GB2037532A

GB2037532A - Image Reproduction Systems With Digital Enlargement

Info

Publication number: GB2037532A
Application number: GB7940987A
Authority: GB
Original assignee: Crosfield Electronics Ltd
Current assignee: Crosfield Electronics Ltd
Priority date: 1978-11-28
Filing date: 1979-11-28
Publication date: 1980-07-09
Also published as: GB2037532B

Abstract

In image reproduction apparatus of the kind in which an original on a drum is scanned by an analysing head and the resulting electric signals are converted into digital form, stored, thereinafter extracted, and reconverted to analogue form to control an output drum exposing scanner, it is ensured that the store capacity is utilized to the fullest extent compatible with other limiting factors by calculating a trial output resolution (Ro/p), based on the store capacity and the required picture height; deriving a trial input sampling frequency (Fi/p) from the trial output resolution and from the required enlargement (E), output drum rotary speed (S), output drum circumference (C), and ratio of output drum diameter to input drum diameter (D); comparing the maximum rate of analogue-to-digital conversion (FADC) with the maximum input sampling rate for a minimum signal-to-noise ratio (FS/N) and selecting the lower of these rates as a limiting input sampling frequency; comparing the calculated trial input sampling frequency (Fi/p) with the said limiting input sampling frequency and using the smaller as the actual input sampling frequency from which are calculated multiplier and divider factors for controlling the read and write rates for the store and the rate of traverse of analysing and exposing heads parallel to the axes of the analysing and exposing drums. <IMAGE>

Description

SPECIFICATION Improvements Relating to Image Reproduction Systems This invention relates to image reproduction systems of the kind employing scanning apparatus to convert the light values of the image to be reproduced into electric signals and, in response to such signals after any necessary correction has been carried out, to scan an image recording surface.

One convenient form of scanner is that employing cylinders to support the image to be reproduced and the light-sensitive surface to be exposed. Rotation of the cylinder supporting the image to be reproduced in relation to an analysing system will provide the necessary scanning motion in one direction, scanning in a direction perpendicular to the first being achieved by relative movement between the cylinder and the analysing system in a direction parallel to the cylinder axis. A similar arrangement is used for corresponding scanning motion of the reproducing head in relation to the reproducing cylinder.

In our Patent No. 1,166,091, we have described a digital enlarging system. In this system, digital signals corresponding to the output of the analysing head are written into a line store, from which they are read at a rate which is determined by the required magnification, among other factors; the digital output signals derived from the store are converted back into analogue form before being applied to an exposing head. In the example shown in Patent No. 1,166,091, the analysing and exposing cylinders are mounted on a common shaft and therefore rotate at the same speed. Of course, the relationship between the rates of movement of the analysing and exposing heads parallel to the cylinder axis is also varied with the required magnification.

In the past, it has been the practice to operate such enlarging apparatus always at a given output cylinder resolution or at one of a small number of given output cylinder resolutions. Thus, in a machine operating at a known output cylinder resolution (for example 120 picture elements per centimetre) and having a known speed of rotation of the output cylinder, when the dimension of the picture in the circumferential direction of scanning was known, the required "read" frequency at the store output could be calculated.

Assuming, for example, a picture "height" of 25 centimetres and a picture resolution of 120 picture element per inch, then (25x 120)=3000 picture elements had to be extracted from store in the time required for 25 centimetres of the output cylinder circumference to pass the scanning head.

By extraction of a picture element from store, we mean the extraction of the digital data representing the corrected density value for this picture element.

Once the "read" frequency was known, the "write" frequency at the store input could be calculated by multiplying the output frequency by the enlargement factor. This assumes that the two cylinders are on the same shaft, or at least that they are rotating at the same speed.

The "read" and "write" rates having been entered into the apparatus, the enlargement can then be made. Such a system works well but has the disadvantage that the store is not always fully utilised.

