Apparatus and method for detecting crossmodulation distortion

A method and apparatus for detecting crossmodulation distortion on an optical sound track by detecting variations in the light transmitted through a predetermined area on the sound track, the predetermined area being selected such that its transmittance varies substantially only with the amount of crossmodulation distortion on the sound track. The predetermined area preferably has a dimension along the sound track which is a whole number multiple of, and substantially greater than, the wavelength of a high frequency test signal. Light is also transmitted through a second predetermined area on the sound track which clips the high frequency test signal, thereby providing a low frequency phase reference for determining the relative exposure of the film. In an alternate embodiment, the predetermined areas are divided into primary and secondary areas, mutually spaced by a distance which is out-of-phase with respect to the wavelength of a low frequency test signal. Crossmodulation distortion information is obtained by comparing the light transmitted through both areas. Instead of single predetermined areas, multiple areas spaced by whole number multiples of the low frequency signal wavelength can be used to improve the resolution of the system's frequency response. A mask with defined apertures controls the light paths. Testing for crossmodulation distortion can be done by simply moving the film by hand a short distance through the test equipment. Because crossmodulation is continuously monitored, diagnostic testing is also possible.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to sound recording for motion pictures, and more 
particularly to a method and apparatus for detecting crossmodulation 
distortion on optical sound tracks. 
2. Description of the Prior Art 
Variable area motion picture sound tracks exhibit a form of distortion 
peculiar to optical sound tracks if the negative and print are not exposed 
and developed properly. Such distortion is known as crossmodulation 
distortion. The common method of testing for crossmodulation distortion 
involves the recording of a high frequency test signal, amplitude 
modulated by a low frequency test signal on the optical sound track. For 
35 mm motion picture sound tracks, 8000-9000 hertz and 400 hertz are 
commonly used for the high and low frequency test signals, respectively. 
A segment of a film strip 2 having recorded thereon such a test sound track 
in dual bilateral format is shown in FIG. 1. One wavelength of the low 
frequency test signal is denoted by dimension A. The amplitude modulated 
high frequency test signal appears within each lobe 3 of the low frequency 
signal. 
If there is no crossmodulation distortion, only the originally recorded 
signal with frequencies present only around the high frequency test signal 
will be present when the sound track is played back. For example, with a 
high frequency signal of 9,000 hertz and conventional amplitude modulation 
used to create the crossmodulation signal, the resultant frequencies will 
be at 8,600, 9,000 and 9,400 hertz. The presence of crossmodulation 
distortion results in the presence of not only the above three frequencies 
during playback, but also a spurious signal at 400 hertz. FIG. 2 depicts 
an enlarged portion of the high frequency test signal image. For zero 
crossmodulation distortion, the test signal appears as an amplitude 
modulated sine wave, shown in dashed lines 4. The portion of the sound 
track within this signal envelope is darkened, as indicated by the 
diagonal lining. The presence of crossmodulation distortion causes the 
image boundary to shift, for example to the boundary indicated by solid 
line 6. In this case the lined area between lines 4 and 6 would also be 
dark on the sound track, distorting the recorded signal and leading to 
lower quality sound reproduction. 
If both the recording and printing of the crossmodulation test signals are 
done properly, the amplitude modulated signal will play back as recorded 
and the sound track will exhibit no distortion. If, however, either the 
recording or printing was imperfect, the playback will contain a residual 
crossmodulation signal having a frequency equal to that of the low 
frequency modulating signal. The level of the spurious residual signal 
provides a measure of the distortion level. Its phase relative to the 
phase of the low frequency modulating test signal indicates whether the 
print and/or negative has been overexposed or underexposed. 
The crossmodulation distortion test that has been used in the industry 
involves the recording of several feet of test signal on a film, and then 
running the film through a conventional optical sound track playback 
device at a controlled speed to measure the crossmodulation distortion 
signal with a filter tuned to the frequency of the low frequency 
modulating signal. Because this technique is cumbersome and time 
consuming, it is performed only infrequently. Furthermore, it provides 
only an average indication of crossmodulation distortion, rather than a 
continuous indication of the distortion level at discrete locations along 
the track. This can result in certain crossmodulation distortion being 
missed, and also severely limits the usefulness of the testing technique 
as a diagnostic tool for determining the cause of the distortion. Further 
details on crossmodulation distortion testing may be obtained from a 
publication by Howard M. Tremaine, Audio Cyclopedia, sections 
18.283-18.299, Howard W. Sams & Co., 1977 (2d edition). 
