RCA SelectaVision VideoDisc SFT 100 Service Data page 7

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ELECTRONIC/MECHANICAL SYSTEMS DESCRIPTION
ELECTRONICS: System Control and Signal Processing
Fig. 1-4 is an electrical systems block diagram of the RCA SFT100
VideoDisc player. Most electronic circuits in the VideoDisc player
can be separated into two basic functional categories: SYSTEM
CONTROL and SIGNAL PROCESSING.
The system control electronics are on the PW 500 circuit board,
which is mounted above the VideoDisc turntable. A microcom-
puter integrated circuit is the heart of the system control function.
The microcomputer receives input commands from the user-oper-
ated function switches and, in turn, controls the operation of the
Player. The system control microcomputer also decodes the
Digital Auxiliary Information (DAXI) on the VideoDisc to develop
the elapsed play time readout and to control forward movement of
the pickup arm assembly during the "play" mode.
The signal processing circuits are equipped with severai integr-
ated circuits and discrete devices. These are mounted on the PW
3000 circuit board and on the pickup arm assembly. The signal
processing circuits detect the vídeo and audio information on the
VideoDisc. demodulate it and process it through a comb-filter cir-
cuit, and then modulate it onto either a Channel 3 or Channel 4
television RF carrier. This modulated television RF signal is then
connected through coaxial cable to any NTSC television receiver.
Functional Operation
Operation of the VideoDisc player is totally controlled by the
system control microcomputer. When the user selects an operat-
ing mode — PLAY, RAPID ACCESS FORWARD or REVERSE,
VISUAL SEARCH FORWARD or REVERSE, PAUSE, or LOAD —
input commands related to that mode are fed to the microcom-
puter. The micracomputer decodes these input commands and, in
turn, uses the decoded information to "direct" other system con-
troi electronics to establish the electrical conditions needed to
perform the selected operation mode. The state of atl signal pro-
cessing circuits is controlled by the Not Squelch (SQ) output of
the microcomputer. When the Not Squelch line goes to a logic
"Lo" state, all of the signal-processing electronic circuits are dis-
abled (squelched).
The system control microcomputer also has direct control over
the pickup arm assembly. This involves: — the servo motor opera-
tion, moving the arm forward (toward center of disc) during nor-
та! play; — the stylus lifter operation. raising and lowering the
stylus as the various functions are initiated: — and the stylus
kicker circuits. enabling the system to provide the VISUAL
SEARCH feature. The microcomputer also controls the direction
of the servo system. In the RAPID ACCESS REVERSE, and
VISUAL SEARCH REVERSE operating modes, the microcomputer
instructs the servo system to operate in the reverse mode.
The system contro) microcomputer also generates the elapsed
play time display. The time display information is developed from
a Digital Auxiliary Information (DAXI) signal. This signal is pre-
recorded on the VideoDisc on line 17 of each vertical field. The
БАХ! signal includes a field identification number that is decoded
by the system control microcomputer. This decoded information
is used by the microcomputer to develop the elapsed time dis-
play. The DAXI code is not present in the RAPID ACCESS FOR-
WARD and REVERSE operating modes because the stylus is
lifted from the disc. Therefore, during these two modes of opera-
tion the time display must be artifically maintained so that the ap-
proximate elapsed time of the program material can be tracked
while the stylus is lifted and the arm is moved in either direction
across the disc. This is accomplished by a "photo internipter" cir-
cuit. This circuit computes the approximate elapsed time by
tracking the position of the arm relative to the disc radius.
The signal processing electronics on the pickup arm assembly
detect information recorded on the VideoDisc. The arm also con-
tains components for providing the features of VISUAL SEARCH
FORWARD and REVERSE as well as Locked Groove protection.
They are: the "stylus kicker" coils which will cause the stylus to
skip two grooves of the VideoDisc; the "armstretcher" transducer
which corrects for timebase variations in the recovered chromi-
nance and luminance signals.
The primary function of the pickup arm signal-processing
electronics is to detect the information recorded on the
VideoDisc. This is accomplished by modulating a 910-MHz UHF
resonator circuit with the capacitance changes on the VideoDisc
surface. The variations in capacitance on the VideoDisc surface
causes the 910-MHz resonator center frequency to be modulated.
This, in turn, amplitude modulates a fixed 915-MHz oscillator sig-
nal. This signal is then peak detected, with the resultant signal
representing the capacitance variations on the VideoDisc. The
signal is then preamplified and AFT controlied before being ap-
plied to the remaining signal processing electronics. The Arm
Output (AO) signal contains the video and audio FM-moduiated
carrier information and all of the information necessary for player
control.
The AO signal is fed to the system control electronics (PW 500
board) and to the signal processing electronics (PW 3000 board).
On the signal processing electronics board the AO signal is ap-
plied to two FM demodulator ICs, one for audio and the other for
video. The audio demodulator IC converts the AO signal audio
carrier information into a discrete audio signal. The audio signal
is fed to the sound modulator, which frequency modulates a 4.5-
MHz sound carrier that is fed to the RF Modulator IC.
The audio demodulator IC also contains a Defect Detector cir-
Cuit. This circuit prevents audio noise if the audio carrier of the
AO signal is momentarily interrupted by microscopic debris on
the disc surface.
Before the AO signal is applied to the Video Demodulator ІС. it is
passed through a NonLinear Aperture Correction (NLAC) circuit.
The NLAC circuit removes the 716 kHz audio modulation from the
video information. It does this by phase inverting the audio
modulation, and then adding it back to the original signal. This
cancels out the audio modulation in the carrier information.
The video FM carrier, with the audio modulation removed is ap-
plied to the Video Demodulator ІС which demodulates the video
carrier. The video demodulator also contains a defect detection
circuit, which allows a portion of the previous horizontal line to be
inserted when a defect caused by loss of carrier occurs.
The output of the video demodulator, being composite video with
"buried" subcarrier chroma, is then applied to a comb-filter cir-
cuit. The comb filter dynamically separates chrominance and
luminance information from the composite video information.
The output of the comb filter is "combed" chrominance and'
"combed" luminance. The combed chrominance output signal
contains low frequency luminance information and the DAXI sig-
nal which is transmitted with each vertical field. After bandpass-
ing the 1 to 2 MHz chroma signal, the two remaining signals (low
frequency luminance and DAXI) are separated by low pass filters.
The low frequency luminance information is recombined with the
"combed" luminance information to provide the luminance out-
put. Vertical Detail Output (VDO) containing the БАХ! signal is
supplied via the DAXI buffer IC to the system contrcl microcom-
puter.
The luminance and chrominance information is coupled from the
comb-filter circuit to the video converter circuit. The video con-
verter up-converts the 1.53-MHz chrominance information to 3.58-
MHz. The 3.58-MHz chroma and the luminance information are
then combined. The composite video signal is then supplied to
the RF modulator where the audio FM carrier is added and a ВЕ
signal on Channel З or Channel 4 is developed for output to а
standard NTSC television receiver.
Also developed in the video converter stage is the drive signal for
the "armstretcher" time base corrector circuit. The correction
Signal is developed by comparing the up converted 3.58-MHz
chroma information with a crystal controlled 3.58MHz reference
oscillator. Any phase or frequency difference between the two
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