简单的音频重采样

void _x_audio_out_resample_mono(short* input_samples, int in_samples,

    short* output_samples, int out_samples)

{

  int osample;

  /* 16+16 fixed point math */

  uint32_t isample = 0;

  uint32_t istep = ((in_samples-2) << 16)/(out_samples-2);

#ifdef VERBOSE

  printf ("Audio : resample %d samples to %d\n",

          in_samples, out_samples);

#endif

  for (osample = 0; osample < out_samples - 1; osample++) {

    int s1;

    int s2;

    int16_t os;

    uint32_t t = isample&0xffff;

   

    /* don't "optimize" the (isample >> 16)*2 to (isample >> 15) */

    s1 = input_samples[(isample >> 16)];

    s2 = input_samples[(isample >> 16)+1];

   

    os = (s1 * (0x10000-t)+ s2 * t) >> 16;

    output_samples[osample] = os;

    isample += istep;

  }

  output_samples[out_samples-1] = input_samples[in_samples-1];

}

typedef short HWORD;

typedef int WORD;

typedef unsigned int UWORD;

typedef unsigned short UHWORD;

#define MAX_HWORD (32767)

#define MIN_HWORD (-32767)

#define Np 15

#define Pmask ((1<<Np)-1)

static inline HWORD WordToHword(WORD v, int scl)

{

    HWORD out;

    WORD llsb = (1<<(scl-1));

    v += llsb; /*
 round */

    v >>= scl;

    if (v>MAX_HWORD) {

#ifdef DEBUG

        if (pof == 0)

          fprintf(stderr, "***
 resample: sound sample overflow\n");

        else if ((pof % 10000) == 0)

          fprintf(stderr, "***
 resample: another ten thousand overflows\n");

        pof++;

#endif

        v = MAX_HWORD;

    } else if (v < MIN_HWORD) {

#ifdef DEBUG

        if (nof == 0)

          fprintf(stderr, "***
 resample: sound sample (-) overflow\n");

        else if ((nof % 1000) == 0)

          fprintf(stderr, "***
 resample: another thousand (-) overflows\n");

        nof++;

#endif

        v = MIN_HWORD;

    } 

    out = (HWORD) v;

    return out;

}

static int   SrcLinear(HWORD X[], HWORD
 Y[], double factor, UWORD *Time, UHWORD
 Nx){    HWORD iconst;    HWORD *Xp, *Ystart;    WORD v,x1,x2;        double dt; /* Step through input signal */     UWORD dtb; /* Fixed-point version of Dt */    UWORD endTime; /* When Time reaches EndTime, return to user */        dt = 1.0/factor; /*
 Output sampling period */    dtb = dt*(1<<Np) + 0.5; /*
 Fixed-point representation */        Ystart = Y;    endTime = *Time + (1<<Np)*(WORD)Nx;    while (*Time < endTime)    {        iconst = (*Time) & Pmask;        Xp = &X[(*Time)>>Np]; /*
 Ptr to current input sample */        x1 = *Xp++;        x2 = *Xp;        x1 *= ((1<<Np)-iconst);        x2 *= iconst;        v = x1 + x2;        *Y++ = WordToHword(v,Np); /*
 Deposit output */        *Time += dtb; /*
 Move to next sample by time increment */    }    return (Y - Ystart); /*
 Return number of output samples */}