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Journal of the Society of Motion Picture Engineers (Volume 53)
Direct- Positive Variable-Area
Recording with the Light Valve*
Summary By reflecting light from the back surface of the light-valve
ribbons and focusing the ribbon edges at the film plane a bilateral or uni-
lateral type of direct-positive variable-area track is obtained. By relocat-
ing the recording lamp so that light is transmitted through the space between
the ribbons a normal variable-area negative may be obtained.
ANEW FILM RECORDING MACHINE designated as Type RA-1231
was described in the JOURNAL 1 in 1946 employing the tight-loop,
controlled -compliance type of film drive. As shown, the recorder was
equipped with a small, variable-density type of modulator. Since
then, there have been described a 200-mil push-pull density modu-
lator 2 and a light- valve-type, double-width, push-pull variable-area
modulator 3 both of which operate with the same basic film-pulling
unit as was shown with the small modulator. In this paper is de-
scribed the fourth in the series of modulators for use in the Type RA-
1231 recorder; i.e., a simple, compact, standard variable-area modu-
lator. As in the other modulators, the ribbon light valve is employed
as the basic modulating element, the field of application of this device
being extended in this modulator to the recording of direct-positive
variable-area sound track as well as the standard negative variable-
area track.
The direct-positive recording facility is of particular interest in
connection with those black-and-white and color processes in which
the composite prints are obtained by photographic reversal from
positive sound track and picture films. 4 - 6 The direct-positive sound
track thus eliminates the intermediate sound print which ordinarily
would be required with such processes. The direct-positive sound
track also holds promise in certain television applications where the
tonal scale on the television screen is reversed electrically to give a
positive picture on the film.
* Presented October 26, 1948, at the SMPE Convention in Washington.
150 BROWDER August
For recording direct positive, the aperture between the light-valve
ribbons must be projected on the film as a dark area so that it will
develop out to clear film. The aperture image must, of course, be
bounded by an exposed area which will develop out black on the film.
Thus as the ribbon aperture is made smaller in response to noise-
reduction bias, the percentage of clear area in the developed track be-

Fig. 1 Basic optical system for direct-positive recording.
comes less and the ground noise in the reproduced film is corre-
spondingly reduced. In principle, this reversal of clear and exposed
area from the situation obtained with the standard negative recording
system is accomplished as follows :
As shown in Fig. 1, the light valve is equipped with a high-quality
apochromatic objective lens. Light from the recording lamp is
directed off the inclined slit mirror into this objective lens, the working
distances being such that an image of the lamp filament is formed at
the plane of the light-valve ribbons. As in previous light valves, the
width of the ribbon is considerably larger than the thickness. This
1949 DmECT-PosiTivE RECORDING 151
thin, flat ribbon is suspended in the fixed magnetic field so that electri-
al currents flowing lengthwise through the ribbon will cause it to move
ewise in the plane of the flat surface. For this light valve, the foil
m which the ribbon is to be sheared is polished optically smooth and
iven a mirror-quality surface finish. When stretched between the
bbon clamp carriages, the ribbon appears as a thin strip mirror whose
surface is perpendicular to both the optical axis and the axis of the
lamp-filament image. The width of this strip mirror is defined by the
accurately straight edges of the ribbon and is much less in extent than
the length of the spatial image of the lamp filament, with the result
that the actual area of illumination of the ribbons remains constant
as the ribbons are modulated. There is thus formed by the polished
ribbon surface a mirrored image of just that portion of the filament
image which it intercepts.
Since the ribbon moves laterally across the filament image in re-
sponse to modulation currents it, in effect, scans the image directing
the reflected light back toward the light-valve objective lens. By
means of optical expedients to be described later, the individual coils
of the image of the lamp filament are made to blend with one another
so that the filament appears as a uniformly illuminated rectangle of
light. Thus each reflected element of the image appears identical to
any other differing only in lateral position as determined by the in-
stantaneous displacement of the light-valve ribbon.
Two of these reflecting ribbons are employed in this light valve
They are arranged side by side and accurately adjusted so that their
reflecting surfaces are coplanar. These ribbons may be connected to
record either a bilateral or a unilateral type of variable-area sound
track. 6 In the unilateral case one of the ribbons is connected to move
in response to noise-reduction currents while the other moves in re-
sponse to speech currents. As seen from the light-valve objective
lens there appear two brightly illuminated patches, the inner edges
of which move in response to speech and noise-reduction currents
to define the lateral extent of the embraced dark area. This situation
when projected onto the film is correct for the recording of direct-
positive variable-area sound track. The inclined mirror by which
the light from the recording lamp is directed into the light-valve ob-
jective lens contains a narrow, rectangular, clear slit extending across
its width through which the light can pass to the film.
As shown in Fig. 1, the light proceeds from the recording lamp to the
inclined slit mirror which directs all but that lost through the slit
upward to the light-valve objective lens. This lens forms a spatial
image of the lamp filament at the plane of the light-valve ribbons.
The reflecting surfaces of the ribbons in turn form virtual images of
sections of the filament image, which virtual images are limited in
extent by the width of the ribbons and are located also at the ribbon
plane. The light-valve objective lens then picks up these reflected
images and reprojects them, this time through the clear slit in the
inclined mirror and to the film. The cylindrical lens located near the

