Improved threshold detector.

Field of invention

0001This invention concerns threshold detectors of the type used to convert a varying amplitude electrical signal (such as a video signal obtained by scanning a photosensitive region having a light image focussed thereon) into an electrical signal made up of constant amplitude pulses the durations of which correspond to the durations of the corresponding amplitude excursions of the original video signal.

0002The invention also lies in a scanner when fitted with an improved-detector.

Background to the invention

0003The process of detection just described is most simply achieved by comparing the varying amplitude electrical signal with a reference signal (commonly referred to as a threshold) and generating a constant amplitude pulse only when the amplitude of the varying amplitude signal bears a particular relationship to the volume of the threshold signal. Typically this relationship is when the amplitude of the one signal equals the value of the threshold signal but any relationships may be used as convenient.

0004Where the value of the threshold signal is constant, errors can occur when the amplitude of the varying amplitude signal varies due to factors such as uneven illumination, as well as due to different picture content in the scanned image.

0005Uneven illumination is the single most significant factor since this will tend to produce a white level amplitude of the video signal which varies along the length of each line scan. Typically the amplitude corresponds to a higher brightness level in the control region of each line with amplitudes corresponding to less bright levels at the end regions of each line.

0006It is an object of the present invention to provide an improved detector by which this problem is overcome.

Prior art

0007It is known to measure the mean amplitude level of an amplitude varying video signal to obtain a threshold voltage which varies in sympathy with the mean brightness level of the video signal, and compare the instantaneous video signal amplitude therewith.

0008It is also known to store signals derived from the scanning device whilst the latter is subjected to so-called uniform illumination and generate a compensating signal therefrom which when combined with a constant threshold voltage will produce a varying amplitude threshold signal.

Summary of the invention

0009According to the present invention there is provided a threshold detector comprising <ul id="ul0001" list-style="none"><li>(1) a comparator for comparing a varying amplitude video signal (produced by scanning) with a reference voltage</li><li>(2) an input for the video signal, and</li><li>(3) a reference voltage generating circuit comprising <ul id="ul0002" list-style="none"><li>(a) a counter synchronized to the scanning producing the video signal such that the counter produces a particular sequence of digital values during each line of scan</li><li>(b) a memory device containing a number of memory locations at least equal to the number of different values produced by the counter during each line of scan, each digital value causing a different one of the memory locations to be addressed, to output a digital value stored thereat; and</li><li>(c) digital to analogue conversion circuit means for converting the digital outputs from the memory into an analogue signal which constitutes the threshold signal for the comparator.</li></ul></li></ul>

0010Preferably the memory device is a programmable memory and typically a so-called erasible programmable memory (EPROM) device is employed.

0011The memory device may include a programming aid associated therewith so that its contents can be adjusted to accommodate lamp ageing, variations introduced by the substitution of one lamp for another and the like.

0012The invention also comprises a scanning device for producing a binary signal corresponding to the information content of a document or the like presented thereto which includes: <ul id="ul0003" list-style="none"><li>(1) lamp means for illuminating the document</li><li>(2) imaging means for forming an image of the illuminated document on the photosensitive surface of a camera which is scanned electrically to produce a varying amplitude electrical video signal, and</li><li>(3) a threshold detector circuit as aforesaid which receives the video signal and produces therefrom a series of constant amplitude pulses as a so-called detected video signal.</li></ul>

0013The invention will now be described by way of example with reference to the accompanying drawings in which <ul id="ul0004" list-style="none"><li>Fig 1 shows schematically a circuit of a scanner and improved threshold detector embodying the invention, and</li><li>Fig 2 shows graphically a video signal and programmed threshold signal according to the invention. In the figure a camera 10 is caused to scan an image formed thereon by a lens 12. The scanning may be entirely electronic as in a standard television camera, or may be linear, with frame scanning being achieved by relative movement of the camera or lens or object which is imaged.</li></ul>

0014In the embodiment shown a plane object such as a document page 14 is illuminated by a lamp 16 and reflector 18.

0015Fall-off in light level at the extremes of the scanning can produce variation in the mean value of the video signal as shown in Fig 2(a).

0016Fig 2(b) shows an ideal video signal in which the mean light level is constant.

0017In order to compensate for the varying light level component of the video signal in Fig 2(a), a threshold voltage (see Fig 2(c)) with which it is to be compared must vary in sympathy with the mean signal variation.

0018To this end the threshold voltage is generated during each line of scan from digital values stored at different locations in a memory 20. Each location is addressed in turn by a counter 22 which is reset by the load signal derived from the scanning camera control circuit 24. This produces the signals needed to control the camera 10 which in the embodiment shown is assumed to be a linear camera having a line of photoresistance (typically charge coupled devices) arranged in the direction of line scanning (each device in the line being addressed in turn following the appearance of a 'load' signal) and the camera is moved systematically in a perpendicular direction to present successively different lines of the image to the line of devices, the perpendicular movement being synchronized with the load signals so that the scanning corresponds to a series of consecutive lines.

0019A clock pulse generator (not shown) causes the counter 22 to step during each line scan and the digital output signals appearing at the output of the memory 20 are converted into an analogue signal by a digital to analogue converter 26.

0020The output from the latter appears as shown at Fig 2(c) and is applied to one input of a comparator 28, the video signal (corresponding to Fig 2(a) being applied to the other output.

0021A typical resulting output signal is shown at Fig 2(d).

0022In a modification of the invention, a signal, obtained from the scanning may be digitized and stored in the memory and these values used to produce the varying threshold signal of Fig 2(c). Where these values are obtained from scanning a white document, the values will correspond to any uneveness in the illumination in the line of scan direction.