Texas Instruments TMS320C64X manual Complex Forward Mixed Radix 16- x 16-Bit FFT With Truncation

			DSP_fft16x16t
	Complex Forward Mixed Radix 16- x 16-Bit FFT With Truncation
DSP_fft16x16t
Function
		void DSP_fft16x16t(const short * restrict w, int nx, short * restrict x, short * re-
		strict y)
Arguments		w[2*nx]	Pointer to complex Q.15 FFT coefficients.
		nx	Length of FFT in complex samples. Must be power of 2 or 4
			, and 16 ≤ nx ≤ 32768.
		x[2*nx]	Pointer to complex 16-bit data input.
		y[2*nx]	Pointer to complex 16-bit data output.
Description		This routine computes a complex forward mixed radix FFT with truncation and
		digit reversal. Input data x[ ], output data y[ ], and coefficients w[ ] are 16-bit.
		The output is returned in the separate array y[ ] in normal order. Each complex
		value is stored with interleaved real and imaginary parts. The code uses a
		special ordering of FFT coefficients (also called twiddle 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.	*/

/*−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−−*/ #if TMS320C6X

C64x+ DSPLIB Reference 4-107