Fig. 2 Optical system schematic of variable-area modulator.
film gathers the beam of light from the slit and converges it to the
narrow line required for the recording image.
The complete optical system of this modulator is depicted sche-
matically in Fig. 2. The recording lamp is of the pref ocused type rated
at 5 amperes and 10 volts having a single-helix, curved, horizontal
filament. The axis of the filament is rotated at 45 degrees to the
optical axis which has the effect of reducing the pitch of the filament
helix to the point where there are no dark spaces between the coils
and the whole length of the filament appears uniformly bright. Some
gain in optical efficiency and a considerable saving in space is achieved
by locating the lamp fairly close to the light valve employing an
auxiliary lens to throw a virtual image of the lamp filament back to the
proper distance for projection by the light-valve objective lens to the
ribbon plane. In order to maintain the recording lamp in its normal
vertical operating position the light valve is located immediately
Ee the slit mirror with the light-valve objective lens facing down-
. The slit mirror is a front-surface, aluminized, glass mirror with
a,r slit across its width. The width of this slit together with the
reduction afforded by the objective cylinder lens determines the
height of the recording image. The clear slit in the mirror casts a
shadow which disappears at the ribbon plane but reappears in the
reflected beam from the ribbons in such a way that the slit ordinarily
would be in its own shadow and no light would be available for re-
cording. By displacing the slit from the optical axis through slightly
more than half its width, the shadow is thrown to the opposite side of
the optical axis making the full-intensity beam available for recording.
The mounting of the apochromatic objective follows previous practice
in that it is located within the light-valve structure. Since the light
valve is intended to be a readily replaceable component, this arrange-
ment confines any slight, mechanical misregistration of the light valve
with its mounting to the image space of the objective lens where such
displacements are not subject to optical magnification.
The reflected light from the ribbon passes through the clear slit to a
front-surfaced plane mirror whose function is to turn the recording
beam through 90 degrees to a horizontal axis. The objective cylinder
lens is located near the film and is adjusted to focus an image of the
clear slit in the above mirror onto the emulsion of the recording film.
Separation of the cylinder lens into two components is resorted to as
shown in order to minimize the cylindrical equivalent of spherical
Recording of a standard negative track is accomplished by bringing
the light in through the edge of the light valve as shown in Fig. 2.
Upon reaching the optical centerline of the valve, the beam is de-
flected downward by a prism to the condenser lens. An image of the
lamp filament is focused by the condenser lens slightly beyond the
ribbon plane in order to minimize the effects of the filament stria tions
further. The rear-illuminated aperture between the ribbons is then
projected by the light-valve objective lens through the slit and to the
film. As in the direct-positive setup, the cylinder lenses focus the slit
onto the film emulsion to define the height of the recording image.
Light for phototube monitoring is obtained from the horizontal
section of the recording beam by a thin, inclined, clear-glass wafer
which subtracts a fraction of the beam by Fresnel reflection directing
it back to the monitoring phototube. In the viewer a similarly
located but oppositely inclined mirror throws the reflected fraction of
the recording beam forward allowing the spatial image of the light-
valve ribbons to be inspected wih a microscope eyepiece.
The folded-magnet structure employed in previous variable-area
light valves 3 is used here, as are the beryllium-copper ribbon clamp
carriages with their adjustments for ribbon spacing, height, and tuning

ZOO 500 1000
Fig. 3 Frequency response of light valves.
tension. A box-shaped Alnico magnet comprises the main body of the
case with Permendur pole-pieces closing the ends and carrying the
magnetic flux to the center of the valve where the ribbon gap is
located. One of these pole-pieces supports the objective lens as well
as the clamp carriages while the other contains the condenser lens and
the prism arrangement by which light is directed to the ribbons for
negative recording.
The surface quality of the light-valve ribbons used for direct-posi-
tive recording must, of course, be quite good. Of the several ma-
terials suitable for use in the fabrication of the light-valve ribbon,
aluminum offers'definite advantages from the standpoint of electrical
performance 7 although unfortunately it? softness presents some
problems in the achievement of a satisfactory surface finish. However,
by careful maintenance of the rolls with which the foil is worked, ex-
cellent foil surfaces have been obtained comparable in reflectance to
an aluminized mirror.
The relationship between the amplitude of ribbon displacement
and applied voltage at the a 600-ohm primary of the light-valve match-
ing coil is shown in Fig. 3. The somewhat higher peak exhibited by
the bilateral light valve is due to the use of a series resistor in the
Simplex noise-reduction circuit with which this valve is used. The
light valve looks back into a circuit of relatively high impedance so
that the effectiveness of the electromagnetic damping is somewhat

