GB2143636A - A circuit arrangement and method for detecting markers in a microfilm scanner - Google Patents

Abstract

A circuit arrangement for detecting markers in a microfilm scanner comprises a comparator (8) for receiving an output from a photo-detector (1) and a threshold value as calculated in a microprocessor (6) and converted in a D/A converter (7). The output signals from the photo-detector are also fed to the microprocessor (6), via an analogue switch (3) and A/D converter (5), from which signals the threshold value is calculated as a function of previous values both of baseline signals generated when a marker is absent, and of deviation signals generated when a marker is present. An indication from the comparator that the output from the photo-detector exceeds a threshold level alerts the microprocessor to the fact that a marker exists. <IMAGE>

Description

SPECIFICATION A circuit arrangement for detecting markers in a micro-film scanner According to the present invention, there is provided a circuit arrangement for detecting markers in a microfilm scanner, in which scanner the microfilm passes between a light source and a photo-detector providing an output in response to the light received from the light source, the circuit arrangement comprising :: means for converting the output of the photo-detector when no marker is present to a baseline value; means for converting the output of the photo-detector when a marker is present to a deviation value; means for comparing the deviation in the output of the photo-detector from the baseline caused by the presence of a dark spot in a frame of microfilm and indicating the existence of a marker if the deviation from the baseline exceeds a threshold, the circuit arrangement being such that the said threshold is a function of all preceding baseline values and all preceding deviation values.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic diagram of an example of a circuit arrangement according to the present invention; Figures 2 and 3 are circuit diagrams of an amplifier and a comparator respectively of the arrangement; Figure 4 shows how a digital to analogue converter of the arrangement may be realised; and Figure 5 shows an arrangement which may be used to perform analogue to digital conversion.

The circuit arrangement to be described is for detecting the presence of markers of 'blips' on microfilm, where the microfilm passes between a light source and a photo-electric device such as a phototransistor.

Referring to Figure 1, the circuit arrangement comprises seven elements, namely: a) a phototransistor 1; b) a linear amplifier 2; c) an analogue switch 3, to serve as a sample/hold cicuit in conjunction with a capacitor 4 at its output; d) an analogue to dital (A/D) converter 5, which acts as a means of converting the sampled voltage, proportional to light level, into a digital word; e) a microprocessor system 6, for the mathematical or logical manipulation of the above digital word, and also for controlling the interrogation of the AiD converter 5, analogue switch 3 status, level of output at a digital to analogue (D/A) converter 7 and handling of blip detection via an interrupt input f) a comparator 8, for comparing the incoming voltage (proportional to light level) with a predetermined threshold supplied via the D/A converter 7; and g) the D/A converter 7 to set the threshold voltage as required by the microprocessor system 6.

The phototransistor 1 could be an NPN silicon photodarlington transistor type OP560 made by TRW Optron, a subsidiary of TRW Inc., 1201 Tappan Circle, Carrollton, Texas, 75006, U.S.A.; the amplifier 2 could be as shown by Figure 2, being for example one amplifier of a low power quad operational amplifier of one of the LM124 series made by National Semi-conductor; the analogue switch 3 could be one of the types MC14051B to MC14053B made by National Semiconductor; the analogue to digital converter 5 could be one of the series ADC0801 to ADC0804 made by National Semiconductor; the comparator 8 could be as shown in Figure 3, being for example one of the LM139 series made by National Semiconductor; and the analogue to digital converter 7 could be realised as shown in Figure 4, reference numeral 8 denoting an analogue switch.

A baseline value for the input may be achieved by interrogating the A/D converter 5, and storing the result when a piece of clear microfilm passes between the light source and the phototransistor 1. This value may be updated on a regular basis using an averaging algorithm to avoid spurious results occurring due to dirty or damaged film.

When a deviation from the baseline value occurs, usually caused by a dark spot on the film, the value will be stored in a separate memory location within the microprocessor system 6, to be updated on a regular basis as described above.

The criterion for a true deviation will be a comparison of the immediate value with a predetermined theshold to be calculated as follows. The average value, for a deviation (or in the case of no recorded deviation, zero) will be subtracted from the average value for the baseline. The result will be halved and added to the average value for a deviation.

or: T = (B - D/2) + D Where: B = Average value for baseline D = Average value for deviation T = Threshold value This assumes that the numerical value will increase proportionally with light level.

