Page:Alan Turing - Proposed Electronic Calculator (1945).pdf/6

 Alternatively we may consider the delay line simply as providing a delay, as its name implies. We may put a signal into the line, and it is returned to us after a certain definite delay. If we wish to make use of the information contained in it when it comes back after being delayed we do so. Otherwise we just delay it again, and repeat until we do require it. This aspect loses sight of the fact that there is still a storage medium of some kind, with a variety of states according to the information stored.

There are, of course, other forms of delay line than those using acoustic waves.

(iii) Technical proposals for delay line. Let us now be more specific. It is proposed to build ‘delay line’ units consisting of mercury or water tubes about 5’ long and 1” diameter in contact with a quartz crystal at each end. The velocity of sound in either mercury or water is such that the delay will be 1.024 ms. The information to be stored may be considered to be a sequence of 1024 ‘digits’ (0 or 1), or ‘modulation elements’ (mark or space). These digits will be represented by a corresponding sequence of pulses. The digit 0 (or space) will be represented by the absence of a pulse at the appropriate time, the digit 1 (or mark) by its presence. This series of pulses is impressed on the end of the line by one piece-crystal, it is transmitted down the line in the form of supersonic waves, and is reconverted into a varying voltage by the crystal at the far end. This voltage is amplified sufficiently to give an output of the order of 10 volts peak to peak and is used to gate a standard pulse generated by the clock. This pulse may be again fed into the line by means of the transmitting crystal, or we may feed in some altogether different signal. We also have the possibility of leading the gated pulse to some other part of the calculator, if we have need of that information at the time. Making use of the information does not of course preclude keeping it also. The figures above imply of course that the interval between digits is 1 μs.

It is probable that the pulses will be sent down the line as modulation on a carrier, possibly at a frequency of 15 Mc/s.

(iv) Effects of temperature variations.–The temperature coefficient of the velocity of sound in mercury is quite small at high frequencies. If we keep the temperatures of the tanks correct to within one degree Fahrenheit it will be sufficient. It is only necessary to keep the tanks nearly at equal temperatures. We do not need to keep them all at a definite temperature: variations in the temperature of the room as a whole may be corrected by altering the clock frequency.

 4. Arithmetical Considerations.

(i) Minor cycles.–It is intended to divide the information in the storages up into units, probably of 32 digits or thereabouts. Such a storage will be appropriate for carrying a single real number as a binary decimal or for carrying a single instruction. Each sub-storage of this kind is called a minor cycle or word. The longer storages of length about 1000 digits are called major cycles. It will be assumed for definiteness that the length of the minor cycle is 32 and that of the major 1024, although these need not yet be fixed.

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