Fig. 4 Square-wave response of light valve.
reduced. The unilateral valve, on the other hand, looks back directly
into the low-impedance secondary of the matching transformer with a
consequent improvement in the electromagnetic damping characteris-
tic. Fig. 4 shows an oscillogram of the monitoring phototube output
as the unilateral valve is driven through its matching transformer
with a square-wave generator having a fundamental frequency of 800
cycles per second As used in a recording system, the resonant peaks
are usually flattened by means of a light-valve equalizer which im-
proves the square-wave response also.
The only mechanical difference in the setup for recording negative
track and that for recording direct positive is in the manner in which
the light from the recording lamp is conducted into the light valve.
By mounting the recording lamp on a movable slide, it becomes
possible to effect this change merely by moving the recording lamp from
one position to the other. Fig. 5 is a photograph of the modulator
with the light valve in place and the lamp moved up to the position
for recording negative sound track. Fig. 6, in which the modulator is
mounted in its recorder, shows the lamp moved down in position to
record direct-positive sound track. The lamp bracket is mounted in a
dovetail slide equipped with adjustable stops at each end of its travel.
After the stops have been adjusted and locked for a particular lamp,
the transition from direct-positive to negative recording may be
accomplished in a matter of
seconds merely by moving the
The light valve is a completely
self-contained component of the
modulator, the mounting being
arranged so that it may be re-
moved readily from the modula-
tor proper for inspection or re-
placement. Registry of the face
of the light valve with the modu-
lator mounting plate is accom-
plished by constructing this plate
of soft steel so that the leakage
magnetic flux from the valve
serves to hold the valve face
tightly against this plate. A
single milled slot in the valve face
and a dowel in the mounting
Fig. 5-Modulator and light valve. P Me loCate the Valve in the
right and left directions while
registry tabs at the back of the mounting plate bear against the rear
edge of the valve to locate it fore and aft. After the valve is pushed
home against the registry tabs, a clamping lever is swung into
place and tightened to augment the magnetic clamping force further.
Release of this lever then^ allows the valve to be slid across the steel
mounting plate and finally lifted clear.
The objective cylinder lenses are mounted in a cylindrical cell
which is spring-loaded against a threaded ring. Calibrations on the
periphery of this ring serve as a reference in making film tests to estab-
lish the optimal focus setting. Provision is made for a limited range

of adjustment of this cell about its longitudinal axis for setting the
azimuth of the recording image to the precision required in variable-
area recording.
The monitoring facilities are designed as self-contained accessories
which may be installed into the modulator in place of the right-hand
small circular cover seen in Fig. 5. The phototube monitoring attach-
ment is an assembly of deflecting glass and a field lens for producing a
variable-intensity spot on the plate of the phototube. This assembly

Fig. 6 Modulator and light valve in recorder.
and the chassis containing the monitoring phototube and amplifier
constitute the complete phototube monitoring unit. The visual
monitoring accessory consists of a microscope eyepiece with its
mounting for installation into the modulator. A graduated scale is
located in the focal plane of this eyepiece which enables the setting of
the noise-reduction current to obtain a given width of bias line.
Both the unilateral and the bilateral light valves require an input
level of +20 dbm* into the primary of the matching transformer for
operation to 100 per cent modulation. This figure applies, of course,
for either direct-positive or negative operation.
* Decibels with respect to 0.001 watt.
For direct playback of the direct-positive track, maximum cancella-
tion of the cross-modulation products is obtained by exposing EK-
1372 or EK-5372 variable-area film to a total density 8 of 1.3. A fre-
quency film recorded at constant modulation of the light valve and
exposed to this optimal density exhibits a frequency characteristic as
shown in Fig. 7 for both the 35-mm modulator and the 16-mm version
which uses a narrower recording slit to improve the frequency response.
+ 5
200 SOO 1000 2000
K>000 20000
Fig. 7 Frequency response of direct-positive film.
(1) G. R. Crane and H. A. Manley, "A simplified all-purpose film recording
machine," /. Soc. Mot. Pict. Eng., vol. 46, pp. 465-475; June, 1946.
(2) J. G. Frayne, T. B. Cunningham, and V. Pagliarulo, "An improved 200-mil
push-pull density modulator," /. Soc. Mot. Pict. Eng., vol. 47, pp. 494-519;
December, 1946.
(3) Lewis B. Browder, "A variable-area light-valve modulator," /. Soc. Mot.
Pict. Eng., vol. 51, pp. 521-534; November, 1948.
(4) G. C. Misener and G. Lewin, "An application of direct-positive sound
track in 16-mm release processing by duplication method," J. Soc. Mot. Pict. Eng.,
vol. 46, pp. 167-168; March, 1946.
(5) "Recent American standards for 16-mm and 8-mm emulsion position,"
/. Soc. Mot. Pict. Eng., vol. 49, pp. 547-558; December, 1947.
(6) John G. Frayne, "Variable-area recording with the light-valve," J. Soc.
Mot. Pict. Eng., vol. 51, pp." 501-521; November, 1948.
(7) T. E. Shea, W. Herriot, and W. R. Goehner, "The principles of the light
valve," J. Soc. Mot. Pict. Eng., vol. 18, pp. 697-732; June, 1932.
(8) Dorothy O'Dea, "Comparison of variable-area sound recording films,"
J. Soc. Mot. Pict. Eng., vol. 45, pp. 1-10; July, 1945.
35-Mm and 16-Mm Portable
Sound- Recording System*
Summary A new low-cost portable sound-recording system suitable for re-
cording a standard variable-density sound track on 35-mm or 16-mm film in
synchronism with a motion picture film is described. The basic system in-
cludes a two-channel mixer, a main amplifier including associated noise-re-
duction circuits, a compact recorder, and a power unit for operating the
entire system from a 115-volt, alternating-current supply. An optional
inverter providing for operation from 96-volt batteries and an alternating-
and direct-current multiduty motor-control unit are also available. The vari-
ous circuit facilities and the performance of the components and system
with respect to sensitivity, signal-to-noise ratio, harmonic distortion, and
flutter are discussed in detail. The mechanical and electrical design of the
equipment make possible a high-quality sound product for both speech and
music recordings, the performance specifications being consistent with the
requirements and standards of major Hollywood studios.
ANEW LOW-COST lightweight portable sound-recording system
suitable for recording a standard variable-density sound track on
35-mm or 16-mm film in synchronism with a motion picture film is
now available in the Western Electric 300 Type recording system.
The assembled system, as shown in Fig. 1, consists of three electronic
units, a mixer, amplifier, and power unit, each assembled in an
attractive duralumin case, plus a compact recording machine also well
adapted for portable application.
Although limitations on weight and size are imposed by the porta-
bility requirements, the equipment retains a frequency-response
range and freedom from distortion consistent with the high-quality
requirements of major studio production. In addition, variable
equalization facilities are incorporated for controlling the frequency re-
sponse in the low-, medium-, and high-frequency ranges to obtain the
best possible sound product for either speech or music recordings on
both 16-mm and 35-mm film, and for the wide range of pickup con-
litions found in the studio and on location.
Presented April 24, 1947, at the SMPE Convention in Chicago.
cycle, 115-volt power source with a total drain of less than 2 amperes.
The recording machine motor may be either a 3-phase, 220-volt syn-
chronous type, a conventional 3-phase alternating-current interlock, or
a 96-volt direct-current multiduty motor, depending on the type of
power source provided for driving the motors of the associated
cameras. Alternatively, by means of a supplementary electronic
inverter, shown dotted in Fig. 1, the entire system, including the re-
corder and camera motor, may be operated from 96-volt batteries.
Five 6-conductor-shielded cables terminating in Cannon type "P"
connectors are required for connecting the components together. Ad-
R A- 1290* PUNCH