The threshold now serves two uses: to determine whether an input value is a baseline or deviation by arithmetical comparison of values; and to preset the representation of that threshold through the D/A converter 7, at the input of the comparator 8.

The foregoing describes the method for establishing a threshold value to which the incoming voltage will be compared. This value will be updated on the same regular basis as for the baseline and deviation values, thus compensating for any changes in film density, deviations in lamp intensity or other factors affecting the results.

The incoming voltage is compared with the threshold voltage at the comparator 8, and a deviation past the threshold will be immediately recognised by the microprocessor system 6 via the interrupt input I.

This will have the effect of stopping the microprocessor and attending to the needs of the phototransistor activity. The interrupt may be triggered by either a high to low transition, or a low to high transition as desired, e.g. if the blip image were in positive instead of negative form.

The detection of a blip will have no immediate effect on the threshold value, nor will the return to the baseline. The actual value of either will be added into the appropriate average causing a gradual change if any alterations to the levels have occurred.

In order to detect direction of travel of the blip, by phase comparison, two channels as described are required. In this case, as shown in Figure 4, a spare channel of the microprocessor 6 is connected to the reference input of the comparator 8. A 101lF electrolytic capacitor 10 is connected between this input and ground (or, logic "0"), and a 100 k resistor 11 is connected between this input and a peripheral output part of an interface chip of microprocessor 6. This interface chip 12 is shown in more detail in Figure 5, and is a 6821 1/O chip of a Rockwell 6502 microprocessor, for example.

The analogue to digital conversion is carried out for the channel, and the resultant value compared with the stored theoretical threshold value. If the actual value is low, the output port is set to a logic "1", to provide a charging path for the capacitor and thereby increase the voltage across it. If the value is high, the output port is set to a logic "0", thereby discharging the capacitor to a suitable level.

In this manner the voltage across the capacitor 10 may be controlled to withing 20mV, being updated once every 8mS by the microprocessor's interrupt routine. The interrupt cycle is completely independant of any photocell activity and may be applied to any suitable value other than the threshold for the channel.

For detection of different sized blips, up to four such channels may be used. To cut down on hardware and for reasons of economy, some elements of the system may be common to all channels as will be described with reference to Figure 5. In this Figure four spare channels of a multiplexer are each connected to a reference input of a respective comparator 8a, 8b, 8c, 8d. As hereinbefore described, a 10 > F capacitor 10a, lob, 10c, 10d is connected between each reference input and ground, and a 100k resistor gila, llb, llc, lid is connected between a respective input and one of four lines of a peripheral output port on the 6821110 chip 12.The AID conversion is performed for that channel and the resultant value compared with the theoretical threshold value held in memory. The remainder of the conversion process if as described with reference to Figure 4.

The microprocessor system 6 may be used to control any number of D/A converters 6 or analogue switches 9. An interrupt controller may be used to pass several commands from comparators to the system as no two channels will detect simultaneously.

The A/D converter 5 may be isolated from the amplifier 2 via the analogue switch 3. Thus if several analogue switches 3 are connected together at the input and each channel sampled in turn, only one AID converter 5 is required.

Each channel will now comprise elements as described, being the phototransistor 1, amplifier 2, analogue switch 3, comparator 8 and DIA converter 7.

There will now be outlined a system using such a circuit arrangement. The system comprises a unit for document counting in the forward or reverse directions. A six digit display shows the current total with indicator arrows to denote level one, two or three counting, target mode and audio indication feature.

The basis unit may be expanded by fitting extra keys to a keyboard area such as a 10 key pad, page/ file/batch access, target/count mode etc. Further modifications for computer interfacing and film drive will be discussed later.

Alarm conditions include counter overflow or underflow, warning when the target document is approaching and indication of blip presence if required.

The prime function of the unit is to count the passage of frames of film, or individual documents, and is capable of detecting forward or reverse film motion and has the ability to read either a document edge or a specially prepared 'blip' reproduced photographically on the film. The blip is a small rectangle that appears underneath each document and is approximately 0.7mm by 0.5mm although the length may vary. At a 24 times enlargement, the size at the screen would be 16.8mm by 12mm.

