FIR滤波器的实现方法(转)

源：http://blog.sina.com.cn/s/blog_493520900102uy26.html

内容来自于上篇博文，第七章，FIR滤波器

http://blog.sina.com.cn/s/blog_493520900102uxzu.html

使用 FDATool工具，构建一个Kaiser窗低通滤波器，采样频率8kHz，生成滤波器系数：

1）例程1

文件 fir_fltcoeff.h，以浮点形式表示的量化后的滤波器系数

//define the FIR filter length

#define N_FIR 40

float h[N_FIR] = { ‐ 0.004314029307,‐ 0.013091321622,‐0.016515087727,

‐0.006430584433, 0.009817876267, 0.010801880238,

‐0.006567413713,‐ 0.016804829623, 0.000653253913,

0.022471280087, 0.010147131468,‐0.025657740989,

‐0.026558960619, 0.023048392854, 0.050385290390,

‐0.009291203588,‐ 0.087918503442,‐0.033770330014,

0.187334796517, 0.401505729848, 0.401505729848,

0.187334796517,‐ 0.033770330014,‐0.087918503442,

‐0.009291203588, 0.050385290390, 0.023048392854,

‐0.026558960619,‐ 0.025657740989, 0.010147131468,

0.022471280087, 0.000653253913,‐0.016804829623,

‐0.006567413713, 0.010801880238, 0.009817876267,

‐0.006430584433,‐ 0.016515087727,‐0.013091321622,

‐0.004314029307 };

实际电路用AIC3106，每次采样中断来临后读入一组数据，分别为左右声道的数据，各16位。中断服务程序将左声道的每个数据送入滤波器，然后从DAC电路输出滤波后的结果。右声道数据不变。

文件 ISRs_Plus.c ，中断服务程序：

// Welch, Wright, & Morrow,

// Real-time Digital Signal Processing, 2011

// Modified by Mark Wickert February 2012 to include GPIO ISR start/stop postings

///////////////////////////////////////////////////////////////////////

// Filename: ISRs_fir_float.c

// Synopsis: Interrupt service routine for codec data transmit/receive

// floating point FIR filtering with coefficients in *.h file

///////////////////////////////////////////////////////////////////////

#include "DSP_Config.h"

#include "fir_fltcoeff.h" //coefficients in float format

// Function Prototypes

long int rand_int(void);

// Data is received as 2 16-bit words (left/right) packed into one

// 32-bit word. The union allows the data to be accessed as a single

// entity when transferring to and from the serial port, but still be

// able to manipulate the left and right channels independently.

#define LEFT 0

#define RIGHT 1

volatile union {

Uint32 UINT;

Int16 Channel[2];

} CodecDataIn, CodecDataOut;

float x_buffer[N_FIR]; //buffer for delay samples

interrupt void Codec_ISR()

///////////////////////////////////////////////////////////////////////

// Purpose: Codec interface interrupt service routine

// Input: None

// Returns: Nothing

// Calls: CheckForOverrun, ReadCodecData, WriteCodecData

// Notes: None

///////////////////////////////////////////////////////////////////////

{

WriteDigitalOutputs(1); // Write to GPIO J15, pin 6; begin ISR timing pulse

int i;

float result = 0; //initialize the accumulator

if(CheckForOverrun()) // overrun error occurred (i.e. halted DSP)

return; // so serial port is reset to recover

CodecDataIn.UINT = ReadCodecData(); // get input data samples

//Work with Left ADC sample

//x_buffer[0] = 0.25 * CodecDataIn.Channel[ LEFT];

//Use the next line to noise test the filter

x_buffer[0] = 0.125*((short) rand_int());//scale input by 1/8

//Filtering using a 32-bit accumulator

for(i=0; i< N_FIR; i++)

{

result += x_buffer[i] * h[i];

}

//Update filter history

for(i=N_FIR-1; i>0; i--)

{

x_buffer[i] = x_buffer[i-1];

}

//Return 16-bit sample to DAC

CodecDataOut.Channel[ LEFT] = (short) result;

// Copy Right input directly to Right output with no filtering

CodecDataOut.Channel[RIGHT] = CodecDataIn.Channel[ RIGHT];

WriteCodecData(CodecDataOut.UINT); // send output data to port

WriteDigitalOutputs(0); // Write to GPIO J15, pin 6; end ISR timing pulse

}

//White noise generator for filter noise testing

long int rand_int(void)

{

static long int a = 100001;

a = (a*125) % 2796203;

return a;

}

可以看出，中断服务程序是逐点响应的，每次来一个数据，都要做一次滤波器运算，并将原有的数据前移一位，将新数据追加在缓冲区末尾。

float x_buffer[N_FIR]; // 用于保存现在的新值和过去的数值，长度与滤波器系数相同

x_buffer[0] // 用于保存当前值

x_buffer[1] // 保存过去前一点的值，以此类推

h[0...39] // 滤波器系数

result += x_buffer[i] * h[i]; // 当前点滤波器输出结果，循环从0到39

巴特西

FIR滤波器的实现方法(转)

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