Patent GB-823625-A (GB)
Title: Resolution restorer for electro-optical scanning systems
IPC: H04N1/403
Filed: 1958-02-06
Abstract: 823,625. Copying by scanning. TIME Inc. Feb. 6, 1958 [Feb. 6, 1957], No. 3864/58. Class 40 (3). Relates to apparatus for correcting for loss of resolution in an electro-optical scanning system. The principle of the invention is illustrated in Fig. 6 with reference to an aperture 12 which is scanned across a pattern of light and dark bars with the dimensions shown, and is based on a realization that each width of bar will give rise to a different level of signal as indicated in curve B. By examining the widths of the signal portions at the levels 121a, 121b and 121c it is possible to apply correcting delays to the front and back edges of the portions and to derive therefrom a new signal free from the loss of resolution due to the finite size of aperture 12. The operation involves the derivation in separate channels of square waves C, D and E corresponding respectively to the signal at levels 121a, 121b and 121c (negative version of the waves also being shown at F, G and H), and from these the derivation of impulses as shown in curves I-M corresponding to the front and back edges of the signal portions. These impulses control the production of corresponding but delayed impulses as shown in curves 0 to T. All the delayed front edge and back edges pulses are then combined in common channels to produce impulse trains as shown in curves W and X. One impulse train is applied to operate a trigger circuit in one direction, whilst the other impulse train is applied to operate the circuit in the other direction. The resulting waveform from the circuit is shown in curve Y and corresponds to the original pattern of bars without loss of resolution. The delays applied to impulse trains 0-T are critical and the Specification analyses the required delays for the bar dimensions shown. For example, considering the left-hand black bar, the time duration of the corresponding signal portion, as indicated by the interval between impulses 73a, 94a<1>, is 6 units instead of the correct value of 2. Correction is applied by delaying the front edge by 4 units, pulse 94a, curve 0 and retaining the back edge without delay, pulse 94a<1>, curve P. In the final resulting waveform a corrected impulse 110, curve Y, appears but with an overall delay of 4 units. Correcting delays for the other impulses are derived in a similar manner, allowance being made for the initial delay of 4 units referred to above and the fact that waves D and E are initially delayed with respect to wave C. As illustrated in Fig. 6 the delays are chosen to overcorrect so that the impulses forming wave Y are shorter than they should be. This is done to allow for further loss of resolution which may occur in reproduction. The effect is shown in curve Z where a reproducing aperture 101 of unit dimension is assumed and the impulses forming wave Y are reduced accordingly by one unit. Detailed circuits for carrying out the invention are described with reference to Figs. 5 and 5a (not shown). The examination at levels 121a-121c, curve B, to produce waves C-H is effected with the aid of diode slicer circuits followed by limiting amplifiers. The front and back edge impulses are derived by differentiation and the delays are introduced by multivibrators, although reference is also made to the use of inductancecapacitance delay networks. It is stated that the resolution restoring capacity may be increased by increasing the number of level detecting channels.