FR2715790A1 - Electronic modulator drive for high definition video laser projector - Google Patents

Abstract

The modulator drive has a source (1) which inputs video signals sequentially to a memory (2) driving eight processing circuits (3-1 to 3-8). The digital output from each circuit is converted to an analogue response (4-1 to 4-8) and fed to a high frequency modulator (5-1 to 5-8) which drives an electrode (6-1 to 6-8) of an electro-acoustic modulator (7).This modulates an incoming laser source (9) providing laser signals to a beam divider (10). One portion of the beam is scanned (11) and is available for display. The other is analysed by a photodetector (12) passing feedback to the computer (13) which controls the processing circuits.

Description

r HF enis par; .e iJ QYLOY-O 1in -. . -

  33 145225448 [. . The invention relates to an electronic device for controlling a modulator with one or more channels, such as an acousto-optical modulator, in particular for high definition laser video projector.

  A laser video projector is a device which makes it possible to project a video image on a large surface screen, for example of several natures or tens of square meters. Such a device calls for scanning the screen by one or more laser beams, the amplitude of which is modulated by the video signal: a black and white image requires a single laser beam, while a color image requires at least three laser beams in red, green and blue colors respectively - In the case of a high definition image, the number of pixels which must be scanned atx per second is very high.

  A high definition television standard (HDTV) with 1250 lines, 50 Hz and interlaced frames (European standard) therefore requires, in 16/9 format (2304 pixels or dots per line), a frequency of: 1250 x 2304 x 25 - 72 Megapixels / second.

  In view of the problems posed by operating at such a high frequency in the electronic circuits of the video projector, it has been proposed to process in parallel and simultaneously project j picture lines, which makes it possible to divide by n the line frequency and pixel frequency.

  Patent application FR-A-2 577 371 describes a solution of this type, in which a multi-channel acousto-optical modulator makes it possible to ?. simultaneously modulate laser beams parallel to the mcyen of n video signals corresponding to n image lines.

  Such a multi-channel acousto-optic modulator is a crystal carrying electrodes each permitting locally to induce an accuctic wave as a function of the electrical signal applied to it and thus to locally vary the diffraction characteristics of the crystal. A modulator of this type uses the non-linear properties of the Printed from MIMOSA 1, / 19/2000 12:31:12 page -2- FAX issued by: 33 145ZZ6448 SIE à I l-'oo L. rr 33 145225448 material so that the individual electro-optical response of each channel is not linear.

  A first object of the invention is to provide an electronic device for controlling a modulator with one or more channels which makes it possible to compensate for the intrinsic non-linearity of the response of each channel.

  To this end, the object of the invention is to provide an electronic device for controlling an intrinsically non-linear modulator comprising at least one channel adapted to modulate an incident light ray as a function of an input signal and to produce a diffracted ray of light intensity as a function of said input signal, characterized in that said channel of said modulator is associated with a processing circuit comprising a gain and output adjustment circuit for adjusting said gain so that with an input signal maximum amplitude (corresponds to a diffracted light ray of maximum light intensity, and a linearization circuit for linearising the law of variation of the light intensity;! of said diffracted light ray as a function of the amplitude of said input signal.

  Furthermore, for an image to be suitably restored using a multi-channel acousto-optical modulator of the type. above, it is important to ensure uniformity of light intensities from one line to another.

  Thus, a uniformity of intensities of 1% on the image requires an ejuilibraga of 1% of the yields of the n modulation channels, and this for each color in the case of a color image, whatever the uniform level desired.

  This is more important than an imbalance of one channel compared to the others. Causes a periodic defect every line on the screen in the case of a uniform tint which, in the preferred case of modulation & eight channels, corresponding to a spatial frequency at which the net eye particularly uensible.

  It turns out, in the current state of the art explained above, that this objective of uniformity of intensities Printed from MIMOSA 12/19/2000 12:31] .2 page -3- FAX sent by: 33 145ZZb448 SIH * a LL J A.. . . 33145225i448 271t5790 light from one line to another is not reached and that, more generally, this problem of balancing the yields of n modulation channels arises for any multichannel modulator with non-linear response.

