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SECTION 2
TECHNICAL INFORMATION
2-1. GENERAL
A
videodisc is a prerecorded disc from which a color picture and
sound come o·ut. The disc similar to familiar "audio record", plays
a color picture and sound through any monitor television or stand-
ard color television receiver.
You can watch the slow motion of a fine play in a telecasted
sport program just immediately after that fine play.
A
disc for a
broadcast station can playback such a slow motion picture. The
broadcast use disc is the magnetic record and playback system that
is same as VTRs where magnetic material is coated on the disc, and
the recording and playback are done with a magnetic head. The
videodisc mentioned above is different from the disc being used now
in broadcasting. The videodisc playback systems we are discussing
now are such an optical system and a stylus system which are non-
. magnetic systems. The optical system is that a laser beam is pro-
jected on the videodisc where its reflected beam is picked up, and an
light signal is converted into an electric signal. The stylus system is
that a stylus traces the track on the disc in order to pick up a me-
chanical signal.
From the various videodisc systems employed by ·several com-
panies, the optical system will be described here.
This system is called "VLP (Video Long Playing) system". The
disc is an acrylic disc having a diameter of 30 cm ( 12 inches) same as
familiar audio LP records. The revolution of the disc is I 800 rpm at
CA V (Con.stant Angular Velocity) in NTSC cofor disc. The video
and audio information are recorded onto the high density tracks
which is 600 Jines per I mm. The tracks are hOt grooves like the
ones in 3:n ordinary audio record, but are made of continuous oval
hollows called "pits". The depth of the pit is approximately 0.1µ
and its width approximately 0.4µ. The length of a pit and the
spacing between p'its are continuously varied by the video and audio
signals. The pits form continuous spiral from the inside of the
videodisc to the outside.
He-Ne laser b;am is used for the playback of the videodisc. This
beam traces the pits automatically from the inside of the disc to the
outside, according to the rotation of disc.
An
optics control system
by electronic means is employed so that the laser beam always
traces onto the. center of the track and is focused upon the re-
flecting plane of videodisc.
The projected laser beam is reflected back from the surface of
the videodisc. Thus reflected beam is modulated-by the variation
of the pits.
A
photodiodes catches the reflected laser beam and
create the. electric signal. This· system features that a pickup does
not contact the videodisc and there-fore wear of the disc and the
pickup are not caused. "Special playback features" such as the slow
motion, high speed playback, still, reverse, and random access can
be realized smoothly by making the track jump.
Videodjsc
system,.
{
Transparent type - - - - - - - - - - - - - - THOMSON-CFS
Nan-contact
type
.
(optical system)
[ PHILIPS
.
MCA. PIONEER
Reflection type - - - - - . - - - - ..
MAGNAVOX
SONY
With guide
{
groove for- - - - RCA
tracking
-
Static charge
.
type
Contact
Without
type
J
guide
- - - - - JVC. PANASONIC
(stylus system)
L
groov.
Pieza-electric type - - - - . - - . - - . - . TED
Fig. 2-1. Classification of videodisc system
10
2-2. DISC
2-2-1.
Mastering
SOFTWARE
PLAYBACK
EQUIPMENT
SIGNAL
PROCESS
EQUIPMENT
RECORDING
EQUIPMENT
MANUFACTUR
ORIGINAL
DISC
OEVa.oPMENT
OF
MASTER
Fig. 2-2. Process for making master
Signal Processing Equipment:
Signal processing equipment converts a signal from a software
into an FM signal suitable for recording. The video,signal ·is con-
verted into the FM signal whose center frequency is 8.1 MHz (fre-
quency deviation is 1.7 MH.z). The two channel sound signals are
converted into the FM signals whose center frequencies are 2.3 MHz
and 2.8 MHz (frequency deviations are lO0KHz). The video FM
signal and the sound
FM
signals are added. The amplitude of the
added signal is limited by a limiter so that the signal is shaped into a
rectangular waveform. The resultant signal is a multiplex signal.
The frequency variation of the multiplex signal represents the video
information and its duty variation does the sound information.
The addition ratio of the video carrier and the sound carrier has
been set to 10: 1 in consideration of the linearity of the modulation,
mixed modulation, and S/N ratio. (See Fig.s 2-3 and 2-4.)
VIDEO
AUDIO
FREQUENCY
MODULATION
FREQUENCY
MODULATION
VIDEO
FM SIGNAL
LIMITTER
"~AAW!mHM
\f
V V
V V VU V V V V
V
V \
b ~ N D FM SIGNAL
: : : : S SOUN~.-
;
~
I
I
I I
0
1
t:I
0 0 0 0 0 0 0 0 0 0 0
PIT SPACING
<if}"
RF
Fig. 2·3. Encoding of information onto DISC
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