Finite Impulse Response (FIR) Decimator

Functions
void	arm_fir_decimate_f32 (const arm_fir_decimate_instance_f32 *S, float32_t *pSrc, float32_t *pDst, uint32_t blockSize)
	Processing function for the floating-point FIR decimator.
void	arm_fir_decimate_fast_q15 (const arm_fir_decimate_instance_q15 *S, q15_t *pSrc, q15_t *pDst, uint32_t blockSize)
	Processing function for the Q15 FIR decimator (fast variant) for Cortex-M3 and Cortex-M4.
void	arm_fir_decimate_fast_q31 (arm_fir_decimate_instance_q31 *S, q31_t *pSrc, q31_t *pDst, uint32_t blockSize)
	Processing function for the Q31 FIR decimator (fast variant) for Cortex-M3 and Cortex-M4.
arm_status	arm_fir_decimate_init_f32 (arm_fir_decimate_instance_f32 *S, uint16_t numTaps, uint8_t M, float32_t *pCoeffs, float32_t *pState, uint32_t blockSize)
	Initialization function for the floating-point FIR decimator.
arm_status	arm_fir_decimate_init_q15 (arm_fir_decimate_instance_q15 *S, uint16_t numTaps, uint8_t M, q15_t *pCoeffs, q15_t *pState, uint32_t blockSize)
	Initialization function for the Q15 FIR decimator.
arm_status	arm_fir_decimate_init_q31 (arm_fir_decimate_instance_q31 *S, uint16_t numTaps, uint8_t M, q31_t *pCoeffs, q31_t *pState, uint32_t blockSize)
	Initialization function for the Q31 FIR decimator.
void	arm_fir_decimate_q15 (const arm_fir_decimate_instance_q15 *S, q15_t *pSrc, q15_t *pDst, uint32_t blockSize)
	Processing function for the Q15 FIR decimator.
void	arm_fir_decimate_q31 (const arm_fir_decimate_instance_q31 *S, q31_t *pSrc, q31_t *pDst, uint32_t blockSize)
	Processing function for the Q31 FIR decimator.

Description

These functions combine an FIR filter together with a decimator. They are used in multirate systems for reducing the sample rate of a signal without introducing aliasing distortion. Conceptually, the functions are equivalent to the block diagram below:

Components included in the FIR Decimator functions

When decimating by a factor of M, the signal should be prefiltered by a lowpass filter with a normalized cutoff frequency of 1/M in order to prevent aliasing distortion. The user of the function is responsible for providing the filter coefficients.

The FIR decimator functions provided in the CMSIS DSP Library combine the FIR filter and the decimator in an efficient manner. Instead of calculating all of the FIR filter outputs and discarding M-1 out of every M, only the samples output by the decimator are computed. The functions operate on blocks of input and output data. pSrc points to an array of blockSize input values and pDst points to an array of blockSize/M output values. In order to have an integer number of output samples blockSize must always be a multiple of the decimation factor M.

The library provides separate functions for Q15, Q31 and floating-point data types.

Algorithm:

The FIR portion of the algorithm uses the standard form filter:

    
     y[n] = b[0] * x[n] + b[1] * x[n-1] + b[2] * x[n-2] + ...+ b[numTaps-1] * x[n-numTaps+1]    
  

where, b[n] are the filter coefficients.

The pCoeffs points to a coefficient array of size numTaps. Coefficients are stored in time reversed order.

    
     {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}    
  

pState points to a state array of size numTaps + blockSize - 1. Samples in the state buffer are stored in the order:

    
     {x[n-numTaps+1], x[n-numTaps], x[n-numTaps-1], x[n-numTaps-2]....x[0], x[1], ..., x[blockSize-1]}    
  

The state variables are updated after each block of data is processed, the coefficients are untouched.

