APPARATUS AND METHOD FOR DETECTING CROSSMODULATION DISTORTION

Abstract: Crossmodulation distortion is detected from multiple bands (34) on a film (12). Light is passed through the film (12) where each band (34) has a high frequency signal modulated by a low frequency signal. Mask (18) with two groups of apertures (20 and 22) is positioned between the film (12) and two detectors (14 and 16). A first group of apertures (20) has a longitudinal dimension which is a whole number multiple of, and substantially greater than the wavelength of the high frequency signal and a transverse dimension sufficient to encompass some whole number of bands (34). The second group (22) is positioned and dimensioned such that only light from a partial portion of a selected number of bands (34) passes to provide a phase reference. More than one mask (18) can be used and secondary aperture groups (120 and 122) can also be used.

Description: APPARATUS AND METHOD FOR DETECTING CROSSMODULATION DISTORTION '- BACKGROUND OF THE INVENTION Field of the Invention This invention relates to sound recording for τ_g motion pictures, and more particularly to a method and apparatus for detecting crosεmodulation distortion on optical sound tracks. Description of the Prior Art 15 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 20 crosεmodulation distortion. The common method of testing for cross odulation distortion involves the recording of a high frequency test signal, amplitude modulated by a low frequency test signal on the optical sound track. For 35mm motion picture sound tracks, 5 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 0 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. 5 If there is no crossmodulation distortion, ' only the originally recorded signal with frequenci present only around the high frequency test signal wi be present when the sound track is played back. F example, with a high frequency signal of 9,000 her and conventional amplitude modulation used to crea the crossmodulation signal, the resultant frequenci will be at 8,600, 9,000 and 9,400 hertz. The presen of crossmodulation distortion results in the presen of not only the above three frequencies duri playback, but also a spurious signal at 400 hertz. Fig. 2 depicts an enlarged portion of the. hig frequency test signal image. For zero crossmodulati distortion, the test signal appears as an amplitud modulated sine wave, shown in dashed lines ' 4. T portion of the sound track within this signal envelo ' is darkened, as indicated by the diagonal lining. T presence of crossmodulation distortion causes the imag boundary to shift, for example to -the boundar indicated by solid line 6. In this case the lined are between lines 4 and 6 would also be dark on the soun track, distorting the recorded signal and leading t lower quality sound reproduction. If both the recording and printing of th crossmodulation test signals are done properly, th amplitude modulated signal will play back as recorde and the sound track will exhibit no distortion. If howeve , either the- recording or printing wa imperfect, the playback will contain a residua crossmodulation signal having a "frequency equal to tha of the low frequency modulating signal ' . . ' . The level o the spurious residual signal provides a measure of th distortion level. Its phase relative to the phase o the low frequency modulating test signal indicate whether the print and/or negative has been overexpose or underexposed. The crossmodulation distortion test that has been used in the industry involves the recording of 5 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 _0 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 5 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 0 distortion testing may be obtained from a publication by Howard M. Tremaine, Audio Cyclopedia, sections 18.283 - 18.299, Howard . Sams & Co., 1977 (2d edition) . 5 ' 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 0 a quick and easy -test for crossmodulation distortion on an optical sound track, v.'ithout 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. 5 Another object is the provision of a method and apparatus for continuous tes ting fo crossmodulation distortion along the length of a tes str ip , thereby enabling the use of the test as diagnostic tool . An additional object is the provision of method and apparatus for testing for crossmodulatio distortion which requires only a small amount of fil to be devoted to a test strip, and which eliminates th need to precisely control the speed of the film throug the testing apparatus. Still another object is the provision of method and apparatus for determining whether a fil exhibiting crossmodulation distortion has bee overexposed or underexposed. ■ The above objects are accomplished accordin to the present invention by a method and apparatus fo directing light through specified portions of a optical test sound track, such that useful informatio on crossmodulation distortion is obtained by monitorin the light transmitted through the film during relativ movement between the film and light path. For zer crossmodulation distortion, the amount of ligh transmitted through the film exhibits zero or minimu variation; increasing amounts of crossmodulatio distortion produce increasing amounts of variation i the amount of transmitted light. In the preferre embodiment, light is directed through an area on th film having a dimension along the sound trac substantially equal to a whole number multiple of th wavelength of the high frequency test signal, an approximately one-half the wavelength of the lo frequency modulating signal. With an illuminated fil 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 phas-e 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 th * e 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, t