AVS Special Member
Join Date: May 2008
Location: San Francisco - East Bay area
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It is well-documented that phosphor-based (crt and plasma) and projection lamp video displays drift over time. And the drift is not always in the same direction(s).
It is also well-documented that Panasonic slowly raises the black level of their plasma displays as hours are accumulated. Several years ago there was quite a dust-up about it because Panasonic was not changing the black level very often and when they did, the jump was large enough to be fairly obvious. That annoyed a lot of owners, so rather than figure out a way to not have to raise the black level over time (citing technical/performance reasons), Panasonic changed to more frequent increases of black level in much smaller steps so it would not be so obvious... you still lose the deep blacks over time, but they sneak it up on you rather than make a big change all at once.
"Movies is magic..." Van Dyke Parks
THX Certified Professional Video Calibration
Widescreen Review -- Home Theater & Sound