  Another object of the invention is precisely to solve this problem.

  To this end, the object of the invention is an electronic piloting device of an intrinsically non-linear modulator with several channels each adapted to modulate an incident light ray in function of an input signal and to produce a diffracted ray of light intensity as a function of said input signal, characterized in that each channel of said modulator is: associated with a processing circuit comprising: - a first gain adjustment circuit for adjusting said gain so that at a signal input of maximum amplitude corresponds to a diffracted light ray of maximum light intensity, a linearization circuit for linearizing the law of variation of the light intensity of said diffracted light ray as a function of the amplitude of said input signal, a second input gain control circuit to make the maximum light intensities of the diffused light rays substantially equal across all of said channels ctés in response to the application to all the circuits for processing an input signal of the same amplitude linearized by said linearization circuit and weighted by said first gain adjustment circuit at output. Preferably, said modulator is of the acousto-optical type and said light beam is a laser beam.

  The invention also relates to a lacing video projector comprising in the hands an electronic control device as defined above.

  Other characteristics and advantages of the invention will emerge from the description which will follow from a given embodiment unique by way of example and illustrated by the appended drawings in which: Printod from MIMOSA 1; / 19/2 ) O0O 12:31:12 page -4- FAX sent by: 33 14bZZbqqUl * I '33 145225448 - Figure 1 is a simplified block diagram of a laser video projector comprising an electronic device according to the invention for: control an eight-channel acousto-optical modulator; - Figure 2 is a block diagram of the processing circuit associated with each channel. the acousto-optical modulator; FIG. 3 is a graph illustrating, in the absence of correction, the response curve I f (V) of any channel of the acousto-optical modulator, namely the light intensity liffracted by said channel as a function of the video signal voltage applied to it; - Figure 4 is a graph illustrating the curve I wf (V) after adaptation of the output gain of the processing circuit of Figure 2 so that a maximum video signal voltage corresponds to a maximum light intensity diffracted by the route considered; - Figure 5 is a graph illustrating the response curve I = f (V) after linearization; - Figure 6 is a graph showing, by way of example, the maximum light intensity diffracted by each of the eight channels; of an acousto-optic modulator in response to the application:% of the same maximum video voltage at each of these eight votes: and - Figure 7 is a graph similar to that of Figure 6 showing the in: ensity maximum light diffracted by the eight channels after balancing them.

  Referring to FIG. 1, a source 1 of digital video signals applies digital video signals in line-to-line scanning mode to a memory 2 as well as the necessary synchronization signals., The memory 2 is adapted to sequentially store one or more groups of video signals corresponding to eight lines of the video image.

  Ligneo video signals applied sequentially by source 1 to mAmoirA 2 are reproduced eight by eight by the latter and applied respectively to eight circuits of Printed from MIMOSA 12/19/2000 12:31:12 page -5- FAX transmitted by : 33 Z14bzbqqu IJ *. 33 l4522S448 treatment 3 ", 3 ...... 38 respectively.

  Each of the processing circuits 3 ...... 3, is connected via a digital analog converter 4, ...... 4 and a high-S frequency modulation circuit 5 ... ... 5 to an electrode 6 ...... 6m of an electro-acoustic modulator 7. For clarity of the drawing, only the converters 4X and 49 and the modulation circuits 5, and 5, corresponding to the circuits of treatment 31 and 3e respectively: have been represented in FIG. 1.

  In each heute-frequency modulation circuit 5. 5., the analog signal coming from the corresponding converter 4, ..... 4, modulates a high-frequency signal coming from a pilot oscillator 8 common to the eight high modulation circuits. frequency 5 ...... 5 ,. The high-frequency signal thus modulated is amplified by each high-frequency modulation circuit 5 ...... 5, before its application to the corresponding electrode 6 ...... 6 of the acousto-optical modulator 7.

  Eight laser rays applied by a laser source 9 to the acoustooptical modulator 7 are diffracted by the latter and the light intensity of each diffracted royon is a function of the voltage represented by the digital value of the video signal applied line, the corresponding channel i. The eight light rays diffracted by the modulator 7 are applied, by means of a separating blade 10, on the one hand to a scanning device 11 ensuring the formation of an image in a pack of eight lines, and on the other part to a photosensitive detector 12 such as a photodiode making it possible to measure the light intensity of each of the eight rays diffracted by the acousto-optical modulator 7.