measurement of both crossmodulation distortion a phase is accomplished by directing light throug primary and secondary areas on the sound 1 track. T primary and secondary areas are mutually spaced by predetermined distance which is out-of-phase wit respect to the wavelength of the low frequency tes signal. The pattern of out-of-phase variations i light transmittance detected through the primary an secondary areas prov ' ides • an indication of th distortion level. A second, similarly spaced set o primary and secondary areas, selected to. rectify th high frequency test signal and thereby provide a phas reference for . the distortion signal, can also b illuminated. Again, light can be passed through plurality of spaced areas on the film for bot measurement of crossmodulation distortion and phas reference, to enhance frequency response. The invention also comprehends the use of a opaque mask member to provide the light contro function. The mask member is positioned in the ligh path, and has • apertures which " enable the transmissio of light between the . light source and. detector onl through the desired areas of the film sound track. Additional objects and advantages of th invention will be recognized by those skilled in th art from a consideration of the following detaile description of various preferred embodiments, togethe with the appended drawings. DESCRIPTION OF THE DRAWINGS Fig. 1 is an enlarged elevation plan view of a typical sound track with high crossmodulation distortion test signals on a portion of film strip; Fig. -2 is a further enlarged illustration of a portion of the high frequency test signal depicted in Fig. 1, showing the effects of crossmodulation distortion; Fig. 3 is an elevation view illustrating the arrangement of components in one ' embodiment of the invention; Fig. 4 is a plan view of a mask member suitable for use with the embodiment shown in Fig. 3; Fig. 5 is a plan view of the mask member shown in Fig. 4 positioned below overlying the test sound track; Fig. 6 is a schematic diagram of output circuitry for the embodiment shown in Fig. 3; Fig. 7 is an elevation view illustrating another embodiment of the invention; Figs. 8 and 9 are plan views similar to Figs. 4 and 5, respectively showing a mask member suitable for use with the embodiment illustrated in Fig. 7, ' and such mask member positioned below .a test sound track; and Fig. 10 is a schematic diagram of output " BUREΛT^ OMPI circuitry for the embodiment of the invention shown Fig. 7. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Referring to Fig. 3, one embodiment of t subject apparatus for detecting crossmodulati distortion is shown. Light from a light source 8 directed through a collimating lens 10 onto a fi strip 12 having a conventional optical sound tra with a high frequency test signal modulated by a l frequency test signal. Positioned below film strip are first and second light detectors 14 and 1 respectively, which produce electrical " outputs proportion to the amount of incident light. Detecto 14 and 16 may comprise conventional photocells. An opaque mask member 18 is positioned betwe ■film 12 and detectors 14 and 16 to control- t transmission of light from lens 10 onto the lig detectors. Mask member 18 has a series of apertur arranged in groups or cells generally indicated reference numerals 20 and 22, which apertures determi the permissible light path between light source 8 a detectors- 14 and 16. The size and position of t apertures are selected such that only light whi passes through desired target areas on the sound tra is transmitted to the light detectors, all lig outside of- the desired target areas being excluded fr the light path between source 8 and detectors 14 a 16. Support members 24 and 26 hold mask 18 in plac while amplifiers 28 and 30 are respectively connect to detectors - 14 and 16 to amplify .the signal representing the amount of light received by t detectors. O. 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 desire . target areas of the sound track reaches detectors 14 and 16. As shown in Fig'. .4, the first gr ' oup 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. Referr ing now to Fig . 4 in conj unction with Fig. 5, the apertures of mask member 18 are shown relation to a test sound track. In Fig. 5 it assumed that the light follows a generally collimat path through the film and mask member to the lig detector, and that each of these elements are align so that the aperture areas are equal to the targ areas on the film. Adjustment of the apertu dimensions would be required should the light follow converging or diverging rather than a collimated pat or if the film and mask • member were- out-of alignme with respect to. the light .source. The critical fact is that the light eventually reaching the detecto represent only that light which is transmitted throu the desired target areas of the film.. The light from lens ' 10 is not perfect collimated, but rather is somewhat fuzzy at its edge This, together with the fact that Light is transmitt along an area of the sound track which is. substantiall greater than the wavelength of. the high frequency te signal, minimizes and ideally eliminates any variatio in trans ittance due to the high frequency componen of the crossmodulation signal. Each of the first target areas on the fil included .in. the controlled light path, corresponding t apertures 20a-20d, are characterized by a dimensio along the sound track (parallel to axis line 32) whic is preferably substantially equal to a whole numbe multiple of the high frequency signal wavelength. I the transverse direction across the sound track apertures 20a-20d encompass a whole number of signa bands, a signal band being defined herein as th smallest unit across the sound track to transmit a amount of light which is constantly proportional