Fig. 1 Basic 300 Type recording system block schematic.
ditional cables are required for the microphones and for connection to
the power source. Six varieties of the "P"-type connectors are used in
order to minimize the possibility of mispatching cables. The assign-
ment of terminals on the connectors has been arranged to prevent
damage to components in the event cables are accidentally connected
to the wrong receptacle.
The microphone cables normally are 100 feet long. The system will
also accommodate a 100-foot separation between mixer and amplifier.
The amplifier, recorder, and power unit are intended to be located
close together so that all may be under the operating control of the
recordist. The power-supply cable may be as long as necessary, pro-
viding the terminal voltage at the power-supply input is within the
range of 100 to 130 volts.

162 TEMPLIN August
The RA-1283 mixer is shown schematically in Fig. 2. Two micro-
phone inputs are provided, working into separate preamplifiers.
These amplifiers are arranged to operate from microphones having a
30-ohm impedance such as the Western Electric RA-1142 (cardioid)
type. The preamplifiers have a gain of 33 decibels, an overload level
of dbm* for 1 per cent total harmonic distortion, and a background
noise level of 93 dbm at the amplifier output.
Separate variable dialog equalizers are provided in each preamplifier.
This feature is desirable not only to provide greater flexibility but be-
cause of special requirements in a low-cost system that it be possible to
use the original film for release negative rather than undergo a costly
re-recording process. In applications of this type, background music
might be recorded simultaneously with the picture and the low-
frequency equalization for the music would generally be considerably
less than for the dialog. As another example, in a two-microphone
setup where acoustics differ markedly in the two positions, the
equalizers may be adjusted so that they are balanced in the original
recording, thus eliminating the necessity of frequency-response re-
adjustment during a subsequent re-recording process.
The desired equalization characteristic is obtained by series and
shunt elements in the plate circuits of the preamplifiers. The rela-
tively high impedance of the plate circuit allows a smaller space and
weight for the components than would be possible in the 600-ohm
output circuit. The dialog-equalization characteristics are described
in detail in a later portion of this paper.
Separate mixer potentiometers follow the preamplifiers. These are
standard 600-ohm bridged-T type having l 1 /^ decibel per step ( 3 /4
decibel per step when bridging two contacts) and tapered to full cutoff
at the maximum counterclockwise position. The mixer potentiome-
ters terminate in a combining network, the output of which is carried
to the mixer output terminals.
Either phototube or direct monitoring may be selected by the
mixer operator providing the optional RA-1278 phototube monitor
assembly is installed in the recorder.
A volume-indicator, direct-monitor line is bridged from the main
amplifier output. The volume-indicator circuit includes a high-
speed meter and a control switch providing a range from +2 to +24
* Decibels with respect to 0.001 watt.
dbm for 0-decibel meter deflection. Direct-monitor level for the
mixer operator is adjustable by means of a bridged-T continuously
variable attenuator which provides a level from 10 dbm to cutoff for
100 per cent modulation of the light valve. An auxiliary direct-
monitor circuit, at a fixed level of 18 dbm for 100 per cent light-
valve modulation, is provided for the microphone-boom operator.
The RA-1283 mixer requires 0.6 ampere at 8.3 volts direct current
for the vacuum-tube heaters and the volume-indicator meter lamp, and
5 milliamperes at 220 volts direct current for the plate supply.
A front view of the mixer, with cover removed, is shown in Fig. 3.
The mixer potentiometer and dialog equalizer for one microphone
channel are on the left and for the other on the right. In the central

