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Re: [Phys-l] 240 and 600 Hz

The frame rate of traditional movies is actually 24Hz, but I understand that
films would sometimes have shutters that would strobe the picture at a
higher rate. Actually TV is 30Hz while movies are 24. Or rather TV is 60Hz
but that is for half frames interlaced and each half frame has some motion
in between them.

The disparity between 24 and 30 f/s is resolved by having 3 frames in
succession and then two frames with interlaced mixtures of f3&4 then f4&5 to
convert 24f/s to 30f/s. This is called 3/2 pulldown and it results in some
jerkiness in the picture, but is still much smoother than just doubling the
4th frame to produce 5 frames from 4.

As to whether a high refresh rate is necessary, it depends on the
individual. Apparently some people are very sensative to flicker and are
bothered by the 60Hz fluorescent light flicker. So some fluorescents use a
higher frequency of AC. But even then, tests have shown that most people
judge the higher refresh rate to produce a better picture. 120Hz is the
minimum needed for reproduction of traditional 30Hz TV and 24Hz film, so 240
is just double that.

Higher refresh rates would probably generally not be noticeable on
conventional phosphor screens because of the persistence of the phosphors.

Incidentally the old idea of persistence of vision has been deprecated.

So in the end the real test is for you to look at the set and see if you can
notice a difference. Our senses vary quite a lot and the old models still
in the textbooks do not reflect modern research. For example some women
have 4 different cones and have "super" colore vision. Sorry men, you never
have this!

John M. Clement
Houston, TX


Some new High Definition TV sets are designed for very high
image refreshment rates, such as 240 Hz and even 600 Hz. This
seems to be unnecessary; the refreshment rate for traditional
movies has been 30 Hz. Because of this I think that anything
above 60 Hz is just waste of money. Do you agree?

Ludwik

http://csam.montclair.edu/~kowalski/life/intro.html