Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/5

 (7) Various ‘trees’ required in connection with LC and CA for the selection of the information required at any moment. These trees require much more valve equipment than LC and CA themselves.

(8) The clock (CL). This provides pulses, probably at a recurrence frequency of a megacycle, which are applied, together with gating signals, to the grids of most of the valves. It provides the synchronisation for the whole calculator.

(9) Temperature control system for the delay lines. This is a somewhat mundane matter, but is important.

(10) Binary to decimal and decimal to binary converters. These will have virtually no outward and visible form. They are mentioned here, lest it be thought they have been forgotten.

(11) Starting device.

(12) Power supply.

 3. Storages.

(i) The storage problem. As was explained in § 1 it is necessary for the calculator to have a memory or information storage. Actually this appears to be the main limitation in the design of a calculator, i.e. if the storage problem can be solved all the rest is comparatively straightforward. In the past it has not been possible to store very large quantities of information economically in such a way that the information is readily accessible. There were economical methods such as storage on five-unit tape, but with these the information was not readily accessible, especially if one wishes to jump from point to point. There were also forms with good accessibility, such as storage on relays and valves, but these were quite prohibitively uneconomical. There are now several possibilities for combining economy with accessibility which have been developed, or are being developed. In this section we describe the one which will most probably be used in the calculator.

(ii) Delay line storage. All forms of storage depend on modifying in some way the physical state of some storage medium. In the case of ‘delay line storage’ the medium consists of mercury, water, or some other liquid in a tube or tank, and we modify its state of compression at various points along the tube. This is done by forcing supersonic waves into the tube from one end. The state of the storage medium is not constant as it would be for instance if the storage medium were paper or magnetic tape. The information moves along the tube with the speed of sound. Unless we take some precautions the sound carrying the information will pass out of the end of the tube and be lost. We can effectively prevent this by detecting the sound in some way (some form of microphone) as it comes out, and amplifying it and putting it back at the beginning. The amplifying device must correct for the attenuation of the tube, and must also correct for any distortion of form caused by the transmission through the tube, otherwise after many passages through the tube the form will be eventually completely lost. We can only restore the form of the signal satisfactorily if the various possible ideal signal forms are quite distinct, for otherwise it will not be possible to distinguish between the undistorted form of one signal and a distorted form of another. The scheme actually proposed only recognizes 21024 distinct states of compression of the water medium, these being sequences of 1024 pulses of two different sizes, one of which will probably be zero. The amplifier at the end of the line always reshapes the signal to bring it back to the nearest ideal signal.

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