Fig. 3 RA-1283 mixer front view.
portion, below the volume-indicator meter, from left to right, are the
phototube, direct switch, volume-indicator range switch, and monitor
volume control.
A rear view of the mixer is shown in Fig. 4. On the left side is the
mixer monitor jack. On the rear are receptacles for two microphone
cables, low- and high-level interconnecting cables to the R A- 1282
amplifier, and auxiliary monitor.
The cover, not shown in the photographs, can also be latched to the
under side of the mixer when the equipment is in use, thus avoiding
the possibility of its being left behind as the mixer is moved about from
one setup position to another. The mixer is 15 3 /4 inches long by 6
inches high by 8 l /z inches deep, including cover, and weighs 1 7 J /2 pounds.
164 TEMPLIN August
The RA-1282 amplifier is shown schematically in Fig. 5. This unit
contains the main system amplifier, a 1000-cycle test oscillator, vari-
able low-, middle-, and high-frequency equalization, a noise-reduction
circuit for automatically biasing down the light-valve spacing in
accordance with the envelope of the modulating signal, a volume-indi-
cator circuit, and the recorder lamp-control circuit.
The amplifier section is a 3-stage unit having either 60, 70, or 80
decibels gain. By connecting an internal strap the gain may be in-
creased 5 decibels on each of the above steps. The power output is
+ 18 dbm for 1 per cent total harmonic distortion and +22 dbm for 5

Fig. 4 RA-1283 mixer rear view.
per cent total harmonic distortion. A maximum output of approxi-
mately +25 dbm prevents damage to the light valve under conditions
of excessively high input signals. The background noise level at the
amplifier output with the mixer turned OFF is 56 dbm on low gain,
51 dbm on medium gain, and 41 dbm on high gain. Under
normal operating conditions the signal-to-noise ratio of the amplifier is
65 decibels or well above that of the mixer preamplifiers.
The first stage of the amplifier uses a 403-B or -C vacuum tube con-
nected as a pentode with cathode feedback. The feedback may be re-
duced by installing the short-circuiting strap mentioned above to
raise the amplifier gain by 5 decibels. This strap is located con-
veniently on the subpanel on the top side of the main chassis. The

166 TEMPLIN August
first stage of gain is resistance-capacitance-coupled to the second
stage, the latter having a grid pot for controlling the gain in three 10-
decibel steps. This control is located on the front panel.
The second and third stages utilize a 403-B or -C and 6AK6 Type
vacuum tube, respectively, with feedback around the two stages from
plate to cathode.
A high-pass filter, located at the input to the first stage provides
optimum low-frequency cutoff for either 16-mm or 35-mm recording
or may be switched out of the circuit. An inductance-capacitance
resonant circuit in the cathode of the second stage provides adjustable
mid-frequency equalization. High-frequency equalization is located
in the 600-ohm output circuit of the amplifier. These will be described
in detail later.
The test oscillator is used to check continuity, to determine light-
valve overload level, to set noise reduction, and otherwise align the
system. It uses a 6AS6 vacuum tube in a modified Transitron circuit.
A Western Electric 400A germanium crystal is used in the cathode
circuit of this tube, its steep resistance-versus-current characteristic
acting to stabilize the oscillator. The oscillator frequency is approxi-
mately 1000 cycles per second and its output level sufficient to over-
load the light valve on medium-gain step. Continuously variable
oscillator output control is provided. The oscillator heater is paral-
leled with those of the other tubes so that the test tone is available
immediately when the oscillator OFF-ON switch supplies plate voltage
to the oscillator. The noise-reduction circuit comprises three tubes
used as audio amplifier and rectifier, carrier oscillator-modulator,
and modulated-carrier power amplifier, respectively.
The first stage uses a 6AQ6 duo-diode-triode. The triode section
amplifies the signal input, which is bridged from the main amplifier
output. An input potentiometer serves as a noise-reduction margin
control. An isolating resistance between the amplifier output and the
noise-reduction input allows the latter to be short-circuited without
affecting the main amplifier output when the light-valve switch is in
the OFF position. This prevents noise-reduction signals from modu-
lating the valve when unmodulated biased and unbiased test tracks
are being made at the end of a take. The duo-diode portion of the
tube serves to rectify the signal. This is then filtered to provide a
varying direct current proportional to the envelope of the input signal.
The attack and release times of the noise-reduction system are also
established by the elements of this filter.

In the second stage is a 6AS6 tube operating in a 30-kilocycle
Hartley oscillator circuit, with suppressor-grid modulation obtained
from the signal envelope described above.
The output amplifier stage uses a 6AK6 vacuum tube. A grid
potentiometer provides a control of the bias current and thus controls
the amount of noise reduction being applied to the light valve. The
output is transformer-coupled to a full-wave copper-oxide rectifier
which is filtered to provide a modulated direct current varying in-
versely with the envelope of the signal.
A volume-indicator meter and range switch similar to that provided

Fig. 6 RA-1282 amplifier front view.
in the mixer and a lamp-current control circuit are also included in
this unit. The lamp-current meter may alternatively be switched to
read the value of noise-reduction bias current.
A front view of the amplifier is shown in Fig. 6. In the upper left
are the amplifier-gain switch and oscillator-gain potentiometer. Below
these are the oscillator OFF-ON switch and lamp operate-hold switch.
The left-hand meter indicates lamp or noise-reduction-bias current as
selected by the switch in the lower center. The lamp-current rheostat
is located below the associated meter. The right-hand meter is the
volume indicator with the volume-indicator range switch located im-
mediately below. In the upper right are the noise-reduction input

(or margin) and noise-reduction bias-control pots. Below them are
the noise-reduction and light-valve OFF-ON switches.
Three receptacles are provided on both the left and right ends for
interconnection cables to the associated units, one of the recorder
cables being required only when the optional phototube amplifier is
installed in the recorder. A monitor jack for the recordist is also
located on the right side. When the top cover is in place, the side
flaps are held closed over the receptacles, thus protecting them from
dirt and physical damage.