to th 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 -12- provide a phase reference for the low frequency signal. Phase reference apertures 22a-22d coul spaced respectively by any known distance apertures 20a-20d within the limits of the sound tr However, it is preferable for convenience of ou analysis that the crossmodulation distortion si obtained through apertures 20a-20d be either dire in-phase or 180 degress out-of-phase with the p reference signal through apertures 22a-22d. The la apertures are accordingly- displaced from apert 20a-20d by a whole number multiple of one-half wavelength of the low frequency test signal. Further elements of the distortion detec apparatus are indicated in Fig. 5. A wall 36 prov ' a stop which facilitates simple insertion of the f Wall 36 is positioned so that the film can be prop aligned with respect to the optical elements merel sliding it into the distortion detecting appar until it abuts the wall. Sprockets 38 of a spro wheel extend through corresponding sprocket holes the film to move the film under the light beam. sprocket wheel can -be motor controlled, alternately the film can simply be pulled under light beam by hand, or the ' mask or even the l source moved or oscillated. In the ebodiment shown, which is designed 35 mm sound track, apertures 20a-20d are approxima equal in length to the width of the sound track, w is 0.09 inches. They are 0.02 inches wide, w compares with a wavelength of 0.045 inches for hertz low frequency test and crossmodulation resi signals. ■ • The output circuit which provides an indica ~BU 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 "BUREAU O PI V/1PO follower ' circuits are respectively connected to th negative and -positive input terminals of a differentia amplifier 68 through resistors 70 and 72. The negativ input to amplifier 68 is connected through resistor 7 in a feedback circuit to the amplifier's output, whil the positive input terminal is connected throug resistor 76 to ground. At the output of amplifier 6 is. a meter 78 which indicates the difference ' betwee the signals held by the positive and negative pea follower circuits. - Since this difference i proportional to the variation in light transmitted t photocell 14 as the test ' sound track is moved wit respect ' to the beam from light source 8 , it provides a indication of the amount of any residua crossmodulation distortion on the sound track. The output from phase reference .photocell 1 is processed through amplifier 30, and has its ' DC bia removed by capacitor 79 and resistor 80. It is the applied to a comparator 81, which determines whethe the " applied signal is positive or negative. The outpu of comparator 81 is connected to the D (data) input o a D type flip flop circuit 82. The clock input t flip-flop 82 is provided by a comparator 83 an one-rshot circuit 84. Comparator 83 has its positiv input connected to the .output of amplifier 28, and it negative input connected to the positive input o amplifier 46. When ' the output of positive pea follower amplifier 41 exceeds its input, one-sho circuit 84 delivers a pulse to the clock input o flip-flop 82, which then captures the polarity of th output of comparator 81. Flip-flop 82 is connected- t an output device such as a pair of lamps 86 and 88, on or the other of which will light to indicate whethe the crossmodulation distortion signal from photocell 1 O 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 prio 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 variation 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 he 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 f spurious signals caused by pinholes, dust, etc Accordingly, it is within the scope of the invention provide a low pass system which responds to a frequencies substantially below the high frequen level, rather than a band pass system as describ herein. Referring now to Figs. 7-10, another embodime of the invention is shown which enhances the resoluti of the crossmodulation distortion detection. A ma member 90 is shown in Fig ' . 8- which is similar to ma member 18 of Fig. 4. The same reference numerals a used to indicate corresponding portions of both mask Primary groups of large and small apertures 20 and 2 respectively, are provided in mask 90 with tbe sa dimensions and relative spacing as for mask 18. Spac on the side of aperture group 20 away from group 22 a predetermined distance, which is out-of-phase wi respect to the low frequency test signal-, is secondary group of apertures 120. The latter apertur are substantially identical to those , in prima aperture group 20. A secondary, group . of pha reference apertures 122- is provided on the opposi side of primary phase reference aperture group 22, a is displaced from group 22 by the same spacing between primary and secondary groups 20 and 12 Secondary aperture ' groups 20 and 122 are ea preferably spaced from their respective prima aperture groups 20 and 22 a whole number multiple one-half by the wavelength of the low frequency te signal. Fig. 7 is similar to Fig. 3, showing ma member 90 positioned in the testing apparatus. addition to photocells 14 and 16, additional photocells 114 and 1Ϊ6 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 correspnding 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 group 120 and 20, thereby enabling a direct compariso between the derived crossmodulation distortion signa and the phase reference signal. A different spacin could be used between the phase reference an distortion amplitude apertures, with phase compensatio apparatus added to the output stage to compensate fo the difference in phase. Output circuitry for this embodiment is show in Fig. 10, in somewhat more block diagram form tha the circuit of Fig. 6. ' Difference amplifier 13 produces a signal proportional to