  The photosensitive detector 12 is connected to a microcomputer 13 which controls the processing circuits 3, ..... 3a.

  Referring also to FIG. 2, each processing circuit 3i (i - 1, 2 ....... 8) comprises a digital circuit 14 for adjusting the input gain Kl, a circuit 15 for linemizing the light intensity curve I Printed from MIMOSA 12/19/2000 12:31:12 pqcge -6- FAX enls by: .J1 1bLLbéu9 "ln 33 145225448 dlffractée by the caral i of the modulator 7 according to the voltage V of the line video signal applied by memory 2, to channel 1, and a digital circuit 16 for adjusting output gain K. The circuit 15 is a LUT table in which are stored digital values intended for apply a transformation of the form to the input signal: Z Arc sin / V (1) The circuits 14 and 16 are, for example, multipliers to which the microcomputer 13 applies the coefficients KL and K2 in digital form. , in the form of a contactor connected in parallel with the linearization circuit 15, u n means of inhibltion of this table by 1f micro-computer 13 as will be described below.

  The process of adjusting the response I e f (V) for each of the channels i of the acoustic modulation device will now be described with reference also to FIGS. 3 to 7.

  The first step in this process is to index the individual responses of each channel 1 of acousto-optical modulation. To do this, we proceed as follows; - by means of the microcomputer 13, we adjust the input gain K1 to the value 1, so as to use all the dynamics of the table linearization 15: - we inhibit the linearization cable 15, function symbolized on the fiqure 2 by closing the contactor 17, in order to apply tol what to circuit 16 the digital video signal from the same memory 2; - A digital input video signal is applied to the nircult 14 in the form of a ramp between O and the maximum value that this signal can take, namely 255 in the cRs where the video signal is coded on eight bits; this gives a response curve I = Io sin '(-V LJ dens which the value V0 corresponding to the maximum output Printed from MIMOSA 12/19/2000 12:31:12 page -7- FAX sent by: 33 14qzzbq4q 1 33 l45225448 diffraction depends on a18 flies considered as shown in Figure 3; - one adjusts by means of the microcomputer 13 the output gain K. so that the maximum value of the digital video signal ent: -e (V - 255) corresponds to the maximum light intensity ID of the ray dliffracted by the modulator 7 and detected by the phctodetector 12: the response curve IF (V) is then normalized and corresponds to the formula: 1 - I $ 1n2 <), in which the value Vo is equal to the full dynamic range of the video signal, that is to say A Z55 in the case under consideration of a signal coded on eight bits; - the linearization formula (1) is applied to the input video signal applied to circuit 14, by passing it

by table LUT 15.

  We repeat the operations defined above for each of the flights 1 to 8 including the response curve! - f (V) is thus linêar.Lsée, with a different maximum I0 for each flight i of modulation.

  We then proceed to a uniformity adjustment by applying to the eight processing circuits 3 ...... 3a a digital video signal of maximum amplitude (V - 255) with the output gains K2 set as defined above and via the linearization tables 15.

  By means of the photocell 12, the light intensities I diffracted by the eight channels of the modulator 7 are measured, which gives a distribution of the intensities I of the type shown in FIG. 6.

  We then set the :; input gains K1 of the eight circuits 14 to equalize the light irradiance I on all of the channels based on the channel with the weakest light intensity as shown in FIG. 7.

  Measurements carried out on several types of acoustoopl modulators: iquAs 7 show that the adjustment process described above makes it possible to obtain uniformity of illumination at 1% of the maximum level, whatever the uniform level applied to the eight channels.

  The adjustment process described above can be set Printed from MIMOSA 12/19/2000 12:31:12 paqe -8- FAX issued by: Ji lqLzbqqu zn. - 33 145223448 implemented automatically by the microcomputer 13 without disturbing the projectiton by using the dead time at the start of frame scanning to send test signals on the modulators, to an ins': ant where the laser beams are located outside of the projection field and are masked by a mechanical cover at the level of the scanning device 11.

