Texas Instruments TMS320C64X manual DSPfft16x16r

DSP_fft16x16r

DSP_fft16x16r	Complex Forward Mixed Radix 16 x 16-bit FFT With Rounding
Function
		void DSP_fft16x16r(int nx, short * restrict x, const short * restrict w, const un-
		signed char * restrict brev, short * restrict y, int radix, int offset, int nmax)
Arguments		nx	Length of FFT in complex samples. Must be power of 2 or 4, and
			≤16384
		x[2*nx]	Pointer to complex 16-bit data input
		w[2*nx]	Pointer to complex FFT coefficients
		brev[64]	Pointer to bit reverse table containing 64 entries. Only required for
			C code. Use NULL for assembly code since BITR instruction
			is used instead.
		y[2*nx]	Pointer to complex 16-bit data output
		radix	Smallest FFT butterfly used in computation used for
			decomposing FFT into sub-FFTs. See notes.
		offset	Index in complex samples of sub-FFT from start of main FFT.
		nmax	Size of main FFT in complex samples.
Description		This routine implements a cache-optimized complex forward mixed radix FFT
		with scaling, rounding 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 as interleaved 16-bit real and imaginary parts. The code uses a special ordering of FFT coefficients (also called twiddle factors).

This redundant set of twiddle factors is size 2*N short samples. As pointed out in subsequent sections, dividing these twiddle factors by 2 will give an effective divide by 4 at each stage to guarantee no overflow. The function is accurate to about 68dB of signal to noise ratio to the DFT function as follows.

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