bit_allocate.c

关于ＡＣ３的解码程序

	}

	if(audblk->cplinu)
	{
		start = audblk->cplstrtmant; 
		end = audblk->cplendmant; 
		fgain = fastgain[audblk->cplfgaincod];
		snroffset = (((audblk->csnroffst - 15) << 4) + audblk->cplfsnroffst) << 2 ;
		fastleak = (audblk->cplfleak << 8) + 768; 
		slowleak = (audblk->cplsleak << 8) + 768;

		ba_compute_psd(start, end, audblk->cpl_exp, psd, bndpsd);

		ba_compute_excitation(start, end , fgain, fastleak, slowleak, 0, bndpsd, excite);

		ba_compute_mask(start, end, fscod, audblk->cpldeltbae, audblk->cpldeltnseg, 
				audblk->cpldeltoffst, audblk->cpldeltba, audblk->cpldeltlen, excite, mask);

		ba_compute_bap(start, end, snroffset, psd, mask, audblk->cpl_bap);
	}

	if(bsi->lfeon)
	{
		start = 0;
		end = 7;
		fgain = fastgain[audblk->lfefgaincod];
		snroffset = (((audblk->csnroffst - 15) << 4) + audblk->lfefsnroffst) << 2 ;
		fastleak = 0;
		slowleak = 0;

		ba_compute_psd(start, end, audblk->lfe_exp, psd, bndpsd);

		ba_compute_excitation(start, end , fgain, fastleak, slowleak, 1, bndpsd, excite);

		/* Perform no delta bit allocation for lfe */
		ba_compute_mask(start, end, fscod, 2, 0, 0, 0, 0, excite, mask);

		ba_compute_bap(start, end, snroffset, psd, mask, audblk->lfe_bap);
	}
}


static void ba_compute_psd(sint_16 start, sint_16 end, sint_16 exps[], 
		sint_16 psd[], sint_16 bndpsd[])
{
	int bin,i,j,k;
	sint_16 lastbin = 0;
	
	/* Map the exponents into dBs */
	for (bin=start; bin<end; bin++) 
	{ 
		psd[bin] = (3072 - (exps[bin] << 7)); 
	}

	/* Integrate the psd function over each bit allocation band */
	j = start; 
	k = masktab[start]; 
	
	do 
	{ 