File content as of revision 3:4098b9d3d571:

/* ----------------------------------------------------------------------
 * Project:      CMSIS DSP Library
 * Title:        arm_fir_decimate_f32.c
 * Description:  FIR decimation for floating-point sequences
 *
 * $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
 */

/**
 * @defgroup FIR_decimate Finite Impulse Response (FIR) Decimator
 *
 * 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:
 * \image html FIRDecimator.gif "Components included in the FIR Decimator functions"
 * When decimating by a factor of <code>M</code>, the signal should be prefiltered by a lowpass filter with a normalized
 * cutoff frequency of <code>1/M</code> 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 <code>M-1</code> out of every <code>M</code>, only the
 * samples output by the decimator are computed.
 * The functions operate on blocks of input and output data.
 * <code>pSrc</code> points to an array of <code>blockSize</code> input values and
 * <code>pDst</code> points to an array of <code>blockSize/M</code> output values.
 * In order to have an integer number of output samples <code>blockSize</code>
 * must always be a multiple of the decimation factor <code>M</code>.
 *
 * The library provides separate functions for Q15, Q31 and floating-point data types.
 *
 * \par Algorithm:
 * The FIR portion of the algorithm uses the standard form filter:
 * <pre>
 *    y[n] = b[0] * x[n] + b[1] * x[n-1] + b[2] * x[n-2] + ...+ b[numTaps-1] * x[n-numTaps+1]
 * </pre>
 * where, <code>b[n]</code> are the filter coefficients.
 * \par
 * The <code>pCoeffs</code> points to a coefficient array of size <code>numTaps</code>.
 * Coefficients are stored in time reversed order.
 * \par
 * <pre>
 *    {b[numTaps-1], b[numTaps-2], b[N-2], ..., b[1], b[0]}
 * </pre>
 * \par
 * <code>pState</code> points to a state array of size <code>numTaps + blockSize - 1</code>.
 * Samples in the state buffer are stored in the order:
 * \par
 * <pre>
 *    {x[n-numTaps+1], x[n-numTaps], x[n-numTaps-1], x[n-numTaps-2]....x[0], x[1], ..., x[blockSize-1]}
 * </pre>
 * The state variables are updated after each block of data is processed, the coefficients are untouched.
 *
 * \par 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.
 *
 * \par 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.
 *
 * \par
 * 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
 * <pre>
 *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};
 * </pre>
 * where <code>M</code> is the decimation factor; <code>numTaps</code> is the number of filter coefficients in the filter;
 * <code>pCoeffs</code> is the address of the coefficient buffer;
 * <code>pState</code> is the address of the state buffer.
 * Be sure to set the values in the state buffer to zeros when doing static initialization.
 *
 * \par 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.
 */

/**
 * @addtogroup FIR_decimate
 * @{
 */

  /**
   * @brief Processing function for the floating-point FIR decimator.
   * @param[in] *S        points to an instance of the floating-point FIR decimator 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 input samples to process per call.
   * @return none.
   */

void arm_fir_decimate_f32(
  const arm_fir_decimate_instance_f32 * S,
  float32_t * pSrc,
  float32_t * pDst,
  uint32_t blockSize)
{
  float32_t *pState = S->pState;                 /* State pointer */
  float32_t *pCoeffs = S->pCoeffs;               /* Coefficient pointer */
  float32_t *pStateCurnt;                        /* Points to the current sample of the state */
  float32_t *px, *pb;                            /* Temporary pointers for state and coefficient buffers */
  float32_t sum0;                                /* Accumulator */
  float32_t x0, c0;                              /* Temporary variables to hold state and coefficient values */
  uint32_t numTaps = S->numTaps;                 /* Number of filter coefficients in the filter */
  uint32_t i, tapCnt, blkCnt, outBlockSize = blockSize / S->M;  /* Loop counters */

#if defined (ARM_MATH_DSP)

  uint32_t blkCntN4;
  float32_t *px0, *px1, *px2, *px3;
  float32_t acc0, acc1, acc2, acc3;
  float32_t x1, x2, x3;

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

  /* 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);

  /* Total number of output samples to be computed */
  blkCnt = outBlockSize / 4;
  blkCntN4 = outBlockSize - (4 * blkCnt);

  while (blkCnt > 0U)
  {
    /* Copy 4 * decimation factor number of new input samples into the state buffer */
    i = 4 * S->M;

    do
    {
      *pStateCurnt++ = *pSrc++;

    } while (--i);

    /* Set accumulators to zero */
    acc0 = 0.0f;
    acc1 = 0.0f;
    acc2 = 0.0f;
    acc3 = 0.0f;

    /* Initialize state pointer for all the samples */
    px0 = pState;
    px1 = pState + S->M;
    px2 = pState + 2 * S->M;
    px3 = pState + 3 * S->M;

    /* Initialize coeff pointer */
    pb = pCoeffs;

    /* Loop unrolling.  Process 4 taps at a time. */
    tapCnt = numTaps >> 2;

    /* Loop over the number of taps.  Unroll by a factor of 4.
     ** Repeat until we've computed numTaps-4 coefficients. */

    while (tapCnt > 0U)
    {
      /* Read the b[numTaps-1] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-1] sample for acc0 */
      x0 = *(px0++);
      /* Read x[n-numTaps-1] sample for acc1 */
      x1 = *(px1++);
      /* Read x[n-numTaps-1] sample for acc2 */
      x2 = *(px2++);
      /* Read x[n-numTaps-1] sample for acc3 */
      x3 = *(px3++);

