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DIGITAL COMB FILTER – TOSHIBA TC9090AF.
The operation of a comb filter is to separate the composite video into luminance and reconstruct the chrominance colour sub carrier
signal using digital signal processing techniques. The comb filter IC, IN01 has an additional vertical edge enhancement feature.
Composite video input (Pin 6 of connector E500) and colour sub-carrier (pin 8 of E500) are converted via an 8 bit analogue to digital
converter where the vertical enhancement and digital comb filtering takes place. Separated luminance and chrominance output
components are available via a digital to analogue 8 bit converter at pins 1 & 4 respectively.
SCAN VELOCITY MODULATION.
During transmission the signal sufferers from degradation and also due to the frequency characteristics of the television circuitry.
This normally results in a gently rise or fall in the luminance change areas when black-to-white-to-black patterns are received.
The picture sharpness quality can be assessed by how steep the leading and trailing edges are. The scan velocity modulation circuit
has been designed to improve picture quality, i.e. make the video signal edges steeper, by controlling the horizontal scanning
velocity of the electron beam in the CRT.
The velocity modulation circuit produces a compensation signal by adding the RGB components then differentiating. The
compensation signal is given some current gain, applied to the auxiliary coil (connector E806) on the neck of the cathode ray tube
(CRT).
The VM circuit is located on the CRT base along with the final RGB amplifiers. The RGB signals derive from PL803 pins 4-6 and
into emitter of Q861, through the differentiator C863, R891 arriving at the base of Q862 for voltage amplification and latter stages,
power gain.
DIGITAL DOUBLE SCAN CONVERSION UNIT (FEATURE BOX).
The main feature of this unit is double scan frequency conversion, 100/120Hz interlaced and 50/60Hz progressive scan. Other
features of this unit are CTI (Colour Transient Improvement), horizontal compression, noise reduction, sharpness and vertical zoom.
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These functions are controlled by I
C bus and are provided elsewhere by the picture improvement IC, TDA 9178 (IE01) for digital
models without 100Hz/progressive scan.
Scan rate conversion mode for display
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