According to the present invention, image reproducing apparatus having an image analysing head for deriving picture signal values for elements of an image to be reproduced, analogue-to-digital conversion means for sampling the picture signal values and converting the samples to digital form, a store for storing the digital picture signal values, and a reproducing head for treating an output surface in response to picture signals extracted from the store at a rate dependent on the required output resolution and enlargement, further includes means receiving as an input the required enlargement and the input or required output picture size and further responsive to the store capacity available for the digital signal values, and the speed of movement of the input and output surfaces past the analysing and reproducing heads, respectively, for computing a first sampling rate which is the input sampling rate corresponding to the maximum output resolution for the required output picture height with the available store capacity, means comparing the said first sampling rate with a second sampling rate which is a limiting input sampling rate imposed by the maximum rate of analogue-to-digital conversion or the maximum input sampling rate for a minimum signal-to-noise ratio, and means utilising the lower of the said first and second sampling rates to calculate the required ratios of input sampling rate and output extraction rate to the speed of movement of the input surface and the output surface past the analysing head and the reproducing head respectively.Preferably the apparatus further includes means automatically setting the calculated values into the scanning apparatus to enable it to perform the required enlarging operation. The apparatus may include a microprocessor for computing the necessary division and multiplication factors to provide input sampling rate and analyse advance rate, output sampling rate and expose advance rate. In this way we are able to use an output sampling rate which is the highest possible without one of the said limits operating, i.e. without exceeding the store capacity available, without exceeding the conversion rate of the analogue-to-digital converter, and without exceeding the maximum permissible input sample rate corresponding to a minimum acceptable signal-to-noise ratio of input sampling.

In order that the invention may be better understood, an example will now be described with reference to the accompanying drawings, in which: Figure 1 is a block diagram illustrating image reproduction apparatus to which the present invention can be applied; Figure 2 is a flow diagram illustrating the manner in which the invention is applied to the apparatus of figure 1; and Figure 3 illustrates data processing apparatus used in one image reproducing apparatus embodying the present invention.

In figure 1, an analysing cylinder 10 and an exposing cylinder 12 are mounted on a common shaft 14. On the same shaft there is mounted a coding disc 1 6 which, in combination with a light source 1 8 and a photocell 20, provides timing signals on line 22, synchronised with the rotation of the shaft 14.

The image 24 to be reproduced is mounted on the drum 10 and the light-sensitive surface 26 on which the reproduction is to be formed is mounted on the cylinder 12.

An analysing head 28 derives, in a known manner, signals representing the light values of successively scanned elements of the image 24, these signals being transmitted by way of line 32 to an analogue-digital converter 34. From the analogue-digital converter, corresponding digital signals are transferred to an enlarging store 36.

The digital signals are thereafter extracted from store and applied to a digital-analogue converter 38, from which the corresponding analogue signals are applied to an exposing head 40 scanning the light-sensitive surface 26.

The clock pulses on line 22 are multiplied in a multiplier 41 by a factor p. The resulting pulses are transmitted to four control circuits 42, 44, 46 and 48. In these control circuits the pulses are divided or multiplied by adjustable factors B, A, a, and b respectively. Thus, in control circuit 44, the pulses are divided by A to provide driving signals at the correct frequency for a driving system 50 rotating a lead screw 52 on which the analysing head 28 is mounted for movement parallel to the cylinder axis. In control circuit 42, the pulses are divided by a different factor B to give the required rate of pulsing for a driving system 54 controlling the rate of rotation of a lead screw 56 on which the exposing head 40 is mounted for movement parallel to the cylinder axis.Control circuit 46 multiplies the pulse rate by factor a to give the rate of analogue-digital conversion in circuit 34 and the corresponding rate of reading the converted signals into the enlarging store (the "write" clock pulses). Control circuit 48 multiplies the pulse rate by a factor b to give the required rate of reading signals from the enlarging store (the "read" clock pulses).

The size of the enlarging store 36 need not be equal to the capacity allocated to one line. Thus, that part of the core store used in line storage can be reduced to less than the number of locations required for a compiete line by using the store in a recycling manner as explained in patent No.

1,166,091. In such a case, the recycling capability is taken into account in making the above-mentioned calculations.

In one form of apparatus embodying the invention, more than one output picture is exposed at one time by placing two or more lightsensitive surfaces spaced around the periphery of the same portion of the cylinder so that in each rotation of the cylinder, one line of each of the light-sensitive surfaces is scanned. The microprocessor may control store start addresses and analogue switch functions selecting signals from the store to implement this "simultaneous" exposure of more than one light-sensitive surface.

Furthermore, if desired, it may be programmed for automatic "sequence" scanning of more than one original (side by side) by detecting when scanning of one original has been completed, readjusting its resolution and enlargement automatically, and scanning the next original on the cylinder.

Figure 2, shows a flow diagram of the kind used in one embodiment of the system for carrying the invention into effect.

