1-4. GENERAL DESCRIPTION.
The 455 Composite Subcarrier System is engineered to provide a transparent path for the
complete MTS/BTSC composite signal over any video microwave system. With its extremely
low distortion (less than 0.05% at 1kHz), flat frequency response from 20Hz to 120kHz, and
outstanding long-term stability, the 455 has almost no measurable effect on stereo audio
performance.
Both the modulator and the demodulator are fully frequency synthesized and microprocessor
controlled for ease of operation and the ultimate in versatility. All of the critical operating
parameters, as well as any alarm conditions, are displayed on the front panel LCD. Any operating
frequency from 1 MHz to 9.99 MHz can be selected from the front panel. Also selected from
the front panel is a choice of "wide" or "narrow" deviation settings. The wide setting is used to
provide optimum signal-to-noise performance for STL applications, while the narrow setting
provides BTSC broadcast standard compatibility which (with the unit set to 4.50 MHz) can be
used for direct feeds to cable systems or low power transmitter facilities. In spite of this
convenient user interface, a number of safeguards make it nearly impossible to change the
settings of the unit by accident. A built in non-volatile memory device retains the most recent
configuration of the unit during temporary power outages or even prolonged storage without
power.
A remote control input is provided which can be used to either turn the channel on or off, or to
switch the operating frequency. This feature is extremely useful for redundant or backup system
configurations, and, like all of the 455's features, can be easily configured from the front
panel.
The 455 demodulator incorporates Tectan's patented threshold extension FM detector. This
unique circuit permits the 455 to flawlessly track the incoming FM signal under severe noise
conditions which render other receivers useless. This detector design is also inherently stable
with respect to both amplitude and phase, without resorting to complicated automatic modulation
control circuits.
The video baseband is combined with the subcarrier signals in the transmitter by a low component
count passive directional coupler. This circuit has been designed to put virtually no load on the
video line (
≥
3 K
Ω
), allowing multiple units to be "daisy-chained" without adversely affecting the
signals on the baseband. The low component count and passive nature of this circuit provide
excellent protection against disruption of the baseband due to component failure or loss of power
in the 455.
455 Operation Manual
Page 10
6/23/98