Fig. 7 RA-1282 amplifier chassis top view.
By loosening two screws on each end of the unit, which are located
on the interconnecting-cable receptacle panels, the chassis may be
slipped from its case. A top view of the chassis with cover removed is
shown in Fig. 7.
The high-pass filter (or low-frequency equalization) control switch
is shown on the subpanel at the extreme left. In the center portion of
the subpanel the mid-range and high-frequency equalization controls
are located. By replacing strap A-A with strap B-B the high-fre-
quency equalization may be removed completely from the circuit. The
RA-1282 amplifier is 15 ! /2 inches long by Il 3 /s inches high by 9 inches
deep and weighs approximately 32 pounds.

170 TEMPLIN August
The R A- 1284 power unit provides plate and heater voltages for all
vacuum tubes and 12 volts direct current for the recorder exciter
lamp. The power unit operates from a single-phase, 50 to 60-cycle
power source of 100 to 130 volts. Taps on the input transformer pro-
vide for a power supply nominally 105, 115, or 125 volts.
A schematic of the unit is shown in Fig. 8. A power-supply switch
controls power to the complete system. The 12-volt, direct-current
portion of the unit utilizes a bridge-type selenium rectifier and induct-
ance-capacitance filter. This circuit supplies 4.5 amperes to the

Fig. 9 R A- 1284 power unit front view.
recorder exciter lamp and 0.3 ampere each to the heaters of the two
mixer preamplifiers and the first amplifier stage in the main amplifier.
These three heaters are fed through separate ballast lamps which
regulate the current to the required value independent of power-
supply voltage, variations in lamp current, and length of connecting
cables between the amplifier and the mixer.. The remaining tubes in
the RA-1282 amplifier receive heater supply from a 6.3-volt, alternat-
ing-current transformer in the power unit. The center- tap of the
winding receives a 6-volt bias from resistances R8 and R9, which re-
duces to a negligible value the introduction of hum components from
the heaters.
The plate-supply circuit provides 30 milliamperes regulated at 180
volts for the main amplifier and 15 milliamperes unregulated at
approximately 275 volts for the mixture preamplifiers and the op-
tional phototube monitor unit. Regulation of the 180-volt circuit is
provided by the two gas tubes V2 and VS. The 275-volt supply is
filtered further in the amplifier to provide 5 milliamperes at 220 volts
direct current at the mixer terminals.
A small fan provides forced-draft cooling for the unit, drawing cool
air in past the selenium-rectifier plates and forcing the air out past the
other components. The use of the forced draft allows a considerable
saving in weight and space for the rectifier and still allows a reasonable
safety factor in the event of failure of the ventilating system.

Fig. 10 RA-1284 power. unit chassis bottom view.
Fig. 9 shows a front view of the power unit with the top cover re-
moved. The air intake for the cooling system is in the center, with
the exhaust on the two sides. Also shown on the front panel are the
power switch, power input and plate-supply fuses, and a power pilot
lamp. Receptacles for the two interconnecting cables to the amplifier
unit are shown on the right. A power-input receptacle is similarly
located on the left side. Side flaps protect the receptacles when the
cover is installed, as already described for the RA-1282 amplifier. A
bottom view of the chassis with the cover removed is shown in Fig. 10.
This shows an adjustable resistor on the right for setting the lamp
current within the desired -range, an adjustable resistor on the left for
setting the regulating range of the gas tubes (depending on whether or
172 TEMPLIN August
not the optional phototube monitor assembly is used), and in the
center, the adjustable taps for 105, 115, or 125 volts supply voltage.
The power unit is mounted in a case the same size and of the same
appearance as already described for the R A- 1282 amplifier. The
weight is 38 pounds.
In order that the system may be operated entirely from 96-volt
batteries, an auxiliary inverter unit has been developed. It supplies
60-cycle power at a nominal 115 volts to the RA-1284 power unit, the
total battery drain under full load being approximately 2.3 amperes.
This unit contains a vibrator and associated tapped autotransformer

Fig. 11 RA-1231 Type recorder.
which permits adjustment of output voltages to 115 volts 5 per cent
for battery-supply voltages in the range of 85 to 105 volts. The unit
is compactly constructed for portable operation, its dimensions being
approximately 8 X 9*/2 X 15 ! /2 inches and its weight 40 pounds.
The recording machine, previously described in the JOURNAL/ may
be either the RA-1231 or RA-1231-A Type, depending on whether 35-
mm or 16-mm film is being used. The 35-mm model is shown in Figs.
11 and 12. A conversion-part set allows the recording machine to be
modified at the studio or in the field so that it may be used inter-
changeably with the two widths of film. This recorder includes a
sealed light valve and simplified modulator with prefocused lamp,
producing a standard 100-mil variable-density track for 35-mm re-
cording or standard 80-mil variable-density track for 16-mm recording.
A new fluid damped drive mechanism, also described in the JOUR-
NAL, 2 is utilized. With this drive the 96-cycle flutter is essentially
negligible and the total flutter for all frequency bands from two to 200
cycles per second is approximately 0.05 per cent. The total weight of
the recorder, less film magazine, is 76 pounds. Standard Mitchell 35-
mm and 16-mm magazines have been adapted for use with this re-
corder. A transformer, matching the 600-ohm output of the amplifier
to the light valve, and a noise-reduction Simplex circuit are provided
in the recorder.
The optional RA-1278 phototube monitor assembly, when pro-
vided, is also mounted in the recording machine. The amplifier por-