Instance Structure

The coefficients and state variables for a filter are stored together in an instance data structure. A separate instance structure must be defined for each filter. Coefficient arrays may be shared among several instances while state variable array should be allocated separately. There are separate instance structure declarations for each of the 3 supported data types.

Initialization Functions

There is also an associated initialization function for each data type. The initialization function performs the following operations:

• Sets the values of the internal structure fields.
• Zeros out the values in the state buffer.
• Checks to make sure that the size of the input is a multiple of the decimation factor. To do this manually without calling the init function, assign the follow subfields of the instance structure: numTaps, pCoeffs, M (decimation factor), pState. Also set all of the values in pState to zero.

Use of the initialization function is optional. However, if the initialization function is used, then the instance structure cannot be placed into a const data section. To place an instance structure into a const data section, the instance structure must be manually initialized. The code below statically initializes each of the 3 different data type filter instance structures

    
*arm_fir_decimate_instance_f32 S = {M, numTaps, pCoeffs, pState};    
*arm_fir_decimate_instance_q31 S = {M, numTaps, pCoeffs, pState};    
*arm_fir_decimate_instance_q15 S = {M, numTaps, pCoeffs, pState};    
  

where M is the decimation factor; numTaps is the number of filter coefficients in the filter; pCoeffs is the address of the coefficient buffer; pState is the address of the state buffer. Be sure to set the values in the state buffer to zeros when doing static initialization.

Fixed-Point Behavior

Care must be taken when using the fixed-point versions of the FIR decimate filter functions. In particular, the overflow and saturation behavior of the accumulator used in each function must be considered. Refer to the function specific documentation below for usage guidelines.

Function Documentation

void arm_fir_decimate_f32	(	const arm_fir_decimate_instance_f32 *	S,
		float32_t *	pSrc,
		float32_t *	pDst,
		uint32_t	blockSize
	)

Parameters

[in]	*S	points to an instance of the floating-point FIR decimator structure.
[in]	*pSrc	points to the block of input data.
[out]	*pDst	points to the block of output data.
[in]	blockSize	number of input samples to process per call.

Returns

none.

References arm_fir_decimate_instance_f32::M, arm_fir_decimate_instance_f32::numTaps, arm_fir_decimate_instance_f32::pCoeffs, and arm_fir_decimate_instance_f32::pState.

void arm_fir_decimate_fast_q15	(	const arm_fir_decimate_instance_q15 *	S,
		q15_t *	pSrc,
		q15_t *	pDst,
		uint32_t	blockSize
	)

Parameters

[in]	*S	points to an instance of the Q15 FIR decimator structure.
[in]	*pSrc	points to the block of input data.
[out]	*pDst	points to the block of output data
[in]	blockSize	number of input samples to process per call.

Returns

none

Restrictions

If the silicon does not support unaligned memory access enable the macro UNALIGNED_SUPPORT_DISABLE In this case input, output, state buffers should be aligned by 32-bit

Scaling and Overflow Behavior:

This fast version uses a 32-bit accumulator with 2.30 format. The accumulator maintains full precision of the intermediate multiplication results but provides only a single guard bit. Thus, if the accumulator result overflows it wraps around and distorts the result. In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits (log2 is read as log to the base 2). The 2.30 accumulator is then truncated to 2.15 format and saturated to yield the 1.15 result.

Refer to the function arm_fir_decimate_q15() for a slower implementation of this function which uses 64-bit accumulation to avoid wrap around distortion. Both the slow and the fast versions use the same instance structure. Use the function arm_fir_decimate_init_q15() to initialize the filter structure.

References __SIMD32, arm_fir_decimate_instance_q15::M, arm_fir_decimate_instance_q15::numTaps, arm_fir_decimate_instance_q15::pCoeffs, and arm_fir_decimate_instance_q15::pState.

void arm_fir_decimate_fast_q31	(	arm_fir_decimate_instance_q31 *	S,
		q31_t *	pSrc,
		q31_t *	pDst,
		uint32_t	blockSize
	)

Parameters

[in]	*S	points to an instance of the Q31 FIR decimator structure.
[in]	*pSrc	points to the block of input data.
[out]	*pDst	points to the block of output data
[in]	blockSize	number of input samples to process per call.

