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DA100 (50Hz) Chassis – Article for Television Magazine
Page 57 of 80
Sharp Electronics (UK) Limited - March 2003
Revision 2
Input Selector
Up to five analogue inputs can be connected. Four inputs are for composite video or S-VHS luminance
signal. These inputs are clamped to the sync back porch and are amplified by a variable gain amplifier.
One of these inputs is for connection of S-VHS chrominance signal (it is internally biased and has a
fixed gain amplifier).
signal. These inputs are clamped to the sync back porch and are amplified by a variable gain amplifier.
One of these inputs is for connection of S-VHS chrominance signal (it is internally biased and has a
fixed gain amplifier).
Clamping
The composite video input signals are AC coupled to the IC. The clamping voltage is stored on the
coupling capacitors and is generated by digitally controlled current sources. The clamping level is
referenced to the back porch of the video signal. S-VHS chrominance is also AC coupled. The input pin
is internally biased to the centre of the Analogue to Digital Converter input range.
coupling capacitors and is generated by digitally controlled current sources. The clamping level is
referenced to the back porch of the video signal. S-VHS chrominance is also AC coupled. The input pin
is internally biased to the centre of the Analogue to Digital Converter input range.
Automatic Gain Control
A digitally working automatic gain control adjusts the magnitude of the selected base-band by +6/–
4.5dB in 64 logarithmic steps to the optimal range of the Analogue to Digital Converter. The gain of
the video input stage including the Analogue to Digital Converter is 213 steps/V with the AGC set to
0dB.
4.5dB in 64 logarithmic steps to the optimal range of the Analogue to Digital Converter. The gain of
the video input stage including the Analogue to Digital Converter is 213 steps/V with the AGC set to
0dB.
Analogue to Digital Converters
Two Analogue to Digital Converters 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.
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.
digitally by the control processor; the clock frequency can be adjusted within ±150 ppm.
Analogue Video Output
The input signal of the luminance Analogue to Digital Converter is available at the analogue video
output pin. A source follower must buffer the signal at this pin. The output voltage is 2V, thus the
signal can be used to drive a 75Ω line. The magnitude is adjusted with an AGC in 8 steps together with
the main AGC.
output pin. A source follower must buffer the signal at this pin. The output voltage is 2V, thus the
signal can be used to drive a 75Ω 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 2kHz.
The beam current limiter has an automatic offset adjustment that is active two lines before the first
cut-off measurement line and 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. This is why some faults are ‘masked in
the service mode. After the white drive measurement line, it takes 3 lines to switch back to BCL
limited drives and brightness.
BCL uses a different filter to average the beam current during the active picture. The filter
bandwidth is approximately 2kHz.
The beam current limiter has an automatic offset adjustment that is active two lines before the first
cut-off measurement line and 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. This is why some faults are ‘masked in
the service mode. 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
Vertical fly-back; This pin searches for a negative edge in every field, otherwise the RGB drive signals
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