the differenc between the outputs of distortion amplitude photocel amplifiers 28 and 128. Its output is connected to peak follow and hold circuit 136, which is turn has it output connected to sample and hold circuit 138. comparator amplifier 140 also receives the output o difference amplifier 132, and is connected to rese peak follow and hold, circuit 136 ' to zero volts when th output of difference amplifier 132 becomes less tha zero volts. The output of peak follow and hold circuit 13 is also connected to one input of a comparator 141 which in turn has its output connected to the input t a and one-shot circuit 142. . The other input t comparator 141, is 'connected to the output o difference amplifier 132. When the output of p ' ea follow and hold circuit 136 exceeds its input, one-sho circuit 142 produces a pulse which is delivered t sample and hold circuit 138, causing it to sample th output of peak follower circuit 136. This signal leve is then delivered to an appropriate readout- device suc as meter 144. It is a feature of the invention that 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 ' BUREAU OΛIPI occur to ' those skilled in the art. For example, th implementation shown herein is ' intended for 35mm dua bilateral optical soundtracks; the dimensions an locations of the mask apertures can be adjusted fo other types of films, optical soundtracks or tes signals. Also, many variations in the number, order o spacing of the mask apertures may be made while stil employing the teachings of the invention, or a mask ca be dispensed with altogther and appropriate reflecting focusing or other means employed to direct the ligh through the desired areas on the sound track. It i accordingly intended that ' the- invention be limited onl in terms of the appended claims. _ ... O fa ,

Claims: AMENDED CLAIMS (received by the International Bureau on 30 March 1981 (30.03.81)) (amended) 1. Apparatus for detecting crossmodulation distortio on a film bearing an optical sound track having a high frequency test signal modulated by a low frequency test signal , comprising: a light source, a first light detection means " positioned to receive light transmitted from the source, means for positioning the film with its sound track i the light path between the light source and first light detec means, said positioning means enabling relative movement betw the film and the light path along, the sound track, light control means controlling the transmission of light along said light path through at least one first predet mined area on the sound track, each of said first areas being defined by a position on the sound track and having a width along the sound track equal to at least one. wavelength of the high frequency test signal whereby, during relative movement between the film and the light path along the sound track, the amount of light received by the first detection means varies substantially only in accordance with the amount of crossmodulation distortion on the sound track, and first output means for detecting variations in the amount of light transmitted through the sound track to the fi detection means during such relative movement between the fil and the light path, thereby providing an indication of the crossmodulation distortion on the sound track, (amended) 2. The crossmodu1 at on distortion detecting apparatus of claim 1, said light control means OMPI comprising . an opaqu-e mask member having an aperture therein enabling transmission ■ of light through said first area on -the so.und track to 'the first light detection means. 3,- The - crossmodulation distortion detecting apparatus of. . claim 1, said light .control means controlling .the transmission of .light ' along said light path through .a plurality of first p edetermined discrete ' areas on the sound ' trac ,- each of said first 'areas being 'defined by such position and -dimensions, . .. said first areas being mutually spaced along the sound track at substantially whole number multiples of the wavelength of the ' low frequency test signal. H _ The crossmodulation distortion detecting apparatus of claim 1, further comprising; a second light detection means positioned to receive light transmitted through the film .'from the source, said light control ' means further controlling transmission of light through a second predetermined area on the sound track, said second area being defined by a position on the. high frequency signal portion of the sound track and dimensions such that, during relative movement between the film and the light 'path along the sound track-, the_ amount of light received by the second detection means varies periodically at th frequency of the low frequency test signal and in known .phase relationship therewith, said: second area being displaced a predetermined distance along the • sou_n- - track ' from the first area and . ^ ^ second output means providing an indication ' ' periodic variations in the ' amount of light received the second detecting 'means, and thereby providing phase reference for the crossmodulation distort detected by the first detecting means, to enable relative exposure of the film to be determined. 