BrightEye 90-A HD Up/Down/Cross Converter and ARC with Analog Audio
BrightEye 90-A - Page 40
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
Video Graphics Array. Traditional 15-pin, analog interface between a PC and
monitor.
Word Clock
Use of Word Clock to genlock digital audio devices developed in the audio
recording industry. Early digital audio products were interconnected with a
massive parallel connector carrying a twisted pair for every bit in the digital
audio word. A clock signal, which is a square wave at the audio sampling
frequency, is carried on a 75 ohm coaxial cable. Early systems would daisychain
this 44.1 or 48 kilohertz clock from one device to another with coax cable and Tee
connectors. On the rising edge of this Work Clock these twisted pairs would carry
the left channel, while on the falling edge, they would carry the right channel. In
most television systems using digital audio, the audio sample clock frequency
(and hence the 'genlock' between the audio and video worlds) is derived from the
video genlock signal. But products that are purely audio, with no video reference
capability, may still require Word Clock.
WSS
Wide Screen Signaling is used in the PAL/625 video standards, both in analog and
digital form, to convey information about the aspect ratio and format of the
transmitted signal. Carried in the vertical interval, much like closed captioning, it
can be used to signal a television receiver to adjust its vertical or horizontal sizing
to reflect incoming material. Although an NTSC specification for WSS exists, it
never achieved any traction in the marketplace.
