Hardware Reference
In-Depth Information
dataisbeingstored,DVDstreatallinformation equallyandapplythefullerrorcorrection
to all sectors.
The main error correcting in DVDs takes place in the ECC frame. Parity Outer (column)
and Parity Inner (row) bits are added to detect and correct errors. The scheme is simple
yet effective. The information from the data frames is first broken up into 192 rows of
172 bytes each. Then a polynomial equation is used to calculate and add 10 PI bytes to
eachrow,makingtherows182byteseach.Finally,anotherpolynomialequationisusedto
calculate 16 PO (Parity Outer) bytes for each column, resulting in 16 bytes (rows) being
added to each column. What started out as 192 rows of 172 bytes becomes 208 rows of
182 bytes with the PI and PO information added.
The function ofthe PIand PObytes can be explained with a simple example using simple
parity. In this example, 2 bytes are stored (01001110 = N, 01001111 = O). To add the
error-correcting information, they are organized in rows, as shown here:
Data bits
1 2 3 4 5 6 7 8
——————————————————-
Byte 1 0 1 0 0 1 1 1 0
Byte 2 0 1 0 0 1 1 1 1
——————————————————-
Then one PI bit is added for each row, using odd parity. This means you count up the 1
bits: In the first row there are four, so the parity bit is created as a 1, making the sum an
odd number. In the second row, the parity bit is a 0 because the sum of the 1s was already
an odd number. The result is 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
——————————————- |————-
Next,theparitybitsforeachcolumnareaddedandcalculatedthesameasbefore.Inother
words, the parity bit will be such that the sum of the 1s in each column is an odd number.
The result is 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 1 1 0 | 1