  As a variant, this adjustment process can be implemented manually e: viewing on an oscilloscope (not shown in the drawing) the signal from the photodetector 12, which is then connected to the oscilloscope instead of being at the microphone -computer 13, The adjustment process by means of the control device described above makes it possible to remedy the main drawbacks encountered with multi-channel acousto-optical modulators J namely: - non-linearity of the individual electro-optical response of each acousto-optic modulator channel: unexpectedly, it appeared that this linearity defect could be corrected so as to respect a uniformity = 20 of illumination at 1% on all channels by simple application to the input signal V of the transformation 2 Arc sin JV obtained from the theoretical response of an acousto-optic modulator, without it being necessary to measure the non-l curve on a case-by-case basis inequality of each of the acousto-optical modulator channels; - existence of significant differences, of the order of 20% or more, between the light intensities of the incident laser beams applied by the source 9 to the acousto-optical modulators; - diffraction efficiencies of acoustooptical modulator cases which can frequently vary by 5% between channels.

  As described above, the adjustment of the K1 input and K output gains allows these two types of dispersal to be easily remedied.

  In addition, the; intermodulations between channels, of the order of 5%, result in equivalent static gain differences. the fact that we use a Printed from MIMOSA 12/19/2000 12:31:12 page -9- FAX sent by: Ji 1j5q: Gqq n * n 33 145225448 oscillator 8 common to the eight channels, which avoids the beats due to the phase fluctuations of the intermodulations that would be encountered if independent clocks or oscillators were used. These differences S of gains being purely mtatic, they feel compensated by the adjustment of the output gain K2.

  In the case where it is a question of processing images in color, it will be done: call to three acousto-optical modulators receiving respectively red, green and blue laser rays. With each modulator is associated, for the processing of the digital video line video signal of the corresponding color, the chain of FIG. 1 comprising the memory 2, the processing circuits 3e ...... 3, the digital / analog converters 41 ..... 4, the high-frequency modulation circuits 5. ..... 5 and the pilot oscillator. 3. By way of example, dichroXque mirrors and blades conventionally make it possible to recombine the three bundles of a red, green and blue beams diffracted by lei; three acousto-optical modulators, and applying them to the photodetector 12.

  The device for controlling an acousto-optical modulator described below: u thus makes it possible, in the case of a high definition television picture 8 1250 lines, 2304 pixels per line and 25 images per second, to carry out the processing video signals at a frequency of 9 MHz in Printed from MIMOSA 12/19/2000 12:31:12 pag. , e -10- Printped from MIMOSA 12/19/20110 12: 31: -12 page -10- FAX sent by: 1. blqlbttb In.

  33 145225448 suitable scanning device.

  More generally, it can be applied to any system using a laser to draw: generation of masks or reticiles in microelectronics, printed circuits, flat screens: etc. The invention is applicable for generation three-dimensional drawings, for example in stereolithography.

  It goes without saying that the embodiment described is only an example and it could be modified, in particular by substitution of technical equivalents, without going beyond the scope of the invention.

  Thus, for example, the electronic control device according to the invention is applicable to any multi-channel modulator using the non-linear properties of the modulator material to modulate on each channel an incident light ray as a function of an input signal, for example a multi-channel electro-optical modulator.

  As a variant, the digital circuits of the piloting device can be:. = Wholly or partly replaced by analog circuits.

  The photodiode 12 for measurement and calibration can be replaced by a bariette of photodiodes allowing the simultaneous measurement of the light intensities t diffracted on the n channels of the modulator.

  Printed from MIMOSA 12/19/2000 12:31:12 p; qe -11- FAX issued by: 33 lqbZbq4U -IK.

