Patent GB-1281730-A (GB)
Title: Fourier transform computer
IPC: G06F17/14
Filed: 1969-09-29
Abstract: 1281730 Digital computers; Fourier transforms; A/D converter TIME DATA CORP 22 Sept 1969 [28 Oct 1968] 47724/69 Headings G4A and G4H A digital computer can determine the Fourier transform of a digital or analog input signal. The computer can accept 1 or 2 analog or 1 or 2 digital signals on lines 1 to 4, an A/D converter (Fig. 6, not shown) being used for the analog signals. The digital signals are then transformed in processor 24 controlled by a read only instruction memory requiring no programming and held in store 22 whence they may pass through a D/A converter for display on a pushbutton, operator controlled, cathode ray oscilloscope. The transformation uses the technique of folding (described below) and two embodiments of the processor (Fig. 4 (not shown) and Fig. 5) are described. Applications mentioned include sound vibration, medicine, economics, defence and seismology and the computer may be used for other algorithms. A/D converter (Fig. 6).-Digital inputs on lines 3, 4 pass to magnetic core memory 22 while analog signals on lines 1, 2 pass through the A/D converter and thence to memory 22 after which they are treated in the same way as the digital inputs. The A/D converter is a feedback encoder comprising a ladder network in which the input is sampled and held on a capacitor. The sign is first determined then each bit in series, starting with the highest, is passed to a shift register from whence they are transferred to the memory in a parallel 8 bit word. The sampling period and sensitivity may be controlled by the operator. The processor.-This performs the Fourier transform where g(t) (the input signal) = 1/2##<SP>+#</SP> -# G(jw) e <SP>jwi</SP> dw It can be seen (Fig. 3, not shown) that both sine and cosine waveforms have half wave and quarter wave symmetry i.e. the waveform before one of these points can be "folded" to produce the waveform after said point. By making the fundamental sinusoidal frequency used in the computation equal to the frequency of the period of the input signal, the use of folding can reduce the amount of computation required considerably. Each frequency and the harmonics thereof is folded about its centre frequency (Fig. 2, not shown). In one processor (Computer I) 17 points of a signal are first folded to 9 points (the centre point + eight folds), then folded to 5 points (centre point + 4 folds), then folded to 3 points (Fig. 8, not shown). The other processor (Computer II) folds as for computer I until there is lack of symmetry then uses double complex folds (Fig. 9, not shown). These double complex folds are stated not to reduce computational time but instead to reduce the number of memory accesses, hence the total processing time. Computer I.-This comprises a digital multiplier 33, a function generator 35 (Fig. 7, not shown), which supplies digital representations of sine or cosine functions plus 500 harmonics, all sampled at increments corresponding in time to the timing of the digital data being transferred, adders 40, 42 for addition or subtraction, accumulators 43, 44 and a divider (sealer 47). Some of these circuits may be made of solid state integrated circuitry. The operation comprises nine cycles: input, first fold, computation of transform coefficients using odd cosine functions, computation of coefficients using odd sine functions, second fold, computation of coefficients using even cosine functions in the series f 0 , f 4 , f 8 , f 12  ..., computation using even sine functions in this series, computation of coefficients using even cosines in series f 2 , f 6 , f 10  ... and computation using even series in this series. Computer II (Fig. 4, not shown).-This can only process one data channel, two data channels can be processed by processor duplication. Units 32, 40, 46, 35, 34 are the same as for computer I and unit 43 is slightly modified. It also operates in nine cycles: input, first fold, cycles 3-5, single complex fold, cycles 6-8, double complex fold, and determination of transform coefficients from the last fold.