DSP/Source/FilteringFunctions/arm_fir_q31.c

/* ----------------------------------------------------------------------
 * Project:      CMSIS DSP Library
 * Title:        arm_fir_q31.c
 * Description:  Q31 FIR filter processing function
 *
 * $Date:        27. January 2017
 * $Revision:    V.1.5.1
 *
 * Target Processor: Cortex-M cores
 * -------------------------------------------------------------------- */
/*
 * Copyright (C) 2010-2017 ARM Limited or its affiliates. All rights reserved.
 *
 * SPDX-License-Identifier: Apache-2.0
 *
 * Licensed under the Apache License, Version 2.0 (the License); you may
 * not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an AS IS BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "arm_math.h"

/**
 * @ingroup groupFilters
 */

/**
 * @addtogroup FIR
 * @{
 */

/**
 * @param[in] *S points to an instance of the Q31 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.
 *
 * @details
 * <b>Scaling and Overflow Behavior:</b>
 * \par
 * 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.
 * After all multiply-accumulates are performed, the 2.62 accumulator is right shifted by 31 bits and saturated to 1.31 format to yield the final result.
 *
 * \par
 * Refer to the function <code>arm_fir_fast_q31()</code> for a faster but less precise implementation of this filter for Cortex-M3 and Cortex-M4.
 */

void arm_fir_q31(
  const arm_fir_instance_q31 * S,
  q31_t * pSrc,
  q31_t * pDst,
  uint32_t blockSize)
{
  q31_t *pState = S->pState;                     /* State pointer */
  q31_t *pCoeffs = S->pCoeffs;                   /* Coefficient pointer */
  q31_t *pStateCurnt;                            /* Points to the current sample of the state */


#if defined (ARM_MATH_DSP)

  /* Run the below code for Cortex-M4 and Cortex-M3 */

  q31_t x0, x1, x2;                              /* Temporary variables to hold state */
  q31_t c0;                                      /* Temporary variable to hold coefficient value */
  q31_t *px;                                     /* Temporary pointer for state */
  q31_t *pb;                                     /* Temporary pointer for coefficient buffer */
  q63_t acc0, acc1, acc2;                        /* Accumulators */
  uint32_t numTaps = S->numTaps;                 /* Number of filter coefficients in the filter */
  uint32_t i, tapCnt, blkCnt, tapCntN3;          /* Loop counters */

  /* S->pState points to state array 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 / 3;
  blockSize = blockSize - (3 * blkCnt);

  tapCnt = numTaps / 3;
  tapCntN3 = numTaps - (3 * tapCnt);

  /* 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 three new input samples into the state buffer */
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;
    *pStateCurnt++ = *pSrc++;

    /* Set all accumulators to zero */
    acc0 = 0;
    acc1 = 0;
    acc2 = 0;

    /* Initialize state pointer */
    px = pState;

    /* Initialize coefficient pointer */
    pb = pCoeffs;

    /* Read the first two samples from the state buffer:
     *  x[n-numTaps], x[n-numTaps-1] */
    x0 = *(px++);
    x1 = *(px++);

    /* Loop unrolling.  Process 3 taps at a time. */
    i = tapCnt;

    while (i > 0U)
    {
      /* Read the b[numTaps] coefficient */
      c0 = *pb;

      /* Read x[n-numTaps-2] sample */
      x2 = *(px++);

      /* Perform the multiply-accumulates */
      acc0 += ((q63_t) x0 * c0);
      acc1 += ((q63_t) x1 * c0);
      acc2 += ((q63_t) x2 * c0);

      /* Read the coefficient and state */
      c0 = *(pb + 1U);
      x0 = *(px++);

      /* Perform the multiply-accumulates */
      acc0 += ((q63_t) x1 * c0);
      acc1 += ((q63_t) x2 * c0);
      acc2 += ((q63_t) x0 * c0);

      /* Read the coefficient and state */
      c0 = *(pb + 2U);
      x1 = *(px++);

      /* update coefficient pointer */
      pb += 3U;

      /* Perform the multiply-accumulates */
      acc0 += ((q63_t) x2 * c0);
      acc1 += ((q63_t) x0 * c0);
      acc2 += ((q63_t) x1 * c0);

      /* Decrement the loop counter */
      i--;
    }

    /* If the filter length is not a multiple of 3, compute the remaining filter taps */

    i = tapCntN3;

    while (i > 0U)
    {
      /* Read coefficients */
      c0 = *(pb++);

