tns.c

jpeg and mpeg 编解码技术源代码

#include "all.h"

#define	sfb_offset(x) ( ((x) > 0) ? sfb_top[(x)-1] : 0 )

/* Decoder transmitted coefficients for one TNS filter */
void
tns_decode_coef( int order, int coef_res, short *coef, Float *a )
{
    int i, m;
    Float iqfac,  iqfac_m;
    Float tmp[TNS_MAX_ORDER+1], b[TNS_MAX_ORDER+1];

    /* Inverse quantization */
    iqfac   = ((1 << (coef_res-1)) - 0.5) / (C_PI/2.0);
    iqfac_m = ((1 << (coef_res-1)) + 0.5) / (C_PI/2.0);
    for (i=0; i<order; i++)  {
	tmp[i+1] = sin( coef[i] / ((coef[i] >= 0) ? iqfac : iqfac_m) );
    }
    /* Conversion to LPC coefficients
     * Markel and Gray,  pg. 95
     */
    a[0] = 1;
    for (m=1; m<=order; m++)  {
	b[0] = a[0];
	for (i=1; i<m; i++)  {
	    b[i] = a[i] + tmp[m] * a[m-i];
	}
	b[m] = tmp[m];
	for (i=0; i<=m; i++)  {
	    a[i] = b[i];
	}
    }

}

/* apply the TNS filter */
void
tns_ar_filter( Float *spec, int size, int inc, Float *lpc, int order )
{
    /*
     *	- Simple all-pole filter of order "order" defined by
     *	  y(n) =  x(n) - a(2)*y(n-1) - ... - a(order+1)*y(n-order)
     *
     *	- The state variables of the filter are initialized to zero every time
     *
     *	- The output data is written over the input data ("in-place operation")
     *
     *	- An input vector of "size" samples is processed and the index increment
     *	  to the next data sample is given by "inc"
     */
    int i, j;
    Float y, state[TNS_MAX_ORDER];

    for (i=0; i<order; i++)
	state[i] = 0;

    if (inc == -1)
	spec += size-1;

    for (i=0; i<size; i++) {
	y = *spec;
	for (j=0; j<order; j++)
	    y -= lpc[j+1] * state[j];
	for (j=order-1; j>0; j--)
	    state[j] = state[j-1];
	state[0] = y;
	*spec = y;
	spec += inc;
    }
}

/* TNS decoding for one channel and frame */
void
tns_decode_subblock(Float *spec, int nbands, short *sfb_top, int islong,
	TNSinfo *tns_info)
{
    int f, m, start, stop, size, inc;
    int n_filt, coef_res, order, direction;
    short *coef;
    Float lpc[TNS_MAX_ORDER+1];
    TNSfilt *filt;

    n_filt = tns_info->n_filt;
    for (f=0; f<n_filt; f++)  {
	coef_res = tns_info->coef_res;
	filt = &tns_info->filt[f];
	order = filt->order;
	direction = filt->direction;
	coef = filt->coef;
	start = filt->start_band;
	stop = filt->stop_band;

	m = tns_max_order(islong);
	if (order > m) {
//	    CommonWarning("Error in tns max order");
	    order = m;
	}
	if (!order)  continue;

	tns_decode_coef( order, coef_res, coef, lpc );

	start = min(start, tns_max_bands(islong));
        start = min(start, nbands);
	start = sfb_offset( start );
	
	stop = min(stop, tns_max_bands(islong));
        stop = min(stop, nbands);
	stop = sfb_offset( stop );
	if ((size = stop - start) <= 0)  continue;

	if (direction)  {
	    inc = -1;
	}  else {
	    inc =  1;
	}

	tns_ar_filter( &spec[start], size, inc, lpc, order );
    }
}