SUMMARY OF THE INVENTION 
In view of the above problems associated with the prior art, an object of 
the present invention is the provision of a method and apparatus for 
performing a quick and easy test for crossmodulation distortion on an 
optical sound track, without having to run the film through an optical 
playback system. The testing of every print would thereby be encouraged as 
a routine quality control step. 
Another object is the provision of a method and apparatus for continuous 
testing for crossmodulation distortion along the length of a test strip, 
thereby enabling the use of the test as a diagnostic tool. 
An additional object is the provision of a method and apparatus for testing 
for crossmodulation distortion which requires only a small amount of film 
to be devoted to a test strip, and which eliminates the need to precisely 
control the speed of the film through the testing apparatus. 
Still another object is the provision of a method and apparatus for 
determining whether a film exhibiting crossmodulation distortion has been 
overexposed or underexposed. 
The above objects are accomplished according to the present invention by a 
method and apparatus for directing light through specified portions of an 
optical test sound track, such that useful information on crossmodulation 
distortion is obtained by monitoring the light transmitted through the 
film during relative movement between the film and light path. For zero 
crossmodulation distortion, the amount of light transmitted through the 
film exhibits zero or minimum variation; increasing amounts of 
crossmodulation distortion produce increasing amounts of variation in the 
amount of transmitted light. In the preferred embodiment, light is 
directed through an area on the film having a dimension along the sound 
track substantially equal to a whole number multiple of the wavelength of 
the high frequency test signal, and approximately one-half the wavelength 
of the low frequency modulating signal. With an illuminated film area 
having large dimensions relative to the high frequency wavelength and poor 
edge sharpness, the amount of transmitted light is made substantially 
independent of the high frequency signal. It varies, instead, only in 
response to track density variations at the frequency of the low frequency 
modulating signal, thereby facilitating a determination of the 
crossmodulation distortion. 
To ascertain the phase of the residual crossmodulation distortion signal 
relative to the low frequency test signal, and thereby the relative 
exposure of the film, light is directed to a second light detector through 
a second area of the sound track. The second area is selected such that 
the light received by the second light detector varies, as the film is 
moved, at the periodicity of the low frequency modulating test signal and 
in known phase relationship therewith. The second area is displaced from 
the first area on the film by a predetermined distance, preferably an 
integral number of half periods of the low frequency modulating signal. It 
thereby provides a phase reference for the residual crossmodulation 
distortion signal whereby the relative exposure of the film can be 
determined. 
Rather than directing light through single areas on the film, light can be 
directed through a plurality of discrete areas which are mutually spaced 
along the sound track by whole number multiples of the wavelength of the 
low frequency test signal. The bandwidth of the detecting apparatus is 
narrowed as more areas on the sound track are illuminated, thereby 
rejecting more spurious signals at wavelengths other than that of the 
residual crossmodulation signal being measured. 
In another embodiment of the invention, the measurement of both 
crossmodulation distortion and phase is accomplished by directing light 
through primary and secondary areas on the sound track. The primary and 
secondary areas are mutually spaced by a predetermined distance which is 
out-of-phase with respect to the wavelength of the low frequency test 
signal. The pattern of out-of-phase variations in light transmittance 
detected through the primary and secondary areas provides an indication of 
the distortion level. A second, similarly spaced set of primary and 
secondary areas, selected to rectify the high frequency test signal and 
thereby provide a phase reference for the distortion signal, can also be 
illuminated. Again, light can be passed through a plurality of spaced 
areas on the film for both measurement of crossmodulation distortion and 
phase reference, to enhance frequency response. 
The invention also comprehends the use of an opaque mask member to provide 
the light control function. The mask member is positioned in the light 
path, and has apertures which enable the transmission of light between the 
light source and detector only through the desired areas of the film sound 
track. 
Additional objects and advantages of the invention will be recognized by 
those skilled in the art from a consideration of the following detailed 
description of various preferred embodiments, together with the appended 
drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Referring to FIG. 3, one embodiment of the subject apparatus for detecting 
crossmodulation distortion is shown. Light from a light source 8 is 
directed through a collimating lens 10 onto a film strip 12 having a 
conventional optical sound track with a high frequency test signal 
modulated by a low frequency test signal. Positioned below film strip 12 
are first and second light detectors 14 and 16, respectively, which 
produce electrical outputs in proportion to the amount of incident light. 
