Colour Television - DA100/DW100 Chassis
AUTOMATIC GAIN CONTROL
A digitally working automatic gain control adjusts the magnitude of the selected base-band by +6/-4.5
dB in 64 logarithmic steps to the optimal range of the ADC. The gain of the video input stage including
the ADC is 213 steps/V with the AGC set to 0 dB.
ANALOGUE -TO -DIGITAL CONVERTERS
Two ADCs are provided to digitise the input signals. Each converter runs with 20.25 MHz and has 8
bit resolution. An integrated band-gap circuit generates the required reference voltages for the
converters.
DIGITALLY CONTROLLED CLOCK OSCILLATOR
The clock generation is also a part of the analogue front end. The crystal oscillator is controlled
digitally by the control processor; the clock frequency can be adjusted within ?150 ppm.
ANALOGUE VIDEO OUTPUT
The input signal of the Luma ADC is available at the analogue video output pin. A source follower
must buffer the signal at this pin. The output voltage is 2 V, thus the signal can be used to drive a 75W
line. The magnitude is adjusted with an AGC in 8 steps together with the main AGC.
AVERAGE BEAM CURRENT LIMITING
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 approximately. 2 kHz.
The beam current limiter has an automatic offset adjustment that is active two lines before the first
cut-off measurement line.
The beam current limiter allows the setting of a threshold 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 analogue contrast multipliers
for the analogue RGB inputs, respectively.
The lower limit of the attenuator is programmable, thus a minimum contrast can always be set. During
the CRT measurement, the ABL attenuation is switched off. After the white drive measurement line it
takes 3 lines to switch back to BCL limited drives and brightness.
PROTECTION CIRCUITRY
?Vertical fly-back? and the ?Safety? inputs provide picture tube and drive stage protection.
Vertical fly-back;This pin searches for a negative edge in every field, otherwise the RGB
drive signals are blanked. Drive shutoff during fly-back: this feature can be
selected by software.
Safety;Input pin: this input has two thresholds. Between zero and the lower
threshold, normal functioning takes place. Between the lower and the
higher threshold, the RGB signals are blanked. Above the higher
threshold, the RGB signals are blanked and the horizontal drive is shut off.
Both thresholds have a small hysteresis. The main oscillator and the
horizontal drive circuitry are run from a separate (standby) power supply
and are already active while the TV set is powering up.
Note:
In the DA100 and DW100 chassis the standby supply pin is connected to VCC.
SCAN VELOCITY MODULATION OUTPUT
This output delivers the analogue SVM signal. The D/A converter is a current sink like the RGB D/A
converters. At zero signal level the output current is 50% of the maximum output current.
Sharp Electronics (UK) Ltd
Technical Support
September 99Page 12 |
ACEC
