DCT4 Q31

Functions
RISCV_DSP_ATTRIBUTE riscv_status	riscv_dct4_init_q31 (riscv_dct4_instance_q31 *S, riscv_rfft_instance_q31 *S_RFFT, riscv_cfft_radix4_instance_q31 *S_CFFT, uint16_t N, uint16_t Nby2, q31_t normalize)
	Initialization function for the Q31 DCT4/IDCT4. More...
RISCV_DSP_ATTRIBUTE void	riscv_dct4_q31 (const riscv_dct4_instance_q31 *S, q31_t *pState, q31_t *pInlineBuffer)
	Processing function for the Q31 DCT4/IDCT4. More...

Detailed Description

Function Documentation

riscv_dct4_init_q31()

RISCV_DSP_ATTRIBUTE riscv_status riscv_dct4_init_q31	(	riscv_dct4_instance_q31 *	S,
		riscv_rfft_instance_q31 *	S_RFFT,
		riscv_cfft_radix4_instance_q31 *	S_CFFT,
		uint16_t	N,
		uint16_t	Nby2,
		q31_t	normalize
	)

Initialization function for the Q31 DCT4/IDCT4.

Deprecated:

Do not use this function. It will be removed in future versions.

Parameters

[in,out]	S	points to an instance of Q31 DCT4/IDCT4 structure.
[in]	S_RFFT	points to an instance of Q31 RFFT/RIFFT structure
[in]	S_CFFT	points to an instance of Q31 CFFT/CIFFT structure
[in]	N	length of the DCT4.
[in]	Nby2	half of the length of the DCT4.
[in]	normalize	normalizing factor.

Returns

execution status

• RISCV_MATH_SUCCESS : Operation successful
• RISCV_MATH_ARGUMENT_ERROR : N is not a supported transform length

Normalizing factor:

The normalizing factor is sqrt(2/N), which depends on the size of transform N. Normalizing factors in 1.31 format are mentioned in the table below for different DCT sizes:

DCT Size	Normalizing factor value (hexadecimal)
2048	0x4000000
512	0x8000000
128	0x10000000

riscv_dct4_q31()

RISCV_DSP_ATTRIBUTE void riscv_dct4_q31	(	const riscv_dct4_instance_q31 *	S,
		q31_t *	pState,
		q31_t *	pInlineBuffer
	)

Processing function for the Q31 DCT4/IDCT4.

Deprecated:

Do not use this function. It will be removed in future versions.

Parameters

[in]	S	points to an instance of the Q31 DCT4 structure.
[in]	pState	points to state buffer.
[in,out]	pInlineBuffer	points to the in-place input and output buffer.

Input an output formats

Input samples need to be downscaled by 1 bit to avoid saturations in the Q31 DCT process, as the conversion from DCT2 to DCT4 involves one subtraction. Internally inputs are downscaled in the RFFT process function to avoid overflows. Number of bits downscaled, depends on the size of the transform. The input and output formats for different DCT sizes and number of bits to upscale are mentioned in the table below:

DCT Size	Input format	Output format	Number of bits to upscale
2048	2.30	12.20	11
512	2.30	10.22	9
128	2.30	8.24	7