcorrecterrorsonthe
fly.
BesidestheECCframes,DVDsalsoscramblethedataintheframesusingabit-shifttech-
nique 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)
FourmaintypesofDVDsareavailable,categorizedbywhethertheyaresingle-ordouble-
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 sub-
strates bonded with adhesive. One is stamped with a recorded layer (called Layer 0),
andtheotherisblank.Analuminum coating typically isapplied tothesingle 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 lay-
ers; only the focus of the laser is changed.
