CA1101538A - Facsimile transmitter having improved response - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A facsimile transmitter detects dark-light variations in an illuminated document by means of a photodetector. Fiber optics form an optical pathe from the illuminating means to the document and from the document to the photodetector means with filter means being located in the optical path for attenuating light in the near infrared region of the spectrum. Amplifier means are coupled to the photodetector means and automatic gain control means are associated with the amplifier means for increasing the gain of the amplifier in response to a lesser amount of light detected by the photodetector. The gain control means is adjustable so as to limit the increase in gain when detecting dark regions. Lead networks are coupled to the amplifier means to increase the speed of response to dark-light variations in the document.

Description

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l Back~round of the Invent~on - ~ --

2 This invention relates to facsimile ~ransmitters

3 of the type utllized in facsimile systems including a trans~

4 mitter9 a receiver and a co~nunications network therebetween~
More partlcularly, this invention relates to faesimile trans-6 mitters wherein a doeument is scanned so as to generate 7 eleetrieal information-bearing signals representing the dark-8 light variations in the document. These ~nformation-bearing 9 signals may then be transmitted over the co~munieations net-work to a aesimile reeeiYer where the information-bearing 11 signals ~re eonverted to marks or images on a copy medium l2 so as to form a copy which is a reasonable facsimile of the 13 original document.
14 Tn eommercially avail~ble prior art ~aesimile transmitters manu~actured by th~ assignee of this invention, 16 the amplifier portion of the transmitter is provided w~th an 17 automatie gain eontrol so as to permit an inerease in gain 18 of the amplifier when a diminished amount of light is de-19 tected hy the photodeteeting means of the transmitter. In -2~ other words, the automatie gain control of the amplifier 21 means permits a eorreetion in the gain o~ the amplifier when~
22 due to a malfunction such as reduced illumination of the 23 doeument or decreased sensitivi~y of the photodeteetor, the 24 signal generated by the photode~ector appears as a signal ~5 represent~ng the detection of a darker region on the docu-26 ment. This is of course desirable in that any malfunetion 27 in the transmitter is eorreeted even though that malfunction 28 may be of a relatively minor nature, e.g., the illuminating 29 means in eommunicatlon with the documen~ at the transmitter ~eeomes covered with dust or other debris. However, auto-31 matie gain control o~ this type is undesirabie in that the 32 detection of an actual dark region on the document can - ?. -.,.~

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result in the adjustment of gain for the amplifier which will rnake the actual dark region appear as a white region.
This phenomenon may be better understood with reference to U~ S. Patent ~o. 3,911,207, which is assigned to the assignee of this inventi-on.
As shown in the aforesaid patent, the automatic gain control comprises a feedback path including a fixed resistance in parallel with an electronicallly controlled resistance comprising an FET ~field effect transistor). -The FET resistance is controlled or adjusted in response to the charge on the capacitor which is coupled to the output of the amplifier through a transistor. When the transistor is conductive as during detection of a white signal, an in-creased positive charge on the capacitor renders the FE~
conductive so as to reduce the feedback resistance thereby minimizing the gain of the amplifier. However, when the transistor is nonconductive, the capacitance slowly dis-charges and the FET is turned off so as to increase the resistance in the feedback path of the amplifier means, thereby increaslng the gain. When a dark re~ion of extended width or length is detected, the gain of the amplifier is permitted to rise due to the resistance of the fixed resis-tor in the feedback path to a level approaching white there-by failing to provide a signal which is capable of creating a facsimile of the dark region of extended length or dur-ation.
In commercially available facsimile transmitters manufactured by~the assignee of this invention and compris- ;~
ing fiber optics in the optical path between the document and the photodetector and the document and the source of -~
illumination, filters are utilized to improve the response of the transmitter to dark-light variations on the document.