      /* Fetch 1 state variable */
      x2 = *(px++);

      /* Perform the multiply-accumulates */
      acc0 += ((q63_t) x0 * c0);
      acc1 += ((q63_t) x1 * c0);
      acc2 += ((q63_t) x2 * c0);

      /* Reuse the present sample states for next sample */
      x0 = x1;
      x1 = x2;

      /* Decrement the loop counter */
      i--;
    }

    /* Advance the state pointer by 3 to process the next group of 3 samples */
    pState = pState + 3;

    /* The results in the 3 accumulators are in 2.30 format.  Convert to 1.31
     ** Then store the 3 outputs in the destination buffer. */
    *pDst++ = (q31_t) (acc0 >> 31U);
    *pDst++ = (q31_t) (acc1 >> 31U);
    *pDst++ = (q31_t) (acc2 >> 31U);

    /* Decrement the samples loop counter */
    blkCnt--;
  }

  /* If the blockSize is not a multiple of 3, compute any remaining output samples here.
   ** No loop unrolling is used. */

  while (blockSize > 0U)
  {
    /* Copy one sample at a time into state buffer */
    *pStateCurnt++ = *pSrc++;

    /* Set the accumulator to zero */
    acc0 = 0;

    /* Initialize state pointer */
    px = pState;

    /* Initialize Coefficient pointer */
    pb = (pCoeffs);

    i = numTaps;

    /* Perform the multiply-accumulates */
    do
    {
      acc0 += (q63_t) * (px++) * (*(pb++));
      i--;
    } while (i > 0U);

    /* The result is in 2.62 format.  Convert to 1.31
     ** Then store the output in the destination buffer. */
    *pDst++ = (q31_t) (acc0 >> 31U);

    /* Advance state pointer by 1 for the next sample */
    pState = pState + 1;

    /* Decrement the samples loop counter */
    blockSize--;
  }

  /* 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;

  tapCnt = (numTaps - 1U) >> 2U;

  /* copy data */
  while (tapCnt > 0U)
  {
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;
    *pStateCurnt++ = *pState++;

    /* Decrement the loop counter */
    tapCnt--;
  }

  /* Calculate remaining number of copies */
  tapCnt = (numTaps - 1U) % 0x4U;

  /* Copy the remaining q31_t data */
  while (tapCnt > 0U)
  {
    *pStateCurnt++ = *pState++;

    /* Decrement the loop counter */
    tapCnt--;
  }

#else

/* Run the below code for Cortex-M0 */

  q31_t *px;                                     /* Temporary pointer for state */
  q31_t *pb;                                     /* Temporary pointer for coefficient buffer */
  q63_t acc;                                     /* Accumulator */
  uint32_t numTaps = S->numTaps;                 /* Length of the filter */
  uint32_t i, tapCnt, blkCnt;                    /* Loop counters */

  /* S->pState buffer contains previous frame (numTaps - 1) samples */
  /* pStateCurnt points to the location where the new input data should be written */
  pStateCurnt = &(S->pState[(numTaps - 1U)]);

  /* Initialize blkCnt with blockSize */
  blkCnt = blockSize;

  while (blkCnt > 0U)
  {
    /* Copy one sample at a time into state buffer */
    *pStateCurnt++ = *pSrc++;

    /* Set the accumulator to zero */
    acc = 0;

    /* Initialize state pointer */
    px = pState;

    /* Initialize Coefficient pointer */
    pb = pCoeffs;

    i = numTaps;

    /* Perform the multiply-accumulates */
    do
    {
      /* acc =  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] */
      acc += (q63_t) * px++ * *pb++;
      i--;
    } while (i > 0U);

    /* The result is in 2.62 format.  Convert to 1.31
     ** Then store the output in the destination buffer. */
    *pDst++ = (q31_t) (acc >> 31U);

    /* Advance state pointer by 1 for the next sample */
    pState = pState + 1;

    /* Decrement the samples loop counter */
    blkCnt--;
  }

  /* Processing is complete.
   ** Now copy the last numTaps - 1 samples to the starting 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;

  /* Copy numTaps number of values */
  tapCnt = numTaps - 1U;

  /* Copy the data */
  while (tapCnt > 0U)
  {
    *pStateCurnt++ = *pState++;

    /* Decrement the loop counter */
    tapCnt--;
  }


#endif /*  #if defined (ARM_MATH_DSP) */

}

/**
 * @} end of FIR group
 */