Evening star. [volume] (Washington, D.C.) 1854-1972, 1929-01-13
NEW H|GHSPEED
PICTURE SYSTEM
Messages or Photos Sent
Quickly Over Radio or
Telephone.
Special Dispatch to The Star.

Special Dispatch to The Star.
NEW YORK, Jan. IA new pic
ture fac-simile transmission system
that will transmit and receive at q
distance. a picture in black and white
or halftone of a size of 5 inches by
8 inches in 48 seconds was an
nounced and described recently by v
Zworykin of the Westinghouse Re-
search Laboratories at a meeting OF
the Institute of Radio Engineers.
The device also is capable of trans
uniting and receiving written messages
at the rate of $50 words a minute
The received messages and pictures are
reproduced directly on Photographic
paper from the original picture OF
message. The system does not require
either a special preparation of the
original or a skilled operator for trans
mission or reception.
Older Methods Too Slow.
The existing methods of electrical
picture transmission can be divided
into two classes: one which requires
special preparation of the original be-
fore it can be transmitted. and Nle
other which can transmit the original
directly.
The first one includes the electrical
contact method, now almost abandoned,
which requires the preparation of the
original in such form that the dark
and light of the picture give variable
electrical resistance when explored BY
o travelling contact.
In another form OF transmltter OF
the same class, as in the Belin system,
which is still in commercial use In
France. the picture is embossed with q
special ink so that that it may be repro
duced by a ~microphone in the same
way as an electrical pickup reproduces
monograph records.
The method which requires the prep
aration of a transparent picture either
in negative OF in positive form belongs
also to the same class. In this case the
picture is explored by a sharply defined
pencil of light p'hich passes through 1t
and activates a photo-cell placed behind
it. The variation in optical density
produces a corresponding variation IN
ABSORPTION of the light, and therefore
the photo cell delivers an electric cur
rent varying according to the picture.
New Scanning Beam.
The present requirement Of high
speed transmission, however. rules out
all these methods, due to the time
necessary for preparation of specially
treated originals. In this case only n
solution remains, and this is the scam
ming of the original by a pencil of light
and the utilization of the light reflected
from its surface. The amount of re-
fiected light is directly dependent on
the density of the picture or lettering.
The original source of light Is focused
first on a diaphragm to make the size
of the spot independent of the size OF
the source. The image of the diaphragm,
with necessary reduction, is focused on
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light is gathered by means OF the
parabolic reflector, the focus of which
comedies with the Illuminating point.
Part of the reflector is cut away In
order to pass the light and the remain
ing part is brought into close proximity
to the surface of the picture. In this
case almost all the reflected light is
collected and projected as a more or
less parallel beam by the reflector. a
plane mirror with a small hole for pas
sage of the Illuminating spot interceptS
the reflected light at forty-flve degrees
and divertS it to the photo-cel. The
optical path of the whole system is quite
short and the construction is flexible
for adoption to almost all kinds OF
scanning systems. The overall optical
efficiency is many times greater than
the best possible solution by the first
method.
Receiving Simple.
At the receiving end a standard
bromide Photographic paper 5 inches
by 8 inches is wrapped around the
recording cylinder and a special GLOW
lamp furnishes the light for making
the reproduction.
The new lamp wa developed by MI.
Knowles in the Westinghouse labora-
tory and uses helium glow for record
ing. The glow is restricted to the re-
uuired size by a mask built into the
tube. A discharge of approximately 10
milliampereS at 400 volts Is Sut1cient
at the present speed to produce very
satisfactory ~blackening on the bromide
paper.
For the control of the glow tube
which exposes the Photographic paper,
a vacuum tube Is used. The glow tube
is connected in series with the plate
voltage for the acuum tube. Fluctu-
atoms in voltage upon the grid due to
the picture signal produce correspond
ing variations in the glow tube current.
If the signal as it comes from the audlo
amplifer were applied directly to the
grid of the control tube. a negative pic
ture would result. That this is true
can be verified by following the steps in
the transmission of the picture. When
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In the original picture, a relatively large
amount reaches the photo-cell. The
corresponding photo-cell current is am
plified, producing a loud signal. At the
receiving end this signal would increase
the average plate current of a tube
working on the lower bend of the char
acteriStic curve. Such an increase
would augment the light from the glow
tube, darkening the Photographic paper
instead Of making it lighter.
Reversal Kills Static.
Unless the picture is to be recorded
upon a film and subsequent prints are
to be made it is necessary to reverse
the process. This reversal might take
place at the transmitter, but is unde-
sirable for pictures which are largely
white, as printing. for example. Bursts
OF static would be recorded as black
spots on the white background. On the
other hand, if the reversal occurs at
the receiver these disturbances tend
only to make the white whittier. Hence,
reversal at the receiver is used.
For the reception of the picture sig
nails the radio set may be a standard
receiver. The ampliication in the case
OF weak signals is preferably at radio
frequency, in SO far as possible, in order
TO reduce distortion. Transformer
coupled audio-frequency amplifcation,
however, gives very good results if the
gain is fairly uniform between 1000 and
1000 cycles. To date a standard r. c..
A. short-wave receiver has been used
for all work. This employs a stage of
radio-frequency application with a
screen grid tube, detector and two audio