SarnDling arid Periodic Signals Interpolation njlicii the film is played back is v ~ a 5 y to iiiiplrmtmt w hell each image is projected for the corresponding lcngth iine, altliougli in
Trang 1284 11 SarnDling arid Periodic Signals
Interpolation njlicii the film is played back is v ~ a 5 y to iiiiplrmtmt w hell each
image is projected for the corresponding lcngth iine, altliougli in fa(%, llicly ale
riot there for thc eritire interval T = 1/248, as hc%wen two images the film has to
l w moved There is no pr:wtical interpolatiori filter available foi this optical signal,
as in Figuie 11.9, so the re-occurrences of the spectrum at 24
are iiot removcd and w e p m c e i v d by the eyes as Aiclcr Tlic
fiicker up t o around 60 Hz, but tlie first re-occuiieiice at 24Hz
To avoid this problem, a trick can be used: iristeittl of interrupting
on of an image once for tlie filiri transport it is interrupted twice (see
5) The inkvupting i s done with a niechanical hlirrtl which i s easily
arriple rate (to 48 HE), and pairs of coriseqitential
1 0 implcmcnt It doubl~s t h
sample valiics are equal because they c ~ r r i e from the same original
‘ movement of film
L
The effectiveiicss of the clouble projcction proccdurc can be cxp1;~inctl t2icorc.t-
ically IVe reprrserit the doublr projcction of a11 image as the ilupulsc> responsc of
x i inte~~)olati(~ri filter (see Figure 11.25) and find the spectrum:
(11.53)
(11.55)