Returns

none

Scaling and Overflow Behavior:

This function is optimized for speed at the expense of fixed-point precision and overflow protection. The result of each 1.31 x 1.31 multiplication is truncated to 2.30 format. These intermediate results are added to a 2.30 accumulator. Finally, the accumulator is saturated and converted to a 1.31 result. The fast version has the same overflow behavior as the standard version and provides less precision since it discards the low 32 bits of each multiplication result. In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits (where log2 is read as log to the base 2).

Refer to the function arm_fir_decimate_q31() for a slower implementation of this function which uses a 64-bit accumulator to provide higher precision. Both the slow and the fast versions use the same instance structure. Use the function arm_fir_decimate_init_q31() to initialize the filter structure.

References arm_fir_decimate_instance_q31::M, arm_fir_decimate_instance_q31::numTaps, arm_fir_decimate_instance_q31::pCoeffs, and arm_fir_decimate_instance_q31::pState.

arm_status arm_fir_decimate_init_f32	(	arm_fir_decimate_instance_f32 *	S,
		uint16_t	numTaps,
		uint8_t	M,
		float32_t *	pCoeffs,
		float32_t *	pState,
		uint32_t	blockSize
	)

Parameters

[in,out]	*S	points to an instance of the floating-point FIR decimator structure.
[in]	numTaps	number of coefficients in the filter.
[in]	M	decimation factor.
[in]	*pCoeffs	points to the filter coefficients.
[in]	*pState	points to the state buffer.
[in]	blockSize	number of input samples to process per call.

Returns

The function returns ARM_MATH_SUCCESS if initialization was successful or ARM_MATH_LENGTH_ERROR if blockSize is not a multiple of M.

Description:

pCoeffs points to the array of filter coefficients stored in time reversed order:

    
   {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}    

pState points to the array of state variables. pState is of length numTaps+blockSize-1 words where blockSize is the number of input samples passed to arm_fir_decimate_f32(). M is the decimation factor.

References ARM_MATH_LENGTH_ERROR, ARM_MATH_SUCCESS, arm_fir_decimate_instance_f32::M, arm_fir_decimate_instance_f32::numTaps, arm_fir_decimate_instance_f32::pCoeffs, arm_fir_decimate_instance_f32::pState, and status.

arm_status arm_fir_decimate_init_q15	(	arm_fir_decimate_instance_q15 *	S,
		uint16_t	numTaps,
		uint8_t	M,
		q15_t *	pCoeffs,
		q15_t *	pState,
		uint32_t	blockSize
	)

Parameters

[in,out]	*S	points to an instance of the Q15 FIR decimator structure.
[in]	numTaps	number of coefficients in the filter.
[in]	M	decimation factor.
[in]	*pCoeffs	points to the filter coefficients.
[in]	*pState	points to the state buffer.
[in]	blockSize	number of input samples to process per call.

Returns

The function returns ARM_MATH_SUCCESS if initialization was successful or ARM_MATH_LENGTH_ERROR if blockSize is not a multiple of M.

Description:

pCoeffs points to the array of filter coefficients stored in time reversed order:

    
   {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}    

pState points to the array of state variables. pState is of length numTaps+blockSize-1 words where blockSize is the number of input samples to the call arm_fir_decimate_q15(). M is the decimation factor.

