Fourier Transform Operations > Time Considerations

Transform Calculation Time Considerations

Since some large transforms take a long time on machines without a numeric co-processor, a status indicator in the form of a letter (A, C, D, F, L, M, S, or W) is displayed in the analysis reporting area to indicate the FFT operation currently in progress.

This feature is provided to indicate that the computer is currently active and to discourage you from activating the transform again, thereby queuing up more time-consuming requests and tying up the machine for several minutes. Another reason for providing the indicator is to indicate where the computer is spending its time through the course of a transform calculation. Using the table below as a guide, the “Time-minimizing Element” column defines what can be employed to minimize the time required to make the calculation:

 Letter Time-Minimizing Element Action A Computer type and speed Compute squared magnitudes, averaging by frequency smoothing factors. C Computer type and speed Change variable-point sine-cosine table when number of points changes for DFT of other than a power of 2 points. D Hard disk with fast access time Gather input data from disk or compute output data from IFT. F Computer type and speed Perform basic FFT algorithm or its inverse. L Co-processor Compute exponential look-up table for use in logarithmic plotting. M Computer type and speed Multiply input data by window or multiply transforms during indirect DFT of a number of points which is not a power of 2. S Computer type and speed Form Sine-Cosine table for basic FFT. This is done each time an FFT is needed which is two or more times larger than the previous largest. W Computer type and speed Compute Bartlett, hanning, Hamming, and Blackman windows. Done each time an FFT that is two or more times larger than previous one is performed.

L time, which is proportional to the height of the frequency part of the split screen, can be dramatically shortened by a numeric co-processor.

C time is roughly proportional to the number of transform points, and will show up each time the number of points changes for FFTs.

D time is proportional to the total number of transform points and the number of data points to be averaged per transform point. For forward transforms, it is also proportional to the number of channels in the file and the time

required for disk operations.

F, M, C, W, S, and A times are roughly proportional to the number of transform points and machine instruction times. S and W will have to be recalculated after memory de-allocation occurs to spawn another program, to perform a maximum compression, or to enter the X-Y mode. But, memory is retained after an Exit/Enter Split from the FFT mode to allow reentering with another Exit/Enter Split without a time penalty.