Real FFT Q15 Functions

Macros
#define	RFFTINIT_Q15(LEN, CFFTLEN, TWIDMOD)

Functions
RISCV_DSP_ATTRIBUTE riscv_status	riscv_rfft_init_q15 (riscv_rfft_instance_q15 *S, uint32_t fftLenReal, uint32_t ifftFlagR, uint32_t bitReverseFlag)
	Initialization function for the 8192 pt Q15 real FFT. More...
RISCV_DSP_ATTRIBUTE void	riscv_rfft_q15 (const riscv_rfft_instance_q15 *S, q15_t *pSrc, q15_t *pDst)
	Processing function for the Q15 RFFT/RIFFT. More...

Detailed Description

Macro Definition Documentation

RFFTINIT_Q15

#define RFFTINIT_Q15	(		LEN,
			CFFTLEN,
			TWIDMOD
	)

Function Documentation

riscv_rfft_init_q15()

RISCV_DSP_ATTRIBUTE riscv_status riscv_rfft_init_q15	(	riscv_rfft_instance_q15 *	S,
		uint32_t	fftLenReal,
		uint32_t	ifftFlagR,
		uint32_t	bitReverseFlag
	)

Initialization function for the 8192 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 4096 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 2048 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 1024 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 512 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 256 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 128 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 64 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Initialization function for the 32 pt Q15 real FFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

Generic initialization function for the Q15 RFFT/RIFFT.

Parameters

[in,out]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	fftLenReal	length of the FFT
[in]	ifftFlagR	flag that selects transform direction value = 0: forward transform value = 1: inverse transform
[in]	bitReverseFlag	flag that enables / disables bit reversal of output value = 0: disables bit reversal of output value = 1: enables bit reversal of output

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : fftLenReal is not a supported length

Details

The parameter fftLenReal specifies length of RFFT/RIFFT Process. Supported FFT Lengths are 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192.

The parameter ifftFlagR controls whether a forward or inverse transform is computed. Set(=1) ifftFlagR to calculate RIFFT, otherwise RFFT is calculated.

The parameter bitReverseFlag controls whether output is in normal order or bit reversed order. Set(=1) bitReverseFlag for output to be in normal order otherwise output is in bit reversed order.

This function also initializes Twiddle factor table.

This function should be used only if you don't know the FFT sizes that you'll need at build time. The use of this function will prevent the linker from removing the FFT tables that are not needed and the library code size will be bigger than needed.

If you use NMSIS-DSP as a static library, and if you know the FFT sizes that you need at build time, then it is better to use the initialization functions defined for each FFT size.

riscv_rfft_q15()

RISCV_DSP_ATTRIBUTE void riscv_rfft_q15	(	const riscv_rfft_instance_q15 *	S,
		q15_t *	pSrc,
		q15_t *	pDst
	)

Processing function for the Q15 RFFT/RIFFT.

Parameters

[in]	S	points to an instance of the Q15 RFFT/RIFFT structure
[in]	pSrc	points to input buffer (Source buffer is modified by this function.)
[out]	pDst	points to output buffer

Input an output formats

Internally input is downscaled by 2 for every stage to avoid saturations inside CFFT/CIFFT process. Hence the output format is different for different RFFT sizes. The input and output formats for different RFFT sizes and number of bits to upscale are mentioned in the tables below for RFFT and RIFFT:

Input and Output formats for RFFT Q15

RFFT Size	Input Format	Output Format	Number of bits to upscale
32	1.15	6.10	5
64	1.15	7.9	6
128	1.15	8.8	7
256	1.15	9.7	8
512	1.15	10.6	9
1024	1.15	11.5	10
2048	1.15	12.4	11
4096	1.15	13.3	12
8192	1.15	14.2	13

Input and Output formats for RIFFT Q15

RIFFT Size	Input Format	Output Format	Number of bits to upscale
32	1.15	6.10	0
64	1.15	7.9	0
128	1.15	8.8	0
256	1.15	9.7	0
512	1.15	10.6	0
1024	1.15	11.5	0
2048	1.15	12.4	0
4096	1.15	13.3	0
8192	1.15	14.2	0

If the input buffer is of length N (fftLenReal), the output buffer must have length 2N since it is containing the conjugate part. The input buffer is modified by this function.

For the RIFFT, the source buffer must have length N+2 since the Nyquist frequency value is needed but conjugate part is ignored. It is not using the packing trick of the float version.