  33 145225448 REN..V..CATIQ.S 1. Electronic control device for an intrinsically non-linear modulator comprising at least one channel suitable for modulating an incident light beam as a function of an input signal and producing a diffracted ray of light intensity as a function of said input signal, characterized in that each channel of said modulator is associated with a processing circuit comprising: - a circuit (1 () for adjusting the gain at the output for adjusting said gain (] z) in such a way that an input signal of maximum amplitude (V) corresponds to a diffracted light ray of maximum light intensity (I), and - a Linearisation circuit (15) for linearizing the law of variation of the light intensity (I) of said diffracted light ray as a function of the amplitude (V) of said input sign.

  2. Electronic device for controlling an intrinsically non-linear modulator With several channels each adapted to modulate ur, incident light ray as a function of an input signal and produce a diffracted ray of light intensity as a function of said input signal, characterized in that each channel of said modulator is associated with a processing circuit comprising: - a first gain adjustment circuit (16) for adjusting said gain (K2) so that at an input signal d maximum amplitude (V) corresponds to a diffracted light ray of maximum light intensity (I), and a linearization circuit (15) for linearizing the law of variation of the light intensity (I} of said diffracted light ray as a function of the amplitude (V) of said input signal, and - a second circu..t (14) dc gain adjustment at input (K,) to make the maximum light intensities of the light rays substantially equal across all of said channels diffracted, ert response to the application to all processing circuits of an input signal of the same amplitude linearized by said linearization circuit (15) Printed from MIMOSA 12/19/2000 12:31:12 page -12- FAX listed by: 33 145ZZ6448 SIR e Lt oR 33 145225448 and weighted by said [remier gain control circuit (16) at output.

  3. Device itself: .on claim 2, characterized in that it comprises, between said treatment circuit (31 ..... 3a) and the corresponding channel of said modulator (7), a circuit (5 ... ... 5,) of modulation of a high-frequency signal by the output signal of said control circuit.

  4. Device according to claim 3, characterized in that it comprises a single pilot oscillator (8) applying said high-frequency signal to each of the moduleti.n circuits (5 ....... 5i) associated with the channels respective of said modulator (7).

  5. Device according to any one of claims 2 to 4, characterized in that said gain adjustment circuits (14, 16) and ledi: linearization circuit (15) are digital circuits and said input signal is a digital signol.

  6. Device according to claim 5, characterized in that said processing circuit (3 ...... 3.) Comprises, at output, a digital analog converter (4 ...... 4d).

  7. Device according to any one of claims and 6, characterized in that it comprises a photosensitive detector (12) for measuring said light intensity (1) of the light beam diffracted by each channel of said modulator (7) and a computer ( 13) connected between said detector (12) and said processing circuits (3 ...... 3) to generate the numerical values of said gains (K ", K2) as a function of the light intensities measured by said detector (12).

  8. Device according to claim 7, characterized in that said linearization circuit (15) comprises a table in which are stored predetermined correction coefficients.

  9. Device according to any one of claims 2 to 8, characterized in that said incident light beam is a laser beam.

  Printed from MIMOSA 12/19/2000 12:31:12 page -13- FAX e & set by: 33 lqbZZbqq S 1. - - 33 145225448 10. Device according to any one of claims 2 to 9, characterized in that each signal input signal applied to a processing circuit (3 ...... 3e) is a linked video signal.

  11. Device according to any one of claims 1 & 10, characterized in that said modulator (7) is an acoustooptic modulator.

  12. The stlon device of claim 11, characterized in that said linearization circuit (15) is adapted to apply to the amplitude input signal (V) received & its input a transformetion of the form: V 'I Arc sin V , where V 'is the amplifier, ude of the output signal of said linearization circuit.

  13. Device according to claim 10 for forming a color image from eu at least three laser sources of different colors, characterized in that it comprises at least three acoustooptical modulators _ = o20 (7) which are, pplicated the laser rays respectively from the three said sources (9), and a processing circuit (3x ,. ... 3. associated with each channel of each acousto-opt modulator: .que, 14. Lsier video projector, characterized in that it comprises at least one electronic device according to any one of claims 2 to 13, for controlling at least one mu] .ti-channel modulator.

  15. Device for writing and / or printing on a support, characterized in that it comprises at least one electronic device according to any one of claims 2 to 1; for controlling at least one multi-channel modulator.

  Printed from MIMOSA 12/19/2000 12:31:12 page -14-