Detectors 14 and 16 may comprise conventional photocells. 
An opaque mask member 18 is positioned between film 12 and detectors 14 and 
16 to control the transmission of light from lens 10 onto the light 
detectors. Mask member 18 has a series of apertures arranged in groups or 
cells generally indicated by reference numerals 20 and 22, which apertures 
determine the permissible light path between light source 8 and detectors 
14 and 16. The size and position of the apertures are selected such that 
only light which passes through desired target areas on the sound track is 
transmitted to the light detectors, all light outside of the desired 
target areas being excluded from the light path between source 8 and 
detectors 14 and 16. Support members 24 and 26 hold mask 18 in place, 
while amplifiers 28 and 30 are respectively connected to detectors 14 and 
16 to amplify the signals representing the amount of light received by the 
detectors. 
Mask 18, an enlarged plan view of which is shown in FIG. 4, is preferably 
formed from a thin sheet metal such as 0.004 inch thick berrylium copper 
foil. It is held below the film, preferably with a spacing of about 10 to 
20 thousands of an inch between the mask and film. Alternately, mask 18 
could be positioned above the film, or other means can be envisioned for 
insuring that only light which is transmitted through the desired target 
areas of the sound track reaches detectors 14 and 16. 
As shown in FIG. 4, the first group of apertures 20 consists of four 
elongated apertures 20a, 20b, 20c and 20d, while the second group of 
apertures 22 consists of four generally square apertures 22a, 22b, 22c and 
22d. As explained below, aperture group 20 controls the light transmitted 
to detector 14 so as to provide an indication of the magnitude of 
crossmodulation distortion, while aperture group 22 controls the light 
transmitted to detector 16 so as to provide an indication of the relative 
phase of the crossmodulation distortion, and thus of the film's exposure. 
While four separate apertures are shown in each group 20 and 22, the 
advantages of the invention can be obtained with only a single aperture in 
each group. However, the use of multiple apertures enhances the frequency 
resolution of the system by narrowing its reponse bandwidth, which is 
centered around the frequency of the low frequency modulating test signal. 
Furthermore, aperture group 22 may be omitted entirely; aperture group 20 
will still provide an indication of the magnitude of the crossmodulation 
distortion, although no information will be provided regarding its phase. 
Referring now to FIG. 4 in conjunction with FIG. 5, the apertures of mask 
member 18 are shown in relation to a test sound track. In FIG. 5 it is 
assumed that the light follows a generally collimated path through the 
film and mask member to the light detector, and that each of these 
elements are aligned so that the aperture areas are equal to the target 
areas on the film. Adjustment of the aperture dimensions would be required 
should the light follow a coverging or diverging rather than a collimated 
path, or if the film and mask member were out of alignment with respect to 
the light source. The critical factor is that the light eventually 
reaching the detectors represent only that light which is transmitted 
through the desired target areas of the film. 
The light from lens 10 is not preferably collimated, but rather is somewhat 
fuzzy at its edges. This, together with the fact that Light is transmitted 
along an area of the sound track which is substantially greater than the 
wavelength of the high frequency test signal, minimizes and ideally 
eliminates any variation in transmittance due to the high frequency 
components of the crossmodulation signal. 
Each of the first target areas on the film included in the controlled light 
path, corresponding to apertures 20a-20d, are characterized by a dimension 
along the sound track (parallel to axis line 32) which is preferably 
substantially equal to a whole number multiple of the high frequency 
signal wavelength. In the transverse direction across the sound track, 
apertures 20a-20d encompass a whole number of signal bands, a signal band 
being defined herein as the smallest unit across the sound track to 
transmit an amount of light which is constantly proportional to the light 
transmitted across the entire sound track as the film is moved under the 
incident light. There are four signal bands 34a, 34b, 34c and 34d on the 
sound track shown in FIG. 5. 
