The BenQ DW-1640A performed well with the first batch of discs, as we can
see in the first chart above. Note that with a DVD disc, the number of PI errors
occurring in eight consecutive ECC blocks is defined as "PI-Sum8," and
the maximum value for PI-Sum8 is specified as 280 or less. The average PI
Sum 8 values are lower than the 280 limit and
so the disc is in
a good overall condition.

Things aren't the same with the next set of discs, as we can see in the following
chart. More than the half of the discs showed a high PI sum 8 count,
peaking as the measurements move towards the outer areas of the disc's surface.
This was expected, since the DW-1640A reaches its maximum recording speed
(16x using CAV) there, so the possibility of data being written close to the limits is more likely. In the following graph, the red dotted line indicates
the PI Sum8 upper limit (280).

The MID codes that produced high error rates are Ritek R04, Ritek F1,
MCC 03RG20, Optodisc OR16 and CMC MAG M01, as shown on the graph
above. Note here, that the PI Sum8 errors are handled by the first level of
error correction employed on DVDs. Their count normally represents
their severity, which means that high PIE counts normally trigger Parity Output Errors which can in turn become Parity
Outer Fail (POF) errors. The occurrence of POF is not acceptable for
DVDs, since these are uncorrectable errors. So measuring POF for the "problem" MIDs in the previous
scans is mandatory.

The results above confirm the correlation between high PI Sum 8 and the likelihood of POF. A
pleasant exception is the Daxon AZ3 DVD+R disc, which returned zero POF,
despite the high PI Sum 8 counts.