Functional Description - NEC DTA-14V4THS Service Manual

APPENDIX

FUNCTIONAL DESCRIPTION

For CP-185A
FUNCTIONAL DESCRIPTION OF VIDEO PROCESSOR Vision IF amplifier
The vision IF amplifier can demodulate signals with positive and negative modulation. The PLL demodulator is completely
alignment-free.
The VCO of the PLL circuit is internal and the frequency is fixed to the required value by using the clock frequency of the
-Controller/Teletext decoder as a reference. The setting of the various frequencies is made by the controlling software in
subaddress 27H (33.9 MHz for system L-and 38.9 MHz for all other systems) Because of the internal VCO, the IF circuit
has a high immunity to EMC interference.
1. IF demodulator and audio amplifier
The FM demodulator is realised as a narrow band PLL with external loop filter, which provides the necessary selectivity
without using an extra band pass filter. To obtain good selectivity a linear phase detector and a constant input signal
amplitude are required. For this reason the intercarrier signal is internally supplied to the demodulator via a gain controlled
amplifier and AGC circuit. The nominal frequency of the demodulator is set via a gain controlled amplifier and AGC circuit.
The nominal frequency of the demodulator is tuned to the required frequency (5.5 / 6.0 / 6.5 MHz) by means of a
calibration circuit which uses the clock frequency of the microcontroller as a reference. Selection of the required frequency
is done by the controlling software.
Video switches
The video switch has one input for an external CVBS or Y/C signal. The selected CVBS signal can be supplied to pin 38,
the IF video output. The selection between both signals is realised by the controlling software in subaddress 22H.
The video ident circuit is connected to the selected signal. This ident circuit is independent of the synchronisation.
Synchronisation circuit
The IC contains separator circuits for the horizontal and vertical sync pulses and a data-slicing circuit which extracts the
digital teletext data from the analogue signal.
The horizontal drive signal is obtained from an internal VCO which is running at a frequency of 25 MHz. This oscillator is
stabilised to this frequency by using a 12 MHz signal coming from the reference oscillator of the -Controller/Teletext
decoder.
The horizontal drive is switched on and off via the soft start/stop procedure. This function is realised by means of variation
of the TON of the horizontal drive pulses.
1 1