CMSISDSP
Version 1.5.2
CMSIS DSP Software Library

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 floatingpoint FIR decimator. More...  
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 CortexM3 and CortexM4. More...  
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 CortexM3 and CortexM4. More...  
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 floatingpoint FIR decimator. More...  
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. More...  
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. More...  
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. More...  
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. More...  
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:
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 M1
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 floatingpoint data types.
y[n] = b[0] * x[n] + b[1] * x[n1] + b[2] * x[n2] + ...+ b[numTaps1] * x[nnumTaps+1]where,
b[n]
are the filter coefficients. pCoeffs
points to a coefficient array of size numTaps
. Coefficients are stored in time reversed order. {b[numTaps1], b[numTaps2], b[N2], ..., 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[nnumTaps+1], x[nnumTaps], x[nnumTaps1], x[nnumTaps2]....x[0], x[1], ..., x[blockSize1]}The state variables are updated after each block of data is processed, the coefficients are untouched.
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.void arm_fir_decimate_f32  (  const arm_fir_decimate_instance_f32 *  S, 
float32_t *  pSrc,  
float32_t *  pDst,  
uint32_t  blockSize  
) 
[in]  *S  points to an instance of the floatingpoint 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. 
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  
) 
[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. 
Scaling and Overflow Behavior:
arm_fir_decimate_q15()
for a slower implementation of this function which uses 64bit 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, __SMLAD(), 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  
) 
[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. 
Scaling and Overflow Behavior:
arm_fir_decimate_q31()
for a slower implementation of this function which uses a 64bit 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  
) 
[in,out]  *S  points to an instance of the floatingpoint 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. 
blockSize
is not a multiple of M
.Description:
pCoeffs
points to the array of filter coefficients stored in time reversed order: {b[numTaps1], b[numTaps2], b[N2], ..., b[1], b[0]}
pState
points to the array of state variables. pState
is of length numTaps+blockSize1
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  
) 
[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. 
blockSize
is not a multiple of M
.Description:
pCoeffs
points to the array of filter coefficients stored in time reversed order: {b[numTaps1], b[numTaps2], b[N2], ..., b[1], b[0]}
pState
points to the array of state variables. pState
is of length numTaps+blockSize1
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  
) 
[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. 
blockSize
is not a multiple of M
.Description:
pCoeffs
points to the array of filter coefficients stored in time reversed order: {b[numTaps1], b[numTaps2], b[N2], ..., b[1], b[0]}
pState
points to the array of state variables. pState
is of length numTaps+blockSize1
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  
) 
[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. 
Scaling and Overflow Behavior:
arm_fir_decimate_fast_q15()
for a faster but less precise implementation of this function for CortexM3 and CortexM4. References 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  
) 
[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. 
Scaling and Overflow Behavior:
arm_fir_decimate_fast_q31()
for a faster but less precise implementation of this function for CortexM3 and CortexM4. 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.