ADVANCE INFORMATION
VCT 38xxA
2.11.3.Average Beam Current Limiter
2.11.4.Analog RGB Insertion
The average beam current limiter (BCL) uses the
SENSE input for the beam current measurement. The
BCL uses a different filter to average the beam current
during the active picture. The filter bandwidth is
approx. 2 kHz. The beam current limiter has an auto-
matic offset adjustment that is active two lines before
the first cutoff measurement line.
The VCT 38xxA allows insertion of external analog
RGB signals. The RGB signal is key-clamped and
inserted into the main RGB by the Fast-Blank switch.
The external RGB input can be overlaid or underlaid to
the digital picture. The external RGB signals can be
adjusted independently as regards DC level (bright-
ness) and magnitude (contrast).
The beam current limiter function is located in the
front-end. The data exchange between the front-end
and the back-end is done via a single-wire serial inter-
face.
All signals for analog RGB insertion (RIN, GIN, BIN,
FBLIN) must be synchronized to the horizontal flyback,
otherwise a horizontal jitter will be visible. The
VCT 38xxA has no means for timing correction of the
analog RGB input signals.
The beam current limiter allows the setting of a thresh-
old current. If the beam current is above the threshold,
the excess current is low-pass filtered and used to
attenuate the RGB outputs by adjusting the white-drive
multipliers for the internal (digital) RGB signals, and
the analog contrast multipliers for the analog RGB
inputs, respectively. The lower limit of the attenuator is
programmable, thus a minimum contrast can always
be set. During the tube measurement, the ABL attenu-
ation is switched off. After the white-drive measure-
ment line it takes 3 lines to switch back to BCL limited
drives and brightness.
2.11.5.Fast-Blank Monitor
The presence of external analog RGB sources can be
detected by means of a Fast-Blank monitor. The status
of the Fast-Blank input can be monitored via an I2C
bus register. There is a 2 bit information, giving static
and dynamic indication of a Fast-Blank signal. The
static bit is directly reading the Fast-Blank input line,
whereas the dynamic bit is reading the status of a
flip-flop triggered by the negative edge of the Fast-
Blank signal.
Typical characteristics of the ABL for different loop
gains are shown in Fig. 2–22; for this example the tube
has been assumed to have square law characteristics.
With this monitor logic it is possible to detect if there is
an external RGB source active and if it is a full screen
insertion or only a box. The monitor logic is connected
directly to the FBLIN pin.
Fig. 2–22: Beam current limiter characteristics:
beam current output vs. drive
BCL threshold: 1
Micronas
27
MICRONAS [ MICRONAS ]
