Texas Instruments TMS320C64X manual Preventing overflow

DSP_fft32x32s

-The FFT coefficients (twiddle factors) are generated using the program tw_fft32x32 provided in the directory ‘support\fft’. The scale factor must be 1073741823.5. The input data must be scaled by 2(log2(nx) − ceil[ log4(nx)−1 ]) to completely prevent overflow.

Implementation Notes

	- Bank Conflicts: No bank conflicts occur.
	- Interruptibility: The code is interruptible.
	- Scaling is performed at each stage by shifting the results right by 1,
	preventing overflow.
	- The routine uses log4(nx) − 1 stages of radix-4 transform and performs
	either a radix-2 or radix-4 transform on the last stage depending on nx. If
	nx is a power of 4, then this last stage is also a radix-4 transform, otherwise
	it is a radix-2 transform.
	- See the fft16x16t implementation notes, as similar ideas are used.
Benchmarks	Cycles	(13 * nx/8 + 36) * ceil[log4(nx) − 1] + 6 * nx/4 + 36
	Codesize	928 bytes

C64x+ DSPLIB Reference 4-29