At Borehamwood, we are developing three forms of photographic storage for a series working machine, the form of storage being chosen according to the required number of entries and maximum access time and according to whether serial or random access is needed.
For serial access (that is the reading out of entries in the order in which they have been recorded) and for a large number of entries, we use reels of 35 mm film such as the sample. Tracks across the film carry up to 50 binary digits. We find that the limitation in the number of digits across the track is in the increasing difficulty of making a scan linear in time across the film when reading out. We use 20 tracks to the inch so that on a 1,000 ft. reel of film, which is manageable, we have about 200,000 entries. Three kinds of data may be recorded on a film of this type. The film may carry entries of an analytical function table, it may record the output of the high-speed computer, or it may record a series of experimental measurements in binary form, which may later be fed to a computer.
To save the labour of working out the binary entries in the function table and not having access to a high-speed automatic machine, we have made a kind of Binary-Brunsviga. This is the machine which earlier this morning, Professor Newman said did not exist but which would have helped in the study of the Mersenne numbers before the advent of automatic machines. We can carry out any arithmetic operation in binary notation on this machine including multiplication and division, and in particular, we can use the machine for interpolation. To do this we have a controller which will run a preset number of increments into the accumulator. The state of the accumulator is copied on a lamp display and the photographic film store is made by photography of this lamp display.
The second form of input to the film will be from a high-speed computer at a high digit rate. A row of spark gaps will be triggered by the binary pulse output of the computer. We are planning to record up to one number per millisecond.
A third type of input will be a series of experimental data made available in binary form and recorded in a similar way. Reading of the film is done by a flying spot scanner and a photocell, the digit marks on the film being scanned at the digit rate of the computer. The photocell output gates clock pulses direct to the arithmetic unit or working store. Selection of entries is by counting revolutions of the film drive or by a binary serial number at the beginning of the entry. In this case, the computer programme calls for successive serial numbers and the difference between the number called for and the number read from the film controls a velodyne drive of the film. Thus the film speed adapts itself to the computer programme. This can only be done of course with a regular programme. We have to provide suppression of the reader output when the reader is between two numbers on the film or when the serial numbers on the film and in the computer programme differ.
The second type of photographic store will also handle a large number of entries but affords random access to the entries. A similar film is used but the film is driven until the serial number called for by the computer comes up and then the drive waits for the next instruction. With a long film, access time is quite long and a variation of this store is to transfer the data from the film to close packing on an annular track on a disc of say, 12" diameter. In this way, we can have about 20,000 entries of up to 50 digits. The 50 digits could, for instance be a 32 digit number and a 15 digit serial number for identification. As with the film, the disc is driven by the difference between serial numbers of computer and store, but by using this difference to deflect at right angles to itself the flying spot scan line, very short access time is obtained over entries near together in the store.
The third form of store is a slide affording rapid access to any one of a small number of entries, say from 10 to 100. Selection of entry may be by a binary serial number as in the film stores or in some circumstances, the entry may be defined by adjustment of the deflecting current or voltage of the C.R.T. scanner. The entries may be sets of constants or values of a function and differences.
The advantage of the photographic method of permanent and semi-permanent storage is that we can read at the machine's digit rate, and we hope also to write at this rate.
The use of the photographic disc store affords a useful compromise between the number of entries (say 20,000) and access time (say, one second).
