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FFTR2CC.HLP
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1990-01-17
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4KB
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62 lines
Name: FFTR2CC.ASM
Type: Assembler Macro
Version: 1.0
Last Change: 18-Aug-88
Description: Radix 2, In-place, Decimation-in-time Complex FFT Macro
This macro is a slightly faster version of the macro FFTR2C. It uses a
special pipelining technique which overlaps butterflies to obtain faster
overall execution, at a small cost in extra program length. The remaining
part of this description is identical to the one of the macro FFTR2C.
This macro performs a complete Fast Fourier Transform (FFT) on complex
data. The basic algorithm is the Decimation-in-time (DIT), Radix 2
FFT algorithm using 24 bit fixed-point arithmetic. The algorithm uses
a sine-cosine lookup table for the FFT coefficients (twiddle factors).
FFTR2CC can be called to perform any FFT from 16-32768 points. Simply
call it with the arguments of number of FFT points, location of the
data array and location of the sine-cosine table. All register
initialization is performed by this macro. However, the macro assumes
that registers which should not be altered by the FFT have already been
saved by the main program. This allows the user to fit the FFT macro
into his application and thus control the context switching overhead.
No data scaling is performed and no overflow detection is done.
Modifications to this routine could allow it to be used with the
scaling modes and thus allow dynamic scaling for each FFT pass.
All data and coefficients are complex, with the real part in X Data
memory and the imaginary part in Y Data memory. For an N point FFT,
the data buffer requires N X Data and N Y Data memory locations.
The algorithm is performed "in-place", meaning that only one data
buffer is required for both input and output data. The input
data is assumed to be in normal (time-sequential) order and the
output is in bit-reversed order. By using the reverse-carry
address modifier and a separate output data buffer, the output
data may be easily unscrambled. Other methods also exist to
unscramble the output data without a separate output data buffer.
The FFTR2CC macro uses "twiddle factors" (-cosine and -sine tables)
stored in data memory. For maximum speed, the FFT macro performs
a lookup table operation to get new sine and cosine values for
each group of butterflies. A SINCOS macro is available to
generate these tables. For an N point FFT, N/2 X Data and N/2
Y Data locations are required. Sine and cosine values could be
calculated in real-time to save data memory at the expense of
execution time.
The FFTR2CC macro is faster than the FFTR2A and FFTR2B library
macros. The speed increase is obtained by performing the first
two passes in one combined operation without storing the results
of the first pass. The first two passes do not require any
multiplies since the coefficients are -1, 0 or +1. The last pass
is also split out from the triple nested DO loop and given a separate
DO loop (identical to the FFTR2B macro). The reason this is faster
is that the inner loop always starts with a loop count of 1 on the
last pass. Note that the separate last pass DO loop uses different
addressing modes to increment through the butterflies, thus avoiding
outer loops. Additional details are included in the source file;
however, more algorithm description would be required for complete
understanding by typical users. The FFTR2CC macro can directly
replace the FFTR2A macro using the calling procedure shown in the
FFTR2AT test program.
^Z