/* ----------------------------------------------------------------------
* Copyright (C) 2010 ARM Limited. All rights reserved.
*
* $Date: 29. November 2010
* $Revision: V1.0.3
*
* Project: CMSIS DSP Library
* Title: arm_fir_fast_q15.c
*
* Description: Q15 Fast FIR filter processing function.
*
* Target Processor: Cortex-M4/Cortex-M3
*
* Version 1.0.3 2010/11/29
* Re-organized the CMSIS folders and updated documentation.
*
* Version 1.0.2 2010/11/11
* Documentation updated.
*
* Version 1.0.1 2010/10/05
* Production release and review comments incorporated.
*
* Version 1.0.0 2010/09/20
* Production release and review comments incorporated.
*
* Version 0.0.9 2010/08/16
* Initial version
*
* -------------------------------------------------------------------- */
#include "arm_math.h"
/**
* @ingroup groupFilters
*/
/**
* @addtogroup FIR
* @{
*/
/**
* @param[in] *S points to an instance of the Q15 FIR filter structure.
* @param[in] *pSrc points to the block of input data.
* @param[out] *pDst points to the block of output data.
* @param[in] blockSize number of samples to process per call.
* @return none.
*
* Scaling and Overflow Behavior:
* \par
* 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.
* The 2.30 accumulator is then truncated to 2.15 format and saturated to yield the 1.15 result.
*
* \par
* Refer to the function arm_fir_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_init_q15() to initialize the filter structure.
*/
void arm_fir_fast_q15(
const arm_fir_instance_q15 * S,
q15_t * pSrc,
q15_t * pDst,
uint32_t blockSize)
{
q15_t *pState = S->pState; /* State pointer */
q15_t *pCoeffs = S->pCoeffs; /* Coefficient pointer */
q15_t *pStateCurnt; /* Points to the current sample of the state */
q15_t *px1; /* Temporary q15 pointer for state buffer */
q31_t *pb; /* Temporary pointer for coefficient buffer */
q31_t *px2; /* Temporary q31 pointer for SIMD state buffer accesses */
q31_t x0, x1, x2, x3, c0; /* Temporary variables to hold SIMD state and coefficient values */
q31_t acc0, acc1, acc2, acc3; /* Accumulators */
uint32_t numTaps = S->numTaps; /* Number of taps in the filter */
uint32_t tapCnt, blkCnt; /* Loop counters */
/* S->pState points to buffer which contains previous frame (numTaps - 1) samples */
/* pStateCurnt points to the location where the new input data should be written */
pStateCurnt = &(S->pState[(numTaps - 1u)]);
/* Apply loop unrolling and compute 4 output values simultaneously.
* The variables acc0 ... acc3 hold output values that are being computed:
*
* acc0 = b[numTaps-1] * x[n-numTaps-1] + b[numTaps-2] * x[n-numTaps-2] + b[numTaps-3] * x[n-numTaps-3] +...+ b[0] * x[0]
* acc1 = b[numTaps-1] * x[n-numTaps] + b[numTaps-2] * x[n-numTaps-1] + b[numTaps-3] * x[n-numTaps-2] +...+ b[0] * x[1]
* acc2 = b[numTaps-1] * x[n-numTaps+1] + b[numTaps-2] * x[n-numTaps] + b[numTaps-3] * x[n-numTaps-1] +...+ b[0] * x[2]
* acc3 = b[numTaps-1] * x[n-numTaps+2] + b[numTaps-2] * x[n-numTaps+1] + b[numTaps-3] * x[n-numTaps] +...+ b[0] * x[3]
*/
blkCnt = blockSize >> 2;
/* First part of the processing with loop unrolling. Compute 4 outputs at a time.
** a second loop below computes the remaining 1 to 3 samples. */
while(blkCnt > 0u)
{
/* Copy four new input samples into the state buffer.
** Use 32-bit SIMD to move the 16-bit data. Only requires two copies. */
*__SIMD32(pStateCurnt)++ = *__SIMD32(pSrc)++;
*__SIMD32(pStateCurnt)++ = *__SIMD32(pSrc)++;
/* Set all accumulators to zero */
acc0 = 0;
acc1 = 0;
acc2 = 0;
acc3 = 0;
/* Initialize state pointer of type q15 */
px1 = pState;
/* Initialize coeff pointer of type q31 */
pb = (q31_t *) (pCoeffs);
/* Read the first two samples from the state buffer: x[n-N], x[n-N-1] */
x0 = *(q31_t *) (px1++);
/* Read the third and forth samples from the state buffer: x[n-N-1], x[n-N-2] */
x1 = *(q31_t *) (px1++);
/* Loop over the number of taps. Unroll by a factor of 4.
