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Interlaced Scan (i)
After each exposure, odd (1,3,5…) or even (2,4,6…) lines
of the full frame are alternately captured/output from
the sensor, as so called fields. Two sequentially output
fields make up one frame. Fields are output at twice
the frequency of frames from progressive capture. This
principle is based on the afterglow of CRT phosphors and
the characteristics of human vision, which results in two
fields being perceived as a continuous image once the
frequency is high enough. Due to the higher frequency
of the fields, this scanning method provides a smooth
motion representation. Fast horizontal movement or
quick pans, however, cause jagged vertical edges in the
image, as the two fields composing a frame are captured
one after another. This reduces the quality of keying or
masking in postproduction. Interlaced material usually
has to be de-interlaced (combining fields to frames),
before it can be worked on using today’s post processes.
Interlaced material is denoted with the letter
“i”, attached to the image format:
1080i - interlaced HD with 1920x1080 resolution
•
25i - interlaced material at 25 fps i.e. 50 fields/sec
•
1080/25i as a combination of the above.
•
Some people prefer relating to fields per second
rather than frames per second, e.g. 50i or 60i.
As current cameras also offer progressive
capture of 50 or 60 fps, the declaration of
fields per second can be rather confusing.
Progressive Segmented Frame (PsF)
This is not a scanning method used for capturing,
but a transmission method. Basically, a progressively
captured frame is segmented into two fields, which
are then transmitted similarly to an interlaced signal.
This enables devices that usually work with interlaced
material to also work with progressive material. The
notable difference to an interlaced scan signal is that
both fields originate from the same frame and have not
been captured sequentially. Therefore, transmitting
progressive scan material using PsF means no loss in
image quality. Motion representation and resolution
is exactly the same as in progressive scan material.
Progressive segmented frame material is denoted
with the letters “PsF", attached to the image format:
1080/25PsF - progressive HD with 1920x1080
•
resolution at 25 fps, transmitted as PsF.