With the widths of apertures 20a-20d equal to a whole number multiple of 
the high frequency test signal wavelength and an extended light source, 
the amount of light transmitted to detector 14 will remain substantially 
constant if a distortion-free sound track is passed through the light 
path. Since the effect of crossmodulation distortion on the amount of 
light transmitted through the sound track becomes most pronounced at the 
areas of the sound track where the modulated high frequency signal reaches 
maximum amplitude and becomes correspondingly least pronounced where the 
high frequency signal is at minimum amplitude, the amount of light 
transmitted through the sound track and mask combination will vary in 
proportion to the degree of crossmodulation distortion present as the 
sound track is passed through the light path. 
Apertures 22a-22d Provide a phase reference for the signals corresponding 
to the light transmitted through apertures 20a-20d. They are positioned on 
the mask to extend partially into a high frequency signal portion of the 
sound track, thereby clipping the high frequency signal so as to deliver 
to detector 16 a rectified light signal which varies in amplitude at the 
frequency of the low frequency test signal. While apertures 22a-22d are 
positioned over the center of the sound track in FIG. 5, they could also 
be placed along the edges of the sound track or at any other position that 
would clip the high frequency test signal so as to provide a phase 
reference for the low frequency test signal. Phase reference apertures 
22a-22d could be spaced respectively by any known distance from apertures 
20a-20d within the limits of the sound track. However, it is preferable 
for convenience of output analysis that the crossmodulation distortion 
signal obtained through apertures 20a-20d be either directly in-phase or 
180 degrees out-of-phase with the phase reference signal through apertures 
22a-22d. The latter apertures are accordingly displaced from apertures 
20a-20d by a whole number multiple of one-half the wavelength of the low 
frequency test signal. 
Further elements of the distortion detecting apparatus are indicated in 
FIG. 5. A wall 36 provides a stop which facilitates simple insertion of 
the film. Wall 36 is positioned so that the film can be properly aligned 
with respect to the optical elements merely by sliding it into the 
distortion detecting apparatus until it abuts the wall. Sprockets 38 of a 
sprocket wheel extend through corresponding sprocket holes in the film to 
move the film under the light beam. The sprocket wheel can be motor 
controlled, or alternately the film can simply be pulled under the light 
beam by hand, or the mask or even the light source moved or oscillated. 
In the embodiment shown, which is designed for 35 mm sound track, apertures 
20a-20d are approximately equal in length to the width of the sound track, 
which is 0.09 inches. They are 0.02 inches wide, which compares with a 
wavelength of 0.045 inches for 400 hertz low frequency test and 
crossmodulation residual signals. 
The output circuit which provides an indication of both the amplitude and 
phase of the crossmodulation distortion is shown in FIG. 6. The signal 
produced by photocell 14 is amplified by amplifier circuit 28, at the 
output of which is capacitively coupled by capacitor 39 to a positive peak 
follower circuit and a negative peak follower circuit. Capacitor 39 
removed DC bias from the signal. A resistor 40 is connected between the 
capacitor output and ground. The positive peak follower circuit includes 
operational amplifier 41 having its positive input terminal connected to 
capacitor 39, its negative input terminal connected in a feedback loop 
through diode 42, and its output connected through another diode 44 which 
conducts current towards the positive input to operational amplifier 46. 
The output of amplifier 46 is connected in a first feedback loop back to 
its negative input, and in a second feedback loop through resistor 48 to 
the negative input for amplifier 41. A capacitor 50 and resistor 52 are 
connected to ground from the positive input of amplifier 46, providing 
controlled decay for that input signal. 
The negative peak follower circuit is designed similarly to the positive 
peak follower circuit, and comprises operational amplifier 54 with its 
positive input terminal connected to the output of amplifier 28, feedback 
diode 56, diode 58 between the output of amplifier 54 and the positive 
input of operational amplifier 60, feedback resistor 62, and capacitor 64 
and resistor 66 connected to ground from the positive input of amplifier 
60. The negative peak follower circuit is identical to the positive peak 
follower circuit, with the exception of diodes 56 and 58 being reversed in 
the direction of conduction. 
The outputs of the positive and negative peak follower circuits are 
respectively connected to the negative and positive input terminals of a 
differential amplifier 68 through resistors 70 and 72. The negative input 
to amplifier 68 is connected through resistor 74 in a feedback circuit to 
the amplifier's output, while the positive input terminal is connected 
through resistor 76 to ground. At the output of amplifier 68 is a meter 78 
which indicates the difference between the signals held by the positive 
and negative peak follower circuits. Since this difference is proportional 
to the variation in light transmitted to photocell 14 as the test sound 
track is moved with respect to the beam from light source 8, it provides 
an indication of the amount of any residual crossmodulation distortion on 
the sound track. 