      /* Perform the multiply-accumulate */
      acc0 += x0 * c0;
      acc1 += x1 * c0;
      acc2 += x2 * c0;
      acc3 += x3 * c0;

      /* Read the b[numTaps-2] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-2] sample for acc0, acc1, acc2, acc3 */
      x0 = *(px0++);
      x1 = *(px1++);
      x2 = *(px2++);
      x3 = *(px3++);

      /* Perform the multiply-accumulate */
      acc0 += x0 * c0;
      acc1 += x1 * c0;
      acc2 += x2 * c0;
      acc3 += x3 * c0;

      /* Read the b[numTaps-3] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-3] sample acc0, acc1, acc2, acc3 */
      x0 = *(px0++);
      x1 = *(px1++);
      x2 = *(px2++);
      x3 = *(px3++);

      /* Perform the multiply-accumulate */
      acc0 += x0 * c0;
      acc1 += x1 * c0;
      acc2 += x2 * c0;
      acc3 += x3 * c0;

      /* Read the b[numTaps-4] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-4] sample acc0, acc1, acc2, acc3 */
      x0 = *(px0++);
      x1 = *(px1++);
      x2 = *(px2++);
      x3 = *(px3++);

      /* Perform the multiply-accumulate */
      acc0 += x0 * c0;
      acc1 += x1 * c0;
      acc2 += x2 * c0;
      acc3 += x3 * c0;

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

    /* If the filter length is not a multiple of 4, compute the remaining filter taps */
    tapCnt = numTaps % 0x4U;

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

      /* Fetch  state variables for acc0, acc1, acc2, acc3 */
      x0 = *(px0++);
      x1 = *(px1++);
      x2 = *(px2++);
      x3 = *(px3++);

      /* Perform the multiply-accumulate */
      acc0 += x0 * c0;
      acc1 += x1 * c0;
      acc2 += x2 * c0;
      acc3 += x3 * c0;

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

    /* Advance the state pointer by the decimation factor
     * to process the next group of decimation factor number samples */
    pState = pState + 4 * S->M;

    /* The result is in the accumulator, store in the destination buffer. */
    *pDst++ = acc0;
    *pDst++ = acc1;
    *pDst++ = acc2;
    *pDst++ = acc3;

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

  while (blkCntN4 > 0U)
  {
    /* Copy decimation factor number of new input samples into the state buffer */
    i = S->M;

    do
    {
      *pStateCurnt++ = *pSrc++;

    } while (--i);

    /* Set accumulator to zero */
    sum0 = 0.0f;

    /* Initialize state pointer */
    px = pState;

    /* Initialize coeff pointer */
    pb = pCoeffs;

    /* Loop unrolling.  Process 4 taps at a time. */
    tapCnt = numTaps >> 2;

    /* Loop over the number of taps.  Unroll by a factor of 4.
     ** Repeat until we've computed numTaps-4 coefficients. */
    while (tapCnt > 0U)
    {
      /* Read the b[numTaps-1] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-1] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 += x0 * c0;

      /* Read the b[numTaps-2] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-2] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 += x0 * c0;

      /* Read the b[numTaps-3] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-3] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 += x0 * c0;

      /* Read the b[numTaps-4] coefficient */
      c0 = *(pb++);

      /* Read x[n-numTaps-4] sample */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 += x0 * c0;

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

    /* If the filter length is not a multiple of 4, compute the remaining filter taps */
    tapCnt = numTaps % 0x4U;

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

      /* Fetch 1 state variable */
      x0 = *(px++);

      /* Perform the multiply-accumulate */
      sum0 += x0 * c0;

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

    /* Advance the state pointer by the decimation factor
     * to process the next group of decimation factor number samples */
    pState = pState + S->M;

    /* The result is in the accumulator, store in the destination buffer. */
    *pDst++ = sum0;

    /* Decrement the loop counter */
    blkCntN4--;
  }

  /* 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;

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

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

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

  i = (numTaps - 1U) % 0x04U;

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

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

#else

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

  /* 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);

  /* Total number of output samples to be computed */
  blkCnt = outBlockSize;

  while (blkCnt > 0U)
  {
    /* Copy decimation factor number of new input samples into the state buffer */
    i = S->M;

    do
    {
      *pStateCurnt++ = *pSrc++;

    } while (--i);

    /* Set accumulator to zero */
    sum0 = 0.0f;

    /* Initialize state pointer */
    px = pState;

    /* Initialize coeff pointer */
    pb = pCoeffs;

    tapCnt = numTaps;

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

      /* Fetch 1 state variable */
      x0 = *px++;

      /* Perform the multiply-accumulate */
      sum0 += x0 * c0;

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

    /* Advance the state pointer by the decimation factor
     * to process the next group of decimation factor number samples */
    pState = pState + S->M;

    /* The result is in the accumulator, store in the destination buffer. */
    *pDst++ = sum0;

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

  /* Processing is complete.
   ** Now copy the last numTaps - 1 samples to the start 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 */
  i = (numTaps - 1U);

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

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

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

}

/**
 * @} end of FIR_decimate group
 */