Texas Instruments TMS320C64X manual DSPfft, Complex Forward FFT With Digital Reversal

DSP_fft

DSP_fft	Complex Forward FFT With Digital Reversal
Function
		void DSP_fft (const short * restrict w, int nx, short * restrict x, short * restrict y)
Arguments		w[2*nx]	Pointer to vector of Q.15 FFT coefficients of size 2 * nx
			elements. Must be double-word aligned.
		nx	Number of complex elements in vector x. Must be a power of
			4 and 4 ≤ nx ≤ 65536.
		x[2*nx]	Pointer to input sequence of size 2 * nx elements. Must be
			double-word aligned.
		y[2*nx]	Pointer to output sequence of size 2 * nx elements. Must be
			double-word aligned.
Description		This routine is used to compute an FFT of a complex sequence of size nx, a
		power of 4, with “decimation-in-frequency decomposition” method. The output
		is returned in a separate array y in normal order. This routine also performs
		digit reversal as a special last step. Each complex value is stored as
		interleaved 16-bit real and imaginary parts. The code uses a special ordering
		of FFT factors and memory accesses to improve performance in the presence
		of cache.
Algorithm		This is the C equivalent of the assembly code without restrictions. Note that
		the assembly code is hand optimized and restrictions may apply.

/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/ /* The following macro is used to obtain a digit reversed index, of a given */ /* number i, into j where the number of bits in ”i” is ”m”. For the natural */ /* form of C code, this is done by first interchanging every set of ”2 bit” */ /* pairs, followed by exchanging nibbles, followed by exchanging bytes, and */

/* finally halfwords. To give an example, consider the following number:	*/
/*	*/
/* N = FEDCBA9876543210, where each digit represents a bit, the following	*/
/* steps illustrate the changes as the exchanges are performed:	*/
/* M = DCFE98BA54761032 is the number after every ”2 bits” are exchanged.	*/
/* O = 98BADCFE10325476 is the number after every nibble is exchanged.	*/
/* P = 1032547698BADCFE is the number after every byte is exchanged.	*/
/* Since only 16 digits were considered this represents the digit reversed	*/
/* index. Since the numbers are represented as 32 bits, there is one more	*/
/* step typically of exchanging the half words as well.	*/

/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/

4-98