The output from phase reference photocell 16 is processed through amplifier 
30, and has its DC bias removed by capacitor 79 and resistor 80. It is 
then applied to a comparator 81, which determines whether the applied 
signal is positive or negative. The output of comparator 81 is connected 
to the D (data) input of a D type flip flop circuit 82. The clock input to 
flip-flop 82 is provided by a comparator 83 and one-shot circuit 84. 
Comparator 83 has its positive input connected to the output of amplifier 
28, and its negative input connected to the positive input of amplifier 
46. When the output of positive peak follower amplifier 41 exceeds its 
input, one-shot circuit 84 delivers a pulse to the clock input of 
flip-flop 82, which then captures the polarity of the output of comparator 
81. Flip-flop 82 is connected to an output device such as a pair of lamps 
86 and 88, one or the other of which will light to indicate whether the 
crossmodulation distortion signal from photocell 14 is in-phase or 
out-of-phase with the reference signal at photocell 16. With this 
information, it can be determined whether the film is overexposed or under 
exposed. 
One of the advantages of the present invention lies in the fact that 
information on crossmodulation distortion can be provided from only one or 
a few wavelengths of the low frequency test signal, in contrast to the 
prior art technique in which several feet of film leader was typically 
devoted to crossmodulation distortion testing. Furthermore, the present 
apparatus can be used in a diagnostic capacity to determine malfunctioning 
of the processing equipment. For example periodic defocusing, which was 
only detected with the prior art technique insofar as it affected the 
average value of the crossmodulation distortion, can now be identified and 
measured. Periodic defocus results from variations in the film dynamics, 
and is frequently associated with a periodic condition of poor contact 
between the negative and print in the printing process. In one case, the 
continuous monitoring of crossmodulation distortion made possible by the 
present invention was used to detect a periodic varation in the distortion 
level. This was traced to a defect in a sprocketless printer wheel, used 
to move the film through the processing apparatus, which had a 
circumference equal to the distance between the periodic distortions. 
While ideally the distortion detecting apparatus should respond only to the 
low frequency or crossmodulation distortion signal, which is 400 hertz in 
the described embodiment, in practice there is a gap in the frequencies 
present on the sound track between the low and high test frequency areas 
(except for spurious signals caused by pinholes, dust, etc.) Accordingly, 
it is within the scope of the invention to provide a low pass system which 
responds to all frequencies substantially below the high frequency level, 
rather than a band pass system as described herein. 
Referring now to FIGS. 7-10, another embodiment of the invention is shown 
which enhances the resolution of the crossmodulation distortion detection. 
A mask member 90 is shown in FIG. 8 which is similar to mask member 18 of 
FIG. 4. The same reference numerals are used to indicate corresponding 
portions of both masks. Primary groups of large and small apertures 20 and 
22, respectively, are provided in mask 90 with the same dimensions and 
relative spacing as for mask 18. Spaced on the side of aperture group 20 
away from group 22 by a predetermined distance, which is out-of-phase with 
respect to the low frequency test signal, is a secondary group of 
apertures 120. The latter apertures are substantially identical to those 
in primary aperture group 20. A secondary group of phase reference 
apertures 122 is provided on the opposite side of primary phase reference 
aperture group 22, and is displaced from group 22 by the same spacing as 
between primary and secondary groups 20 and 120. Secondary aperture groups 
20 and 122 are each preferably spaced from their respective primary 
aperture groups 20 and 22 a whole number multiple of one-half by the 
wavelength of the low frequency test signal. 
FIG. 7 is similar to FIG. 3, showing mask member 90 positioned in the 
testing apparatus. In addition to photocells 14 and 16, additional 
photocells 114 and 116 are aligned under aperture groups 120 and 122, 
respectively. The light transmitted through the film sound track from 
light source 8 and lens 10 (not shown) thus follows primary paths 
corresponding to aperture groups 20 and 22, and secondary paths 
corresponding to aperture groups 120 and 122. Amplifiers 28, 30, 128 and 
130 receive signals from photocells 14, 16, 114 and 116, respectively. 
Difference amplifiers 132 and 134 receive inputs from amplifiers 28 and 
128, and amplifiers 30 and 130, respectively. 