References ARM_MATH_LENGTH_ERROR, ARM_MATH_SUCCESS, arm_fir_decimate_instance_q15::M, arm_fir_decimate_instance_q15::numTaps, arm_fir_decimate_instance_q15::pCoeffs, arm_fir_decimate_instance_q15::pState, and status.

arm_status arm_fir_decimate_init_q31	(	arm_fir_decimate_instance_q31 *	S,
		uint16_t	numTaps,
		uint8_t	M,
		q31_t *	pCoeffs,
		q31_t *	pState,
		uint32_t	blockSize
	)

Parameters

[in,out]	*S	points to an instance of the Q31 FIR decimator structure.
[in]	numTaps	number of coefficients in the filter.
[in]	M	decimation factor.
[in]	*pCoeffs	points to the filter coefficients.
[in]	*pState	points to the state buffer.
[in]	blockSize	number of input samples to process per call.

Returns

The function returns ARM_MATH_SUCCESS if initialization was successful or ARM_MATH_LENGTH_ERROR if blockSize is not a multiple of M.

Description:

pCoeffs points to the array of filter coefficients stored in time reversed order:

    
   {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}    

pState points to the array of state variables. pState is of length numTaps+blockSize-1 words where blockSize is the number of input samples passed to arm_fir_decimate_q31(). M is the decimation factor.

References ARM_MATH_LENGTH_ERROR, ARM_MATH_SUCCESS, arm_fir_decimate_instance_q31::M, arm_fir_decimate_instance_q31::numTaps, arm_fir_decimate_instance_q31::pCoeffs, arm_fir_decimate_instance_q31::pState, and status.

void arm_fir_decimate_q15	(	const arm_fir_decimate_instance_q15 *	S,
		q15_t *	pSrc,
		q15_t *	pDst,
		uint32_t	blockSize
	)

Parameters

[in]	*S	points to an instance of the Q15 FIR decimator structure.
[in]	*pSrc	points to the block of input data.
[out]	*pDst	points to the location where the output result is written.
[in]	blockSize	number of input samples to process per call.

Returns

none.

Scaling and Overflow Behavior:

The function is implemented using a 64-bit internal accumulator. Both coefficients and state variables are represented in 1.15 format and multiplications yield a 2.30 result. The 2.30 intermediate results are accumulated in a 64-bit accumulator in 34.30 format. There is no risk of internal overflow with this approach and the full precision of intermediate multiplications is preserved. After all additions have been performed, the accumulator is truncated to 34.15 format by discarding low 15 bits. Lastly, the accumulator is saturated to yield a result in 1.15 format.

Refer to the function arm_fir_decimate_fast_q15() for a faster but less precise implementation of this function for Cortex-M3 and Cortex-M4.

References __SIMD32, arm_fir_decimate_instance_q15::M, arm_fir_decimate_instance_q15::numTaps, arm_fir_decimate_instance_q15::pCoeffs, and arm_fir_decimate_instance_q15::pState.

void arm_fir_decimate_q31	(	const arm_fir_decimate_instance_q31 *	S,
		q31_t *	pSrc,
		q31_t *	pDst,
		uint32_t	blockSize
	)

Parameters

[in]	*S	points to an instance of the Q31 FIR decimator structure.
[in]	*pSrc	points to the block of input data.
[out]	*pDst	points to the block of output data
[in]	blockSize	number of input samples to process per call.

Returns

none

Scaling and Overflow Behavior:

The function is implemented using an internal 64-bit accumulator. The accumulator has a 2.62 format and maintains full precision of the intermediate multiplication results but provides only a single guard bit. Thus, if the accumulator result overflows it wraps around rather than clip. In order to avoid overflows completely the input signal must be scaled down by log2(numTaps) bits (where log2 is read as log to the base 2). After all multiply-accumulates are performed, the 2.62 accumulator is truncated to 1.32 format and then saturated to 1.31 format.

Refer to the function arm_fir_decimate_fast_q31() for a faster but less precise implementation of this function for Cortex-M3 and Cortex-M4.

References arm_fir_decimate_instance_q31::M, arm_fir_decimate_instance_q31::numTaps, arm_fir_decimate_instance_q31::pCoeffs, and arm_fir_decimate_instance_q31::pState.