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Project Ne10
An Open Optimized Software Library Project for the ARM Architecture
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Functions | |
void | ne10_fir_interpolate_float_c (const ne10_fir_interpolate_instance_f32_t *S, ne10_float32_t *pSrc, ne10_float32_t *pDst, ne10_uint32_t blockSize) |
Processing function for the floating-point FIR interpolator. More... | |
L
, the signal should be filtered by a lowpass filter with a normalized cutoff frequency of 1/L
in order to eliminate high frequency copies of the spectrum. The user of the function is responsible for providing the filter coefficients.The FIR interpolator functions provided in the CMSIS DSP Library combine the upsampler and FIR filter in an efficient manner. The upsampler inserts L-1
zeros between each sample. Instead of multiplying by these zero values, the FIR filter is designed to skip them. This leads to an efficient implementation without any wasted effort. 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*L
output values.
The library provides functions for floating-point data types.
y[n] = b[0] * x[n] + b[L] * x[n-1] + ... + b[L*(phaseLength-1)] * x[n-phaseLength+1] y[n+1] = b[1] * x[n] + b[L+1] * x[n-1] + ... + b[L*(phaseLength-1)+1] * x[n-phaseLength+1] ... y[n+(L-1)] = b[L-1] * x[n] + b[2*L-1] * x[n-1] + ....+ b[L*(phaseLength-1)+(L-1)] * x[n-phaseLength+1]This approach is more efficient than straightforward upsample-then-filter algorithms. With this method the computation is reduced by a factor of
1/L
when compared to using a standard FIR filter. pCoeffs
points to a coefficient array of size numTaps
. numTaps
must be a multiple of the interpolation factor L
and this is checked by the initialization functions. Internally, the function divides the FIR filter's impulse response into shorter filters of length phaseLength=numTaps/L
. 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 blockSize + phaseLength - 1
. Samples in the state buffer are stored in the order: {x[n-phaseLength+1], x[n-phaseLength], x[n-phaseLength-1], x[n-phaseLength-2]....x[0], x[1], ..., x[blockSize-1]}The state variables are updated after each block of data is processed, the coefficients are untouched.
ne10_fir_interpolate_instance_f32_t S = {L, phaseLength, pCoeffs, pState};where
L
is the interpolation factor; phaseLength=numTaps/L
is the length of each of the shorter FIR filters used internally, 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 ne10_fir_interpolate_float_c | ( | const ne10_fir_interpolate_instance_f32_t * | S, |
ne10_float32_t * | pSrc, | ||
ne10_float32_t * | pDst, | ||
ne10_uint32_t | blockSize | ||
) |
Processing function for the floating-point FIR interpolator.
[in] | *S | points to an instance of the floating-point FIR interpolator 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. |
Definition at line 712 of file NE10_fir.c.