Specifications

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much higher than the desired passband. This allows the
use of more easily realized anti-alias lters.
PAL – During the early days of color television in North
America, European broadcasters developed a competing
system called Phase Alternation by Line. This slightly more
complex system is better able to withstand the dierential
gain and phase errors that appear in analog ampliers and
transmission systems. Engineers at the BBC claim that it
stands for Perfection At Last.
Pathological Test Pattern – see Checkeld
Progressive – An imaging scanning technique that progresses
through all of the lines of a frame in a single pass.
Computer monitors all use progressive displays. This
contrasts to the Interlace technique common to television
systems.
Return Loss – An idealized input or output circuit will exactly
match its desired impedance (generally 75 ohms) as a
purely resistive element, with no reactive (capacitive
or inductive) elements. In the real world, we can only
approach the ideal. So, our real inputs and outputs
will have some capacitance and inductance. This will
create impedance matching errors, especially at higher
frequencies. The Return Loss of an input or output
measures how much energy is returned (reected back
due to the impedance mismatch). For digital circuits, a
return loss of 15 dB is typical. This means that the energy
returned is 15 dB less than the original signal. In analog
circuits, a 40 dB gure is expected.
RGB – RGB systems carry the totality of the picture
information as independent Red, Green, and Blue signals.
Television is an additive color system, where all three
components add to produce white. Because the luminance
(or detail) information is carried partially in each of the
RGB channels, all three must be carried at full bandwidth
in order to faithfully reproduce an image.
ScH Phase – Used in composite systems, ScH Phase
measures the relative phase between the leading edge
of sync on line 1 of eld 1 and a continuous subcarrier
sinewave. Due to the arithmetic details of both PAL and
NTSC, this relationship is not the same at the beginning
of each frame. In PAL, the pattern repeats every 4 frames
(8 elds) which is also known as the Bruch Blanking
sequence. In NTSC, the repeat is every 2 frames (4 elds.)
This creates enormous headaches in editing systems and
the system timing of analog composite facilities.
SDI – Serial Digital Interface. This term refers to inputs and
outputs of devices that support serial digital component
video. This generally means standard denition at
270 Mb/s. The use of “HD-SDI” is used to indicate High
Denition Serial Digital Video at 1.485 Gb/s.
SMPTE – The Society of Motion Picture and Television
Engineers is a professional organization which has
done tremendous work in setting standards for both the
lm and television industries. The term “SMPTE’” is also
shorthand for one particular component video format -
luminance and color dierence.
Single Mode – A Single mode (or mono mode) optical
ber carries an optical signal on a very small diameter
(9 micron) core surrounded with cladding. The small
diameter means that no internally reected lightwaves
will be propagated. Thus only the original “mode” of the
signal passes down the ber. A single mode ber used
in an optical SDI system can carry a signal for up to 20
kilometers. Single mode bers require particular care
in their installation due to the extremely small optical
aperture that they present at splice and connection
points. See also Multi-mode.
TBC – A Time Base Corrector is a system to reduce the Time
Base Error in a signal to acceptable levels. It accomplishes
this by using a FIFO (First In, First Out) memory. The
incoming video is written into the memory using its own
jittery timing. This operation is closely associated with
the actual digitization of the analog signal because the
varying position of the sync timing must be mimicked by
the sampling function of the analog to digital converter.
A second timing system, genlocked to a stable reference,
is used to read the video back out of the memory. The
memory acts as a dynamically adjusting delay to smooth
out the imperfections in the original signal’s timing. Very
often a TBC will also function as a Frame Synchronizer. See
also Frame Sync.
Time Base Error – Time base error is present when there is
excessive jitter or uncertainty in the line to line output
timing of a video signal. This is commonly associated with
playback from video tape recorders, and is particularly
severe with consumer type heterodyne systems like VHS.
Time base error will render a signal unusable for broadcast
or editing purposes.
Timecode – Timecode, a method to uniquely identify and
label every frame in a video stream, has become one of
the most recognized standards ever developed by SMPTE.
It uses a 24 hour clock, consisting of hours, minutes,
seconds, and television frames. Originally recorded on a
spare audio track, this 2400 baud signal was a signicant
contributor to the development of video tape editing. We
now refer to this as LTC or Longitudinal Time Code because
it was carried along the edge of the tape. This allowed it to
be recovered in rewind and fast forward when the picture
itself could not. Timecode continues to be useful today and
is carried in the vertical interval as VITC, and as a digital
packet as DVITC. Timecode is the true metadata.
Tri-Level Sync – For many, many years, television systems
used composite black as a genlock reference source. This
was a natural evolution from analog systems to digital
implementations. With the advent of High Denition
television, with even higher data rates and tighter jitter
requirements, problems with this legacy genlock signal
surfaced. Further, a reference signal with a 50 or 60 Hz
frame rate was useless with 24 Hz HD systems running
at lm rates. Today we can think of composite black as a
bi-level sync signal – it has two levels, one at sync tip and
one at blanking. For HD systems, Tri-Level Sync, which
has the same blanking level (at ground) of bi-level sync,
but the sync pulse now has both a negative and a positive