In article , Bill Taylor
wrote:
AFAIR from a course that I did sometime ago DigiBeta doesn't record
the raw digits, it does a discrete cosine transform on the picture
information and then records the result of that transform. Most of the
time this is completely reversible and so lossless, but on very
testing material the higher orders can be thrown away, so some very
slight loss could be experienced. In reality the format is pretty much
lossless. Multiple passes have been done experimentally and the
perceptible degradation is either very low or none existent, unlike
analogue formats or some of the more highly compressed formats that
are used these days.

I've only had the sales talk, not a technical course, so I never got a
completely satisfactory explanation for how it is possible for a digital
bit rate compression system with any loss whatsoever to allow a signal to
pass through an unlimited number of times without cumulative losses
showing after a few dozen passes. Yet they showed us some special effects
that required several hundred passes, and split screen displays of
severalhundredth generation against the original, with no visible
degradation at all, and they assured us that none was measureable.

Interpreting what the salesman was able to tell us, I think the system
must only be applying bit rate compression to those parts of the signal
that go above a "threshold" value coresponding to the maximum rate that
it can handle (50 Mb/s?), but passing it untouched the rest of the time.
This would result in the second and subsequent passes being passed
unchanged because they had already been compressed once. Even that "very
testing material" would only suffer whatever losses resulted from the
first pass, and could then be copied an infinite number of times without
becoming any worse. Perhaps somebody who knows more about it could
confirm this?

Rod.