5. Apparatus for. detecting crosεmodulati distortion on a film bear.ing an optical sound tra having a high frequency test signal modulated by a l frequency test signal, comprising: a light source, primary and ' secondary light detection mea positioned to receive light transmitted from t source, means for positioning the film with its sou track in the light path between the light source a said primary and secondary light detection means, sa positioning means enabling relative movement betwe the film and the light path alon ' g the sound track, light . control means controlling t transmission of light along primary and secondary lig paths respectively to said primary ' and secondary lig detection means through respective predetermmin primary and secondary areas on the sound track, sa primary area being defined by a position on the sou track and dimensions such that, during relati movement between the film and the primary light pa along the sound track, the amount of light received the primary light detection means varies substantial only with the amount of crossmodulation distortion the sound track, U o: said secondary area being displaced along the sound track from the primary area ' a. predetermined distance which is out-of-phase with respect to the wavelength of the low fraquency test signal, and being defined by dimensions such that, during said relative movement between the- film " and the ' secondary light path, the amount of light received by the secondary light detection means varies substantially only with the amount of crossmodulation distortion on the sound track, but out of phase with the light received by the primary light detection means, and 'output means for comparing the light received by said primary and secondary light detectors during such relative movement between the film and the light paths, thereby providing an indication of the crossmodulation distortion of the sound track. ζ, _ ■ The crossmodulation distortion detecting apparatus of claim 1 , further comprising: primary and ' secondary phase reference light detection means positioned to receive light transmitted through the film through the source, said light ' control means further controlling the transmission of light through predetermined primary and secondary phase reference areas on the sound track, said primary ' and secondary phase reference areas each being defined by positions on the high frequency signal portion of the sound track and dimensions such that, during relative movement between the film and the light path along the sound . track, the amounts of light received by the primary and secondary phase reference light detection means vary periodically at the frequency of the low frequency test signal and in known phase relationship there¬ with, said primary and secondary phase reference areas being displaced by equal predetermined dis.tances along the sound track from said primary and secondary areas, respectively, and phase reference output means providing an indication of periodic variations in the amount of light received by said primary and secondary phase reference light detecting means, and thereby providing a phase reference for the crossmodulation distortion detected by the primary and secondar light detecting means, to enable the relative exposure of the film to be determined. (amended) 7 - T e method of detecting crossmodulation distortion on a film bearing an optical sound track having a high frequency test signal modulated by a low frequency test signal , comprising forming a light beam, transmitting said light beam through the sound track, controlling the transmission of the light beam so that it passes through at least one predetermined area on the sound track, receiving light transmitted through said predetermined areas on the sound track, providing relative movement between the film and the light path along the sound track, and detecting variations in the amount of received light during relative movement between the film and light path, OMP said first predetermined areas having a width along the sound track equal to at least one wavelength of the high frequency test signal , whereby the detected light variations during relative movement between the film and light path provide an indication of the amount of crossmodulation distortion on the sound track. 8, The method of claim 7, further comprising the steps of transmitting light through a second predetermined area on the sound track, which area exhibits a periodic variation in trans- mittance at the frequency of the low frequency test signal , and of receiving light transmitted through second area to proyide a phase reference for the crossmodulation distortion information transmitted through the first area. (amended) 9 « Tne method of detecting crossmodulation distortion on a film bearing an optical sound track having a high frequency test signal modulated by a low frequency test signal , comprising; forming a light beam, transmitting said light beam through the sound track, controlling the transmission of the light beam so that it passes through at least one predetermined primary area on the sound track, said predetermined areas being of sufficient dimensions to respond substantially only to the crossmodulation distortion signal during relative movement between the film and 1 ight path, receiving the light transmitted through said primary areas on the sound track, transmitting a beam of light and controlling the beam so that it passes through at least one secondary predetermined area on the sound track, each of said secondary areas being displaced along the sound track from respective primary areas by a predetermined distance which is out-of-phase with respect to the wavelength of the low frequency test signal , and being defined by dimensions such that, during relative movement between the film and the light beam, the amount of light transmitted through said secondary areas varies substantially only with the amount of crossmodulation distortion on the sound track, but out-of- phase with the light transmitted through the first areas, receiving the light transmitted through the secondary areas on the sound track, providing relative movement between the film and the light path along the sound track, and comparing the light received from the primary and secondary areas on the sound track to provide an indication of the crossmodulation distortion. OMP I?

Inventors: BEARD T
Applicants: BEARD T
Application year: 1980
Main classification: G11B7
IPC codes: G11B7, G11B27