However, the filters in such transmitters have not been effective to compensate the spectral response so that all areas which look dark to the eye appear darlc to a photocell since the characteristic of the photocell differs substan-tially from the eye itself. More particularly, the filters of such transmitters have not been extremely effective at filtering out visible red light which is characteristic red ball-point markings. As a consequence, a relatively dark appearing line or mark on a document which is written in red ball-point pen will reflect a sufficient amount of visible red light so that the photodetector does not detect the presence of a dark region.
Commercially available prior art facsimile trans-mitters are also somewhat deficient in their speed of re-sponse to a dark-light variation in a document. More particularly, an abrupt change from a dark to a light region may not be accurately reflec~ed in the signals which are generated and transmitted by the facsimile transmitter due to the inability of the cir~uitry to respond to such an abrupt change.
Summary of the Invention It is an object of the invention to provide a facsimile transmitter having a spectral response which is compensated such that regions appearing dark to the eye on the document are detected as dark regions by the photodetector.
In accordance with a particular embodiment of the invention, a facsimile transmitter for use producing a copy at one location which is a facsimile of a document located at another location comprises: means for illuminating the document, photodetector means in optical communication with the document for detecting dark-light variations in the i38 i.l.luoinated document, sa.id photodetector means generating photodetec-tor signals representing said dark-light variations;
means for scanning said photodetector means relative to said document, amplifier means coupled to said photodetector means for amplifying said photodetector signals; modulator means coupled to said amplifier means for generating modulated transmission signals representing said dark-light variations :~
on said document, fiber optic means in the path of optical transmission from said illuminating means to said document and from said document to said photodetector means; and means for substantially limiting light in the visible red region of the spectrum from reaching said photodetector means so as to permit detection of a relatively dark appearing region even thoùgh said dark appearing region is capable of reflecting a substantial amount of light in said visible red region.

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Brlef DescriPtion of the Drawinqs .
Fig. 1 is a block diagram of a facsimile system ~.
depicting a preferred embodiment of the invention, Fig. 2 is a schernatic circuit diagram of the trans- .
mitter depicted in Fig. 1, l53~1 1 Fig. 3 is a sectional view of a fiber optic-photo-2 detector combination in optical communication with a docu~
3 ment on the facsimile transmitter drum shown in Fig. l;
4 Fig. 4 is an enlarged sectional view of the hous- `
ing shown in Fig. 3 for a filter and photodetector;
6 Fig. 5 is a view taken along line 5-5 of Fig. 4;
7 Fig. 6 is a waveform diagram utilized in explain-8 ing the impro~ements afforded by the circuit o Fig. 2;
9 Fig. 7 is a ~ilter characteristic for a gelatin il~er utilized in a preferred embodiment of the invention;
ll and 12 Fig. 8 is a ilter characteristic for a glass-13 gelatin filter combination utilized in a preferred embodi-14 ment of the invention. `~
Detailed Description o~ a Preferred Embodiment 16 Re~erring now to the f.acsimile system shown in 17 Fig. 1, a facsimile transmit~er comprises a drum 12T rota-18 ted by a motor lOT so as to create a relative scanning move-19 ment between a document carried by the drum 12T and a scan~
ning head not shown. As the scanning head is advanced ~1 axially along the drum 12T and the drum rotates about its 22 axis, successive paths on ~he doeument are illuminated and 23 variations in light intensity due to the reflec~ivlty and 24 transmissivity of the document are scanned by a photodetec~
tor 14.
26 In accordance with one very important aspect of 27 the invention, filter means are\associated with the photo-28 detector 14 for filtering out light in the visible red 29 region of the spectrum. As a consequence, the photodetector 14 i6 capable of converting variations in l~ght intensity 31 which are a function o~ the reflec~ivi~y of the scanned 32 document into electrical signais represent-lng light and ~.