In Figure 2, the input parameters are as follows: h: Required output picture height, entered by werator E: Required enlargement, entered by operator M: Enlarging store capacity S: Output drum speed (r.p.m) C: Output drum circumference D: Ratio of output drum diameter to input drum diameter Fads: Maximum frequency of analogue-todigital conversion Fs, Maximum input sampling rate for an acceptable signal-to-noise ratio In figure 2, the output height and the required enlargement are entered into the system and a trial output resolution R01p is calculated in block 100 by dividing the enlarging store capacity by the height. This is clearly the maximum possible output resolution from a consideration of the store capacity alone.A trial output sampling frequency Fo/p is then obtained in block 102 as the product of the output drum speed in revolutions per minute, drum circumference, and output resolution, divided by 60. From this the trial input sampling frequency Fllp is calculated in block 1 04 as the product of the enlargement factor and the output sampling frequency, divided by the ratio of the output to input drum diameter.

Next, the maximum input sampling rate for an acceptable signal-to-noise ratio (FS/N) is calculated in block 106 as a function of the output drum speed, enlargement factor, and ratio of output to input drum diameter.

The next step is to ascertain in block 108 whether FS/N is greater than or equal to Fc, the limiting frequency from the point of view of analogue-to-digital conversion. In this way, it is ascertained which of these two factors is the more restrictive. If FADC is the lower (i.e. if the answer is "yes"), it is then ascertained in block 110 whether the calculated trial input sampling frequency is greater than FADS. If this is so, then the trial input sampling frequency is too high and it is made equal to FADC (block 112) and a new output frequency is calculated (block 114).

If the more restrictive factor is Fs!N, then it is ascertained (block 11 6) whether the calculated trial input sampling frequency is greater than Fs!N.

If this is so, then the input sampling frequency Fl/P is too high and it is made equal to FS/N (block 118) and again a new output sampling frequency is calculated (block 114).

If it is found that the calculated trial input sampling frequency is less than the limiting frequency with which it is compared (FADS or Fs,N, then the trial value of F,/p is used (block 120).

From the values ascertained in this way, the read/write multiplier factors b, a are calculated (block 122), as well as the division factors B, A and these values are transmitted to the frequency multiplier/divider circuit (block 124).

Figure 3 shows the general organisation of apparatus for carrying the invention into effect. In Figure 3 there is shown a microprocessor CP, a programmed read-only memory PROM, and a data read/write memory R/W, each connected to a computer interface bus 1 26. Also connected to the computer interface bus is a peripheral interface adaptor which receives enlargement data from a control panel (for example thumbwheel switches) and supplies the B, A factors to a programmable divider and the b, a factors to a programmable divider for forming part of a phase lock loop frequency multiplier.

Claims

1. Image reproducing apparatus having an image analysing head for deriving picture signal values for elements of an image to be reproduced, analogue-to-digital conversion means for sampling the picture signal values and converting the samples to digital form, a store for storing the digital picture signal values, and a reproducing head for treating an output surface in response to picture signals extracted from the store at a rate dependent on the required output resolution and enlargement, the apparatus further including means receiving as an input the required enlargement and the input or required output picture size and further responsive to the store capacity available for the digital signal values, and the speed of movement of the input and output surfaces past the analysing and reproducing heads, respectively, for computing a first sampling rate which is the input sampling rate corresponding to the maximum output resolution for the required output picture height with the available store capacity, means comparing the said first sampling rate with a second sampling rate which is a limiting input sampling rate imposed by the maximum rate of analogue-todigital conversion or the maximum input sampling rate for a minimum signal-to-noise ratio, and means utilising the lower of the said first and second sampling rates to calculate the required ratios of input sampling rate and output extraction rate to the speed movement of the input surface and the output surface past the analysing head and the reproducing head respectively.

2. Image reproducing apparatus in accordance with claim 1, further including means automatically setting the calculated values into the image reproducing apparatus to enable it to perform the required enlarging operation.

3. Apparatus in accordance with claim 1 or 2, comprising means comparing the maximum rate of analogue-to-digital conversion with the maximum permissible input sampling rate for a minimum signal-to-noise ratio, means for comparing the smaller of these rates with the said first input sampling rate, and means whereby the said first input sampling rate is used to calculate read and write multiplier factors and division factors if the said trial value is less than the selected one of the said two frequencies, and whereby if the said first input sampling rate is greater than the selected one of the said analogue-to-digital conversion rate and maximum rate for a given signal-to-noise ratio, a new input sampling rate is used which is equal to the said selected one of the said two rates, the store output sampling frequency and the said multiplier and division factors being then calculated from the new input sampling rate.

4. Image reproducing apparatus in accordance with any one of the preceding claims, in which the maximum input sampling rate for a given signalto-noise ratio is calculated as a function of the output cylinder speed of rotation, the enlargement, and the ratio of the diameters of the output and input cylinders.

5. Image reproducing apparatus, substantially as herein described with reference to the accompanying drawings.