FIG. 9 is a view similar to FIG. 5, showing a strip of film with a test 
sound track overlaying mask member 90. With aperture group 20 180 degrees 
out of phase with group 120 and aperture group 22 180 degrees out of phase 
with group 122 as shown, it can be seen that as a primary aperture is 
aligned with a portion of the sound track at which the high frequency 
signal lobe is expanding in amplitude, the corresponding secondary group 
of apertures will be aligned with a portion of the test track at which the 
high frequency lobe is decreasing in modulated amplitude. Thus, any 
difference in the amount of light transmitted through aperture groups 20 
and 120 and received by photocells 14 and 114 indicates the presence of 
crossmodulation distortion, with the amount of difference increasing in 
proportion to the amount of distortion present. 
Phase reference aperture groups 22 and 122 will similarly provide a 
difference signal at the output of amplifier 134 which acts as a phase 
reference for the distortion amplitude signal at the output of amplifier 
132. Aperture groups 22 and 122 are located on the mask respectively 
in-phase with aperture groups 120 and 20, thereby enabling a direct 
comparison between the derived crossmodulation distortion signal and the 
phase reference signal. A different spacing could be used between the 
phase reference and distortion amplitude apertures, with phase 
compensation apparatus added to the output stage to compensate for the 
difference in phase. 
Output circuitry for this embodiment is shown in FIG. 10, in somewhat more 
block diagram form than the circuit of FIG. 6. Difference amplifier 132 
produces a signal proportional to the difference between the outputs of 
distortion amplitude photocell amplifiers 28 and 128. Its output is 
connected to a peak follow and hold circuit 136, which is turn has its 
output connected to sample and hold circuit 138. A comparator amplifier 
140 also receives the output of difference amplifier 132, and is connected 
to reset peak follow and hold circuit 136 to zero volts when the output of 
difference amplifier 132 becomes less than zero volts. 
The output of peak follow and hold circuit 136 is also connected to one 
input of a comparator 141, which in turn has its output connected to the 
input to a and one-shot circuit 142. The other input to comparator 141, is 
connected to the output of difference amplifier 132. When the output of 
peak follow and hold circuit 136 exceeds its input, one-shot circuit 142 
produces a pulse which is delivered to sample and hold circuit 138, 
causing it to sample the output of peak follower circuit 136. This signal 
level is then delivered to an appropriate readout device such as meter 
144. It is a feature of the invention that a readout is obtained 
independent of the film speed, allowing the film to be simply moved by 
hand without the speed control required in the prior art. 
Difference amplifier 134 receives the outputs of phase reference amplifiers 
30 and 130, and in turn has its output connected to a comparator 146 which 
detects whether the difference signal is positive or negative. The output 
of comparator 146 is connected to the D input of a D type flip-flop 148. 
The clock input to flip-flop 148 is connected to the output of one-shot 
circuit 142, in common with the clock input to sample and hold circuit 
138. Thus, the polarity of the output of comparator 146 is captured by 
flip-flop 148 at the same time sample and hold circuit 138 receives an 
updated peak signal. The relative phase relationship is displayed by a 
pair of lamps 150 and 152 or other convenient display means, as in the 
first embodiment described above. 
Various type of readouts can be employed with the distortion detecting 
apparatus. For example, the first embodiment described above provides an 
analog reading of distortion level with meter 78, while the second 
embodiment provides a digital readout with meter 144. The readout device 
is preferrably graduated in terms of percentage crossmodulation 
distortion, and with a digital readout the output can be updated at 
intervals corresponding to the wavelength of the low frequency modulating 
signal. Other readout means can be envisioned, such as storing the output 
signals in a computer and providing a printout in graph format. 
While particular embodiments of the invention have been shown and 
described, numerous variations may occur to those skilled in the art. For 
example, the implementation shown herein is intended for 35 mm dual 
bilateral optical soundtracks; the dimensions and locations of the mask 
apertures can be adjusted for other types of films, optical soundtracks or 
test signals. Also, many variations in the number, order or spacing of the 
mask apertures may be made while still employing the teachings of the 
invention, or a mask can be dispensed with altogther and appropriate 
reflecting, focusing or other means employed to direct the light through 
the desired areas on the sound track. It is accordingly intended that the 
invention be limited only in terms of the appended claims.