dark regions on -the document regardless of the capability of the particular r~gion to reflect light in the visible red region. By filtering out light in the visible red re-gion it is possible to detect dark-light variations in the document representing ball-point pen markings, e.g., red ball-point or other markings which appear dark to the eye but provide a high degree of red reflectivity in the pre-sence of white light.
The electrical signals generated by the photo-detector 14 are then amplified at a preamplifier 16. In -, accordance with another important aspect of the invention, the preamplifier 16 includes an automatic gain control with adjustable means so as to limit the increase in gain when detecting dark regions thereby assuring a substantial and continuing difference between signals representing light regions on the document and signals representing dark regions on the document~ In addition, and in accordance with another important aspect of the invention, the ampli-fier 16 includes frequency compensating or differentiating networks so as to increase the speed of response of the transmitter to dark-light (or light-dark) variations on the document.
The electrical signals which are amplified by the ~ ~-preampli~ier 16 are utilized to control a ~C0 (voltage con-trolled oscillator) 18 and thereby generate frequency modu-lated signals representing the information content of the document carried by the drum 12T. The frequency modulated signals are then conditioned by a network 20 before being applied to an acoustical coupler 22 which is associated with a conventional telephone handset 24. A conditioning network 20 in the form of a driver is shown in U. S. Patent 3,850,459.

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The frequency modulated signals are then transmit-ted by suitable means such as conventional telephone lines 25 to a facsimile receiver which is coupled to another con-ventional telephone handset 26 and an associated acoustical coupler 28~
The transmitted FM signal received at the receiver is amplified by a preamplifier 30 and applied to a differen-tiating circuit 32 to generate trigger signals which are ;
applied to an FM demodulator comprising a single-shot multi-vibrator 34 as disclosed in the aforesaid U. S. Patent No.
3,911,207. A detector circuit 36 determines the average DC
value o~ the single-shot output and a writing control signal which is generated at the output of the detector 36 is then applied to a stylus driver. The output from the driver 38 is applied to a stylus 40 associated with a movable head juxtaposed to a copy medium carried by a drum 12R. The relative scanning movemenk betwee~ the copy medium and the ;
head is achieved by rotating the drum 12R by means of a motor lOR.
The output of the detector 36 is also applied to ~ ~-a scanning control circuit ~2. The scanning control circuit 42 utilizes the output from the detector 36 to initiate scanning of the copy medium by applying a signal to a scan-ning drive 44 in response to an FM signal from the transmit-ter. The output of the scanning drive 44 is applied to the motor lOR. Details concerning the facsimile receiver are set forth in U. S~ Patent No. 4,112,468, issued September 5, 1978.

The improved preamplifier 16 of the facsimile trans-mitter shown in Fig. 1 will now be described in detail with reference to the circuit diagram of Fig. 2. The output from .

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1 the photodetector 14 is applied to an input terminal o~ an 2 operational amplifier 114 having an output connected to an 3 input terminal of a succeeding operational amplifier 116 4 through a resistor 118. The output of the operational am-

5 plifier 116 ls then applied to the VCO 18 through a field

6 effect transistor 120 coupled to a suitable blanking pu1se

7 generator synchronized with the rotation o the drum so as

8 to blank out the effect of a docum2nt clamp o~ the drum 12T.

9 In order ~o permit the pre~mplifier to ~d~ust for a malfunction such as reduced Lllumination of the document 11 or decreased sensitivity of the photodetector, automatic 12 gain control is provided. As shown in Fig. 2, the automatic 13 gain control includes a feedback path having a resistor 122 14 connected to the base o~ a transistor 124 which is appropri-ately biased by the +8 volt powcr supply so as to be con-16 duct~ve when the output of the operational amplifier lL6 is 17 driven positive in response to the detection of a white or 18 light region on the document. The base of the transistor 19 124 ls connected to ground through a diode 138.
The feedhack path of the AGC includes a capacitor 21 125 connected to the emitter of the transistor 124 so as ~o 22 be charged to a positive value when the transistor 124 is 23 conductive when a white region is detected. In the absence 24 of a white region, the positive charge on the capacitor is 2S reduced ~y the -8 volt power supply which is connected to 26 the capacitor through a resistor 129 and a poten~iometer 27 130 in series with resistors 1~32 and 134. A power supply 28 fllter capacitor 136 connects the junction of the resistors 29 132 and 134 to ground. The charge on the capacitor 125 is utilized to control conduction ~hrough a field effect tran-31 sistor 126 connected in parallel with a potentiometer 128 32 in the feedback path of the operational ampli~ier 114.

