TEA7650H Philips Semiconductors, TEA7650H Datasheet - Page 7

• Manufacturer Part Number: TEA7650H
• Description: Video signal processor for CD-video/laser vision
• Manufacturer: Philips Semiconductors

Philips Semiconductors
FUNCTIONAL DESCRIPTION
Figures 1, 2 show the block diagram of the Video Signal
Processor (VSP) including the peripheral circuitry for the
video signal processing. The pulse-width modulated FM
signal from the preamplifier is fed, via a DC blocking
capacitor, into the IC (pin 7) at the input to the Modulation
Transfer Function (MTF) circuit which corrects for the
characteristic of the optical reading system.
MTF correction
Due to the finite diameter of the laser beam spot and the
tangential velocity of the track of pits on the disk, the MTF
of the optical system acts like a radius-dependent
low-pass filter for the FM input signal. Although the video
signal can be recovered without correction, the ratio of the
amplitudes of the chrominance and luminance signals
would not then be the same at the most inner and the most
outer part of the disk. This influence of the disk radius is
automatically corrected by the Video Signal Processor.
The principle of correction is to use the deviation of the
demodulated burst signal to generate an error voltage in
order to control the frequency selective MTF circuit. The
burst measurement operates as follows: A burst-key
generator is triggered by the line synchronization pulse
(CS1) to generate a burst-key pulse which activates the
burst gate and rectifier stage. The signal at the rectifier
output (pin 6) is used to control the amplification of the
MTF circuit.
The carrier frequency in the PAL standard is different to
that in the NTSC standard, therefore two separate
resonant circuits are required on pins 8 and 9. They are
selected by the PAL/NTSC system selector (pin 37).
The MTF-corrected FM signal at pin 10 is amplified ( 8 dB)
and fed, via the external filter which removes the audio
frequency components from the signal, into the
demodulator at pin 14.
Demodulation
The FM signal is first fed into a limiter circuit (pin 14) with
automatic slicing level control to suppress the main carrier
in the demodulated signal. The demodulator has two
outputs. The first (internal) clamps the demodulated video
signal on peak-sync by controlling the transconductance of
the demodulator. The FM signal can now be demodulated
during disk start-up, thereby facilitating fast run-in. The
second output signal from the demodulator (pin 18) is
passed through an external 5 MHz low-pass filter to extract
the CVBS signal. The CVBS signal is then fed into the
de-emphasis network to compensate for the pre-emphasis
of the video signal recorded on the disk.
September 1990
Video signal processor for CD-video/laser vision
7
De-emphasis
The de-emphasis circuit consists of an internal inverting
amplifier and an external RC feedback network. Since the
pre-emphasis on the disk in the PAL standard is different
from that in the NTSC standard, the time constants are
switchable. When PAL is selected, the first arm of the
feedback network is active, otherwise both operate in
parallel. The de-emphasized video signal is fed into an
AGC stage (pin 20) where it is clamped on its black level
and amplitude-controlled to a constant level. The signal is
then fed into the data slicer and the drop out switch.
Data slicer
Coded signals on the video disk are extracted by the data
slicer (output pin 25) when the Data Request input is
activated (pin 17).
Drop out compensation
The drop out detector (DOD) in the IC is triggered by every
positive or negative transition of the FM signal. A drop out
is detected when the half-cycle period is outside the limits.
Protection against a drop out is achieved by use of a video
signal delayed by one line. The signal at the output of the
drop out switch is fed out of the IC via a buffer (pin 29) and
then through a delaying device (CCD) before being fed
back into the IC (pin 27). The delayed video signal appears
at the input of an AGC circuit to compensate for gain
tolerances of the delay line and avoids the need for an
external adjustment. When a drop out is detected, the drop
out detector activates the video switch so that the lost
information of the line is substituted by the information of
the preceding line.
The drop out pulse is also present at pin 3 and can be used
for different purposes. This pin can also act as an input to
control the drop out switch by an external signal for test
purposes.
Time error compensation
In a videodisc player timing errors are caused by
deviations of the rotational speed of the motor,
imperfections in the disk and unavoidable tolerances in the
centering of the disk on the turntable. Track eccentricity is
the main cause of timing errors.
To minimize timing errors, it is necessary in the first place
to keep the rotational speed of the disk as constant as
possible. Referring back to the output of the switch in
Fig.1-2, the video signal is also fed into a sync separator
and a chrominance separator with its external resonant
circuits tuned to the chrominance subcarrier
Preliminary specification
TEA7650H