NEC DTE-29U1TH, Руководство по обслуживанию

Страница: 32 / 71

-31-
Service Manual CP-785F
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 (38.9 Mhz for all
system). Because of the internal VCO the IF circuit has a high immunity to EMC interferences.
QSS Sound circuit
The sound IF amplifier is similar to the vision IF amplifier
and has an external AGC decoupling capacitor.
The single reference QSS mixer is realised by a multiplier. In this multiplier the SIF signal is
converted to the intercarrier frequency by mixing it with the regenerated picture carrier from the
VCO. The mixer output signal is supplied to the output via a high-pass filter for attenuation of
the residual video signals. With this system a high performance hi-fi stereo sound processing
can be achieved.
Video switches
The video switch has one input for an external CVBS. The selected CVBS signal can be
supplied to pin 38, the IF video output.
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.
The vertical synchronisation is realised by means of a divider circuit. The vertical ramp
generator needs an external resistor and capacitor. For the vertical drive a differential output
current is available. The outputs are DC coupled to the vertical output stage.