Fig. 12 RA-1231 Type recorder front interior view.
tion of this assembly is shown schematically in Fig. 13. It is a two-
stage unit with feedback around the first stage. One 1620 and one
6J5 vacuum tube are used. The output impedance of the amplifier is
50 ohms to match the mixer monitor circuit. It provides an output
level of 10 to +8 dbm, depending on the type of light valve and lamp
current used. For conditions of extremely low input-signal level the
gain may be increased 10 decibels by means of a strap on the terminal
strip which reduces the feedback. The total harmonic distortion at
maximum operating level is not over 1 per cent. The signal-to-noise
ratio with 100 per cent light-valve modulation is over 45 decibels. A
continuously variable output attenuator is provided for balancing the
output against the direct monitor level. A 600-ohm test input is

Fig. 13 RA-1278 phototube monitor unit schematic.
provided on the phototube socket for making electrical transmission
tests. The amplifier chassis is flexibly mounted to prevent micro-
phonic noise when the recorder is operating.
As shown in Fig. 14 the phototube monitor assembly includes a de-
flector subassembly and a relay lens in addition to the amplifier
chassis. The deflector transmits approximately 10 per cent of the
incident modulated light through an aperture and the relay lens to a
phototube mounted on the amplifier chassis. This chassis is located in
the rear compartment of the recorder.
Several types of driving motors are available. These, with their
associated chain and sprocket kits permit alternative operation syn-
chronously from a 50- or 60-cycle, 3-phase supply, in interlock from a


Fig. 14 RA-1278 phototube monitor unit assembly.
3-phase distributor system, or in a multiduty motor system energized
from a 3-phase, 220-volt alternating-current line or 96-volt batteries.
For multiduty motor operation an optional R A- 1444 control unit has
been developed. This unit provides means for controlling one recorder
motor and two camera motors when operating from either an alter-
nating- or direct-current power source. When working from a 3-phase
alternating-current supply all motors operate synchronously. When
operating from batteries they may be operated individually or in inter-
lock at film speeds of 21 to 25 frames per second for both 16-mm and
35-mm film.

ZOO 500 1000 2000
Fig. 15 RA-1283 mixer dialog-equalizer characteristics.
An RA-1289-A photographic slater and RA-1290-A film punch are
available as accessory items. These mount within the recorder and
are energized from the RA-1284 power unit.
The over-all frequency-response characteristic of the system without
equalization is essentially flat from 50 to 10,000 cycles per second with
the amplifier on each of its gain settings. However, in actual practice
the characteristic is always modified to achieve particular over-all
effects as discussed below.
Fig. 15 shows the frequency-response characteristic of the RA-1283
mixer, using the various steps of dialog equalization contained in each
mixer preamplifier. The setting used in any particular instance will
depend on the acoustic conditions on the set, the presence of wind or
other low-frequency noises, and the quality of the particular voice being
recorded. 3 These equalizers are adjustable in 5 steps of 4 decibels each
at 80 cycles per second with an OFF position. The shape of the family
of curves agrees closely with those in general use in Hollywood
studios. The average setting is 4 or 8 decibels for 35-mm recording
and 12 or 16 decibels for 16-mm recording. For music recording the
dialog equalizer is normally in the OFF position.
Fig. 16 shows the frequency-response characteristic for the high-

"~ -20
, - -

: : -




96 MM

9 *2S



16 MM



I ,










rOO 200 500

Fig. 16 High-pass filter characteristics.
pass filter which is located at the input of the R A- 1282 amplifier.
The high-pass filter is used particularly to eliminate components be-
low the fundamental dialog frequency such as are produced by
resonance effects on the set or other extraneous noise sources. These
unwanted components, which may be subaudible, are thereb}'' pre-
vented from operating the noise-reduction circuit or intermodulating
the audible signals. The filter cuts off rather sharply, with the 6-
decibel loss point at approximately 120 cycles per second for 35-mm
recording, and 180 cycles per second for 16-mm recording. A switch
located on the upper side of the amplifier chassis transfers between
these two positions or removes the filter from the circuit.
It will be observed that the dialog equalizer and high-pass filter
combine to produce the desired over-all response at the low-frequency
end, and that the low-frequency loss from each of these networks is
greater for 16-mm than for 35-mm recording. This is because the
high-frequency cutoff point is at a lower frequency in the 16-mm
application because of the reduced film speed and it is necessary to
reduce correspondingly the low-frequency response to obtain the most
pleasing over-all balance.
Fig. 17 shows the adjustable mid-frequency equalization character-
istics. The mid-frequency equalization requirements are based on ex-
perimental listening tests which have shown that the "presence" in
dialog recording is made more realistic by accentuating the response
+ +
o w 5

16 MM






M) \


STEP 4 (35 MM ) *
STEP 3 (35 MM)

STEP 2 (35MM) N
STEP 1 (35MM) \






200 500 1000 2000
Fig. 17 Mid-range-equalizer characteristics,
in this range. The amount of this equalization varies somewhat in
different studios and also depends on the type of microphone used,
some of which have at least one resonant peak in this portion of the
spectrum. With the Western Electric RA-1142 microphone a 3-
decibel boost at 3000 cycles is an average equalization value for 35-
mm recording. For 16-mm recording, additional pre-emphasis peaking
at approximately 5000 cycles is added to the "presence" equalization
in view of the inherent cutoff at 4500 to 5000 cycles on the reproduced
film. Five steps of equalization of 1 decibel each plus an OFF position
are available for 35-mm recording and one position for 16-mm record-
ing. This control is also located on the upper side of the amplifier
Fig. 18 shows the frequency-response characteristic for the high-
frequency equalization . For 35-mm recording this consists of an equal-
izer having response complementary to the resonance characteristic
of the light valve, thereby making the light- valve modulation corre-
spond to that of the signal over the entire useful frequency range. This
characteristic introduces a loss of 9 decibels at the light-valve resonant
frequency of 9000 cycles. For 16-mm recording this equalizer is
switched to provide a low-pass filter cutting out slightly above 5000