- 10 -J ~538 1 In accordance with this invention, the potentio-2 meter 128 is appropriately set so as to assure ~hat the 3 detection of a dark region of extended length or duration 4 will not result in increase of gain to the point that a dark region, after an extended length of time, will appear 6 as a light region, at least as far as the transmission sig-7 nal is concerned. In this connection, it will be apprecia-8 ted that the gain o the operational ampliier 114 is in-9 creased when a dark region is detected so that the positive charge on the capacitor 125 may be sufficientiy reduced to

11 turn off the ~ransistor 126 an~d permit ~he potentiometer . . ~

12 128 to represent a rather substantial resistive feedback

13 path for the operational amplifier 114. On the other hand,

14 when a white or ~ight region is detected, the charge on the capacitor 125 increases to the point of tunling the field 16 effect transistor 126 on, thereby substantially reducing the 17 total feedback resistance so as to reduce the gain of the 18 operational amplifier 114. In accordance with this inven~-19 tion, the poten~iometer 128 is ,et at a point so that the maximum gain of the operational amplifier is suficiently 21 limited to produ~e a transmission signal which at least 22 represents a gray region if no~ a dark region when a dark 23 region of ex~ended duration is detected.
24 In accordance with another important aspect of the invention, the preampl1fier 16 includes frequency compensat-26 ing or differentiating networks so as to increase the speed 27 of response of the transmitter \to dark-ligh~ (or light-dark) 28 variations. In this connection, a lead network 142 is asso-29 ciated with the feedback resistor 144 of the operational amplifier 116. The frequency compensating network 142 in-31 cludes a parallel RC combination ln series with a resistor 32 148 connected to the ~unction of ~ resistor 145 and a potentiometer 146 where the resistor 145 and the potentio-meter 146 are connected between the +8 volt power supply and ground.
In addition, the preamplifier 16 includes a dif-ferentiating or lead network comprising an RC combination 150 which is connected be-tween the field effect transistor 120 and the VCO circuitry 18 shown in Fig. 1. Other details concerning the facsimile receiver circuitry are shown and described in U. S. Patent No. 3,911,207.
In accordance with another important aspect of the invention, filter means are associated with photodetec-tor 14 which scans the document on the drum 12T. As shown in Fig. 3, a scanning head 200 includes illuminating fiber optics 202 and a reading fiber optic 204 in optical communi-cation with the document on the drum 12T. A reading flber optic is connected to a photodetector-filter mounting 206 which is shown in section in Fig. 4. The mounting 206 in-cludes a fiber optic holder 208 receiving the fiber optic 204 which is covered at the internal end thereof by a gela-tin filter 210 which extends transversely across and in optical communication with the fiber optic 204 on the in-tcrior of the mounting 206. In the preferred embodiment of the invention, the gelatin filter 210 comprises a ~ 38 Wratten gelatin filter manufactured by Eastman Kodak. The photodetector 14 is then mounted adjacent and in optical communication with the reading fiber optic 204 through a gelatin filter 210 at the interior of a jacket 212. The photodetector 14 is held in place by a threaded member 214 having a knurled finger grip 216. The leads 218 for the photodetector 14 extend through the member 214 to the opera-tional amplifier 114 as shown in Fig. 2.