** Repeat until we've computed numTaps-4 coefficients. */
tapCnt = numTaps >> 2;
do
{
/* Read the first two coefficients using SIMD: b[N] and b[N-1] coefficients */
c0 = *(pb++);
/* acc0 += b[N] * x[n-N] + b[N-1] * x[n-N-1] */
acc0 = __SMLAD(x0, c0, acc0);
/* acc1 += b[N] * x[n-N-1] + b[N-1] * x[n-N-2] */
acc1 = __SMLAD(x1, c0, acc1);
/* Read state x[n-N-2], x[n-N-3] */
x2 = *(q31_t *) (px1++);
/* Read state x[n-N-3], x[n-N-4] */
x3 = *(q31_t *) (px1++);
/* acc2 += b[N] * x[n-N-2] + b[N-1] * x[n-N-3] */
acc2 = __SMLAD(x2, c0, acc2);
/* acc3 += b[N] * x[n-N-3] + b[N-1] * x[n-N-4] */
acc3 = __SMLAD(x3, c0, acc3);
/* Read coefficients b[N-2], b[N-3] */
c0 = *(pb++);
/* acc0 += b[N-2] * x[n-N-2] + b[N-3] * x[n-N-3] */
acc0 = __SMLAD(x2, c0, acc0);
/* acc1 += b[N-2] * x[n-N-3] + b[N-3] * x[n-N-4] */
acc1 = __SMLAD(x3, c0, acc1);
/* Read state x[n-N-4], x[n-N-5] */
x0 = *(q31_t *) (px1++);
/* Read state x[n-N-5], x[n-N-6] */
x1 = *(q31_t *) (px1++);
/* acc2 += b[N-2] * x[n-N-4] + b[N-3] * x[n-N-5] */
acc2 = __SMLAD(x0, c0, acc2);
/* acc3 += b[N-2] * x[n-N-5] + b[N-3] * x[n-N-6] */
acc3 = __SMLAD(x1, c0, acc3);
tapCnt--;
}
while(tapCnt > 0u);
/* If the filter length is not a multiple of 4, compute the remaining filter taps.
** This is always 2 taps since the filter length is always even. */
if((numTaps & 0x3u) != 0u)
{
/* Read 2 coefficients */
c0 = *(pb++);
/* Fetch 4 state variables */
x2 = *(q31_t *) (px1++);
x3 = *(q31_t *) (px1++);
/* Perform the multiply-accumulates */
acc0 = __SMLAD(x0, c0, acc0);
acc1 = __SMLAD(x1, c0, acc1);
acc2 = __SMLAD(x2, c0, acc2);
acc3 = __SMLAD(x3, c0, acc3);
}
/* The results in the 4 accumulators are in 2.30 format. Convert to 1.15 with saturation.
** Then store the 4 outputs in the destination buffer. */
*__SIMD32(pDst)++ = __PKHBT((acc0 >> 15), (acc1 >> 15), 16u);
*__SIMD32(pDst)++ = __PKHBT((acc2 >> 15), (acc3 >> 15), 16u);
/* Advance the state pointer by 4 to process the next group of 4 samples */
pState = pState + 4;
/* Decrement the loop counter */
blkCnt--;
}
/* If the blockSize is not a multiple of 4, compute any remaining output samples here.
** No loop unrolling is used. */
blkCnt = blockSize % 0x4u;
while(blkCnt > 0u)
{
/* Copy two samples into state buffer */
*pStateCurnt++ = *pSrc++;
/* Set the accumulator to zero */
acc0 = 0;
/* Use SIMD to hold states and coefficients */
px2 = (q31_t *) pState;
pb = (q31_t *) (pCoeffs);
tapCnt = numTaps >> 1;
do
{
acc0 = __SMLAD(*px2++, *(pb++), acc0);
tapCnt--;
}
while(tapCnt > 0u);
/* The result is in 2.30 format. Convert to 1.15 with saturation.
** Then store the output in the destination buffer. */
*pDst++ = (q15_t) ((acc0 >> 15));
/* Advance state pointer by 1 for the next sample */
pState = pState + 1;
/* Decrement the loop counter */
blkCnt--;
}
/* Processing is complete.
** Now copy the last numTaps - 1 samples to the satrt of the state buffer.
** This prepares the state buffer for the next function call. */
/* Points to the start of the state buffer */
pStateCurnt = S->pState;
/* Calculation of count for copying integer writes */
tapCnt = (numTaps - 1u) >> 2;
while(tapCnt > 0u)
{
*__SIMD32(pStateCurnt)++ = *__SIMD32(pState)++;
*__SIMD32(pStateCurnt)++ = *__SIMD32(pState)++;
tapCnt--;
}
/* Calculation of count for remaining q15_t data */
tapCnt = (numTaps - 1u) % 0x4u;
/* copy remaining data */
while(tapCnt > 0u)
{
*pStateCurnt++ = *pState++;
/* Decrement the loop counter */
tapCnt--;
}
}
/**
* @} end of FIR group
*/