5000 IOOOO 20000
200 500 1000 2000
Fig. 18 Light-valve equalizer and low-pass filter characteristics.
cycles in order to suppress those frequency components beyond the
16-mm useful range.
Typical over-all-electrical-response characteristics for recording on
35-mm film are shown in Fig. 19. For dialog recording the low fre-
quencies are attenuated and the mid-range boosted to obain the most
realistic dialog quality. The light-valve equalizer is used to comple-
ment the effect of light-valve resonance. For music the low-frequency
and mid-frequency equalization are removed. The light- valve
equalizer is left in the circuit to provide an over-all system response
(up to the film) which is flat over the useful frequency range, thereby
obtaining the maximum fidelity for the recorded music.
In Fig. 20 a typical over-all electrical response for 16-mm recording
is shown. As previously described, for dialog recording the 1<>\\
frequencies are attenuated to a greater extent than for 35-mm recording
to obtain the proper balance with the high frequencies which are cut
off at 5000 cycles. The mid-range boost and pre-emphasis are also
shown. For music recording the low frequencies are attenuated to a
lesser extent than for dialog. However, the extreme low frequencies
are eliminated and the mid-frequency emphasis is retained to give the
most desirable over-all response. Fig. 21 shows a transmission sche-
matic and level diagram of the complete system for normal conditions
of operation. Vertical lines on the chart represent fixed gains or losses.
Sloping lines represent variable gains or losses.

Fig. 19 35-mm recording over-all-electrical-response characteristics.
Starting at the microphone, at the left end of the chart, an input
level of 70 dbm is assumed. This corresponds approximately to
that obtained from a Western Electric RA-1142 microphone located 3
feet from an average dialog source.
The signal level at the preamplifier outputs under these conditions
is 37 decibels below their overload point, thus allowing an adequate
margin for extremely high-level signals.
For this input signal level of 70 dbm the normal mixer output
level of 55 dbm is obtained with 12 decibels attenuation remaining
in the mixer pots. The signal-to-noise ratio at the mixer output under
these conditions is 56 decibels.
For the normal mixer output level of 55 dbm, a +15 dbm level
is obtained at the output of the final amplifier stage on medium
amplifier gain step. This level corresponds to 100 per cent modulation
of an average RA-1241 Type light valve. The amplifier signal-to-
noise ratio of 65 decibels is well above that of the mixer unit. The
signal-to-noise ratio of the system for this average recording condition
is 55 decibels at the output of the electrical circuits. An effective
signal-to-noise ratio of approximately 48 decibels can be realized from
35-mm film, and approximately 46 decibels from 16-mm film, with the
application of noise reduction as provided in the system.
The system in normal operation has a margin of 3 decibels between

100 1000
Fig. 20 16-mm recording over-all-electrical-response characteristics.
the overload point of the light valve and the level at which the ampli-
fier distortion reaches 1 per cent. Peak signals which are allowed to
overload the valve for increased effective signal level can extend 7
decibels beyond the light-valve overload point with not over 5 per cent
distortion. For peak signals 2 decibels below light-valve overload, and
under optimum film-processing conditions, intermodulation can be |
held to 6 per cent, corresponding to approximately 1.5 per cent total
harmonic distortion.
Fig. 22 shows typical frequency-response characteristics obtained ;
from a 35-mm and 16-mm print for constant modulation of the light j
valve. The high-frequency losses include all film losses introduced
in the recording and printing processes. They do not include i
8 2 I s g


reproducing scanning losses since the data were obtained by scanning
the print with a microdensitometer.
It is felt that the compactness, sturdy construction, and portability
features of this equipment amply meet the requirements of the in-
dustry for a sound-recording system for general location work. Its
versatility and flexibility of operation are of particular value to the
small studio which from the standpoint of economy and convenience
finds it extremely advantageous to have one type of system meet its

> -10
16 MM
tOOOO 20000
200 500 1000 2000 5<
Fig. 22 Response characteristics from film print with constant light-valve
needs for dialog and music recording on 16-mm or 35-mm film, in the
studio or on location. As indicated from the performance character-
istics described in this paper, the mechanical and electrical design of
the equipment make possible a high-quality sound product, the specifi-
cations for frequency range, signal-to-noise ratio, harmonic distor-
tion, and flutter content being consistent with current requirements of
major Hollywood studios.
(1) G. R. Crane and H, A. Manley, "A simplified all-purpose film recording
machine," /. Soc. Mot. Pict. Eng., vol. 46, pp. 465-475; June, 1946.
(2) C. C. Davis, "An improved film-drive filter mechanism," /. Soc. Mot.
Pict. Eng., vol. 46, pp. 454-465; June, 1946.
(3) D. P. Loye and K. F. Morgan, "Sound picture recording and reproducing
characteristics," J. Soc. Mot. Pict. Eng., vol. 32, pp. 631-34