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1 Fig. 5 shows a sectional view of the mounting 2~6 2 where the flber optic 204 is seen through the gelatin filter 3 210. It has been found that the gela~in filter 210 may be 4 conveniently cemen~ed on the end of the fiber optic holder ~08. After cementing, the mounting may be assembled to the 6 condition shown in Fig. 4.
7 As snown in Fig. 3, the ill~lminating ~iber optics 8 202 eY.tend into a Jacket 220 having a glass filter 222 at 9 the end thereof adJacent the illuminating means or light source 224.
11 It will therefore be appreciated that the filter ;~
12 means includes a gelatin filter as well as a glass filter 13 located in the path of optical communication between the 14 illumi.nating means and the photodetector with the document being located at an intermediate point in that optical path.
16 More particularly, the gelatin filter 210 is interposed 17 between the photodetector and tlhe end o the fiber optic 18 means at one end o the path of optical communication and 19 the glass filter 222 is interposed between the light source 224 and the other end of the fiber optic means in the path 21 of optical communication.
22 In accordance with this invention, the gelatin 23 ~ilter 210 substantially attenuates light in the visible red 24 region, i.e., light in the range of 625 to 725 millimicrons.
In thls connection, Fig. 7 shows the transm~ssion character-26 istic of the gelatin filter as a function of wavelength.
27 As khe frequency increases above 625 millimicrons, the 28 ~ransmittance o the filter increases reaching a minimum 29 at approximately 500 millimicrons. In the violet region above 450 millimicrons, the transmittance decreases rapidly.
31 In further accordance wlth this lnven~ion, the 32 gel~in ~ilter 210 is comhlned with the glass filter 222 to ~1~1538 achieve the transmission characteristic shown in Fig. 8.
It will be noted that the glass filter increases the trans-mittance in the 400-600 millimicron visible region to achieve the combined characteristics of Fig. 8. In addition, the glass filter also attenuates light in the near infrared range such as that associated with blue ball-point. Be-cause of the attenuation achieved by the gelatin filter 210 in combination with the glass filter 222, regions of a document which appear dark to the eye will be sensed as dark by the photodetector even though visible light in the red range as well as infrared light is reflected from those regions.
It is also possible to combine the glass filter and the gelatin filter into a single filter at one point in the optical path. It is further possible to incorporate the desired filter characteristics in the fiber optics them-selves and completely eliminate the gelatin filter and the glass filter. In the alternative, l:he light source 224 may be properly chosen so as to limit or reduce the amount of visible red and infrared light which reaches the photode-tector.
Details concerning the relationship of the fiber optics to the document on the drum 12T are disclosed in United States Patent ~o. 3,872,239.
Reference will now be made to the waveforms of Fig. 5 to illustrate the effectiveness of the circuitry in Fig. 2.
The waveform a indicates the electrical signal response at the output of the amplifier 114 for an ideal system upon detecting a narrow width black line with sharp 3~

1 well-defined edges which this invention seeks to simulate.
2 Wavefor~ b indicates the response from ~he same narrow linP
3 through a standard uncompensated amplifier and optical sys-4 tem, and waveform c is the response through the compensated amplifier 116 in accordance with this invention. It will 6 be noted that the white or black transi~ion slope is steep-7 er and the amplitude is greater in waveform c than in wave-8 form b thus achieving a full black condition in a shorter 9 period o~ time thus providing better resolution of the well-defined edge on the scanned ine line on the original docu~
11 ment. The black-to-white transition is also steeper pro-12 viding an improved resolution on the black-to-white transi-13 tion The white overshoot does no~ reduce copy quality 14 since whiter-than-white signals are simply printed as white.
The reduced amplitude o~ the waveform b is due primarily to 16 the resolving power o the optics.
17 In the case of waveform b, ~he circle o least 18 con~usion or optical aperture at the end of ~he fiber optic 19 204 exceeds the width of the blaclc line being scanned. In waveform d, the circle of least confusion is approximately 21 equal to the black line and in waveform e the circle of 22 least confusion is less than the black line. It will be 23 noted that the slope from white-to-black and black-to-white 24 in waveforms b9 d and e is equal.
In accordance with one important aspec~ of this 26 invention, the amplifier compensating network 142 momentari-27 ly increases the gain o~ the operational ampli~ier 116 on 28 the steeper leading and trailing edges so as to produce the 29 waveform c. A wide black line through the compensated am-pliier would provide the waveorm and a wide gray line 31 with sharp leading and trailing edges would produce wave-32 fo~m g.

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~ 5~ 8 1 In accordance with another important aspect o~
2 the invention, the second compensating netwcrk 150 compris-3 ing a parallel RC combination, provides improved gray scale 4 while preserving the higher gain two-level operation neces-5 sary for improved quality o~ fine printed material such as 6 6 point type. The resistor value of the network 150 ls 7 chosen such that a wide medium gray mark on the scanned 8 do^ument produces the desired l~ediurn gray print on the copy 9 medium.
The video voltage applied to the VC0 is shown i~
11 waveform h. It will be noted the i.nitial transition ~rom 12 white to gray produces a brief overshoot to the black re-13 gion. The width of this black pulse is sufficient to span 14 the width of narrow s~rokes in small type so that essen-tially two-level operation exists until the capacitor o~ the

16 network 150 becomes charged at ~hich time the network acts

17 onLy as a resistive àttenuator as long as the video voltage

18 level remains substantially constant through the remainder 1~ of the gray area. Black and white followers are pr~duced as is typical with heavy video peaking. However, printed 21 copy quality is enhanced while preserving a reasonable gray 22 scale o approximately five levels.
23 It will therefore be understood that the electri-24 cal response of the transmitter section including the photo-detector and the voltage controlled oscillator is sufficient-26 ly fast ~o follow the black-white changes in light level 27 into the photodetector. Of course, the rate at which the 28 light level on the sensi~ive area of the photocell changes 29 ls prlmarily a function of optics. The above-described compensatlng networks in effect reduce the optical aperture 31 or circ~e of least confuslon thus improvin~ the overall 32 resolution o the system.

- l.6 -The ~GC portion of the system does not contribute to improved resolution, however, it does establish and main-tain a white reference level by making adjustments in ampli-fier gain over a limited range. The AGC circuit also main-tains a fixed amplifier output voltage representing white -thus establishing a poi~t from which to start in determin~
ing the various shades of gray continuing into the black region. The time constant of this circuit is such that a white level is quickly sensed and stored in the capacitor 125 to establish a gain sufficient to provide the desired white video voltage output from operational amplifier 116.
This gain is then held reasonably constant for the remainder ~ -of the horizontal scan line. The amplifier gain will slowly increase after the first horizontal scan line in the ab-sence of white on the scanned document.
Although the receiver of the system shown in Fig.
1 has not been describecl in detail, a particularly suitable receiver is disclosed in U. S. Patent ~o. 4,112,468, issued September 5, 1978.
Although specific circuitry and systems have been shown and described, it will be understood that various changes and modifications may be made without departing from the true spirit and scope of the invention as set forth in the appended claims.
This application is a division of application serial number 255,104 filed June 17, 1976.

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Claims (6)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:-

1. A facsimile transmitter for use in producing a copy at one location which is a facsimile of a document located at another location comprising:
means for illuminating the document;
photodetector means in optical communication with the document for detecting dark-light variations in the illuminated document, said photodetector means generating photodetector signals representing said dark-light variations;
means for scanning said photodetector means relative to said document;
amplifier means coupled to said photodetector means for amplifying said phtotdetector signals;
modulator means coupled to said amplifier means for generating modulated transmission signals repre-senting said dark-light variations on said document;
fiber optic means in the path of optical transmission from said illuminating means to said document and from said document to said photodetector means; and means for substantially limiting light in the visible red region of the spectrum from reaching said photodetector means so as to permit detection of a relatively dark appearing region even though said dark appearing region is capable of reflecting a substantial amount of light in said visible red region.

2. The facsimile transmitter of claim 1 wherein said means for substantially limiting comprises filter means for attenuating light in the visible red region.

3. The facsimile transmitter of claim 2 wherein said filter means extends substantially transversely to the axis of said fiber optic means.

4. The facsimile transmitter of claim 3 wherein said filter means comprises a gelatin sheet extending sub-stantially transversely to the axis of said fiber optic means.

5. The facsimile transmitter of claim 4 wherein said gelatin sheet is interposed between said photodetector means and said adjacent end of said fiber optic means.

6. The facsimile transmitter of claim 5 wherein said filter means further comprises a glass filter interposed between said illuminating means and the adjacent end of said fiber optic means for substantially attenuating in-frared light.