The basic sensing arrangement for single level counting comprises two phototransistors 1 placed approximately 10mm apart in the plane of the film movement direction and attached to the viewing screen of the microfilm reader. The waveforms produced by passing a blip across them will indicate the direction of travel by observing which one senses the blip first, i.e. right to left, or left to right.

When the direction has been established, the six digit counter will be instructed to increment or decrement when both transistors are covered. In this way an accurate representation may be made of the current document relative to an origin of zero. A 'CLEAR' key is provided to establish the origin, usually at the beginning of the film.

In case of errors due to damaged or dirty film, two keys are provided, marked + and -, for manually incrementing and decrementing the displayed count. This would be useful if the photographed documents were recorded in a consecutive, numeric sequence and the number appeared on that frame.

If the start number is anything other than zero, the 10 key pad may be fitted to enable entry of up to six digits as the origin. This value may be freely cleared, re-entered or adjusted at any time while the film is static.

With the addition of the 10 keypad and another key marked 'TARGET', a target document may be entered for assistance in retrieval. On selecting the key, an arrow will indication that the target register is now being displayed. The displayed value may be cleared or altered as previously described, a value of zero disabling the warning feature. Re-depression of the key will return the display to the count and retrieval may proceed. As the counter approaches the value of the target register, say, a difference of 75 frames, an alarm will issue four short bleeps to allow the user time to slow the film transfer down until the desired document is displayed.

A further key is to select the audio warning mode and an arrow on the display indicates its status. In this mode a single bleep will indicate the passage of each blip or document and will occur as the display is updated.

Certain applications of microfilm storage require sub-groups of documents to be formed, mainly in the form of documents or pages within a file and files within a batch. To indicate the limits of the group, different sized blips are used, i.e. a 2.5mm length for file and 4.5mm for batches. To detect these special blips further, single phototransistors are placed on the viewer screen alongside the page sensing ones such that only a file blip will cover the page sensing transistors and the additional file sensing transistor, and the batch blip will cover all transistors.

Two extra keys are added to the keyboard area for selection of page, file or batch mode. The two keys will index the level indicator arrow up or down through the levels as required.

As the next level is accessed, the count of that level will be displayed, and also the status of the audio warning mode. For file mode, two bleeps will be issued and for batch mode, three. If the target register is now accessed, the target for that level will be displayed and may be altered as previously described.

Additional features when using two or three level sensing are as follows. Internal option switches will select auto-zeroing of inferior registers, i.e. should the page count be, say, 1024 and a file blip is encountered, then the file counter will be incremented and the page counter cleared down to zero. A similar feature will apply to batch sensing. With the option switched off, a continuous count of pages, filed and batches will occur.

Before the page counter is zeroed by the file blip, the page count will be committed to memory in a stacking fashion. Thus, should the film direction be reversed and a file blip encountered then the last page count recorded will be transferred to the page counter. If this were not the case then a page count underflow error would occur at each file blip in the reverse direction. The same function will apply to batch blips and file counts.

In the target mode of multi-level counting, each level may carrying its own target. If batch 3, file 24, page 300 were programmed in and the display set to batch or level three, then the following would occur on retrieval. As the batch counter incremented to 3, three short bleeps would be issued and the display automatically set to file count. As the file incremented to 24, two short bleeps would be issued and the page count displayed. The normal single level warning mode will apply to page mode as previously described.

Apart from the audible warnings already described, two conditions will cause eight bleeps with a displayed warning.

Should any counter overflow, i.e. count over 999,999, then the display will revert to zero and continue as normal with the addition of a letter '0' in the leftmost part of the display. The 'O' will remain for the duration of the bleeps then disappear, or alternatively, if the clear button is pressed.

Should any counter underflow, i.e. count below zero, then the display will show 999,999 and continue as normal with the addition of a letter 'U' in the leftmost part of the display. The 'U' will cancel as for the 'O'.

The display should be an eight digit numeric type with at least five indicator arrows for status and mode. The digits should be clear, easy to read of at least 12mm in height. Either a blue or green filter should be used, preferably blue to reduce eye-strain.

The keys should be large enough to be operated easily without confusion and should be on a 19mm pitch. A mechanical type is preferable to touch or membrane, again for operator ease or use. Any logical layout may be used, the 10 keypad to be grouped in the normal calculator or telephone type arrangement.

The sensing transistors must be mounted on suitable non-permanent adhesive pads for fixing to the viewer screen. The adhesive should be arranged such that either a rear or front projection viewer may be used. Connection to the unit will be via screened cables and 3.5mm jack plugs. A two point or single level sensing arrangement will have a twin screened cable and stereo jack plug.

The unit will be mains powered and contain its own power pack. Normal operation will be 220/240 volts, 50 Hz with the option of 110/120 volts, 60 Hz for U.S. applications.

The basic unit is capable of having additional hardware fitted to effect a full RS232 standard interface.

When fitted, commands received via the interface, from a computer, will operate as if they were keyboard entries. To increment the display a + character would be sent via the computer, to clear the display, a 'c' etc.

In this way a target frame and search may be effected rapidly by the following command string: T102S. This will select the Target register, enter 102 into it and Search for it.

Monitor status may be relayed to the computer via the RS232 interface, such as current display, level, audio or target mode etc.

Entries may be received either by keyboard or computer; no conflict or mutual exclusion will occur.

An important feature in retrieval is the ability to control the film transport such that a document may be automatically accessed simply by programming the desired location into the target register(s) and selecting the SEARCH function key.

This requires additional hardware to be fitted such that a number of relays may be switched in and out, under software control, to directly alter the motor direction and speed.

Further functions may be added such as single frame movement to the left or right and browsing with pre- programmed time dwellings on each frame.

Claims (12)

1. A circuit arrangement for detecting markers in a microfilm scanner, in which scanner the mircrofilm passes between a light source and a photo-detector providing an output signal in response to the light received from the light source, the circuit arrangement comprising: conversion means for converting the output of the phot-detector to a baseline value when no marker is present and for converting the output signal of the photo-detector to a deviation value when a marker is present; comparator means for comparing with a threhold value the deviation in the output signal of the photo-detector from the baseline caused by the presence of a dark spot in a frame of microfilm and indicating the existence of a marker if the deviation from the baseline exceeds the threshold, the circuit arrangement being such that the threshold is a function of all preceding baseline values and all preceding deviation values.

2. A circuit arrangement according to claim 1, wherein the conversion means comprises an analogueto-digital converter, the circuit arrangement further comprising storage means for storing separately digital baseline and deviation values, and computing means for computing the threshold from such values.

3. A circuit arrangement according to claim 2, wherein the storage means and the computing means are constituted by a microprocessor.

4. A circuit arrangement according to claim 3, wherein the comparator means comprises a comparator which receives at one input the threshold and at another input the output signal from the photodetector and which produces an output signal which serves as an interrupt signal for the microprocessor.

5. A circuit arrangement according to claim 4, further comprising digital-to-analogue conversion means for converting a digital value of the threshold to an analogue signal for inputting to the comparator.

6. A circuit arrangement according to claim 5, wherein the digital-to-analogue conversion means comprises a resistor connected between an output of the computing means and the one input of the comparator, and a capacitor connected between said one input and earth potention whereby a digital HIGH or LOW value outputted from the computing means causes a corresponding analogue increase or decrease respectively in the voltage across the capacitor and hence in the signal applied at said one input.

7. A circuit arrangement substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.

8. A microfilm scanner comprising: a circuit arrangement according to any preceding claim; one or more photo- detector; a light source, the microfilm passing between the light source and the photo-de tector(s); counting means for counting the number of markers detected between two limits; and display means for displaying such a number.

9. A microfilm scanner as claimed in claim 8, having two or more photo-detectors spaced along the line of travel of the microfilm whereby the direction of travel of the microfilm along the line may be determined by ascertaining which of the two or more photo-detectors first detects the presence of a marker.

10. A method of detecting markers in a microfilm scanner, in which method: the microfilm passes between a light source and a photo-detector providing an output signal in response to the light received from the light source; the output signal of the photo-detector is converted to a baseline value when no marker is present and to a deviation value when a marker is present; the deviation in the output signal from the photo-detector is compared with a threshold value, the existence of a marker being indicated if the deviation from the baseline exceeds the threshold; and the threshold is a function of all preceding baseline values and all preceding deviation values.

11. A method as claimed in claim 10, wherein said function is T = (B - D/2) + D where T is the threshold, B is the average of all preceding baseline values, and D is the average of all preceding deviation values.

12. A method as claimed in claim 10, substantially as hereinbefore described with reference to the accompanying drawings.