BrightEye 24 SDI to Analog Converter and Disembedder
Interlace
Human vision can be fooled to see motion by present a series of images, each with
a small change relative to the previous image. In order to eliminate the flicker,
our eyes need to see more than 30 images per second. This is accomplished in
television systems by dividing the lines that make up each video frame (which
run at 25 or 30 frames per second) into two fields. All of the odd-numbered lines
are transmitted in the first field, the even-numbered lines are in the second field.
In this way, the repetition rate is 50 or 60 Hz, without using more bandwidth.
This trick has worked well for years, bit it introduces other temporal artifacts.
Motion pictures use a slightly different technique to raise the repetition rate from
the original 24 frames that make up each second of film—they just project each
one twice.
IRE
Video level is measured on the IRE scale, where 0 IRE is black, and 100 IRE is
full white. The actual voltages that these levels correspond to can vary between
formats.
ITU-R 601
This is the principal standard for standard definition component digital video. It
defines the luminance and color difference coding system that is also referred to
as 4:2:2. The standard applies to both PAL and NTSC derived signals. They both
will result in an image that contains 720 pixels horizontally, with 486 vertical
pixels in NTSC, and 576 vertically in PAL. Both systems use a sample clock rate
of 27 Mhz, and are serialized at 270 Mb/s.
Jitter
Serial digital signals (either video or audio) are subject to the effects of jitter. This
refers to the instantaneous error that can occur from one bit to the next in the
exact position each digital transition. Although the signal may be at the correct
frequency on average, in the interim it varies. Some bits come slightly early, other
come slightly late. The measurement of this jitter is given either as the amount of
time uncertainty or as the fraction of a bit width. For 270 Mb/s video, the
allowable jitter is 740 picoseconds, or 0.2 UI (Unit Interval – one bit width).
Luminance
The “black & white” content of the image. Human vision had more acuity in
luminance, so television systems generally devote more bandwidth to the
luminance content. In component systems, the luminance is referred to as Y.
Multi-mode
Multi-mode fibers have a larger diameter core than single mode fibers (either 50
or 62.5 microns compared to 9 microns), and a correspondingly larger aperture. It
is much easier to couple light energy into a multi-mode fiber, but internal reflec-
tions will cause multiple “modes” of the signal to propagate down the fiber. This
will degrade the ability of the fiber to be used over long distances.
See also Single mode.
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