Ensemble Designs 9670 Audio Automatic Gain and Loudness Control and 9690 Audio Compliance and Monitoring Software - Manual (Page 31)

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Avenue 9670 - page 31
LevelTrack™ Audio Loudness Control AGC Software Option
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 significant 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
Definition 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 film 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 element. This keeps the signal symmetrically balanced so that its DC content is zero. And it
also means that the timing pickoff point is now at the point where the signal crosses blanking and is
no longer subject to variation with amplitude. This makes Tri-Level Sync a much more robust signal
and one which can be delivered with less jitter.
USB
The Universal Serial Bus, developed in the computer industry to replace the previously ubiquitous
RS-232 serial interface, now appears in many different forms and with many different uses. It actually
forms a small local area network, allowing multiple devices to coexist on a single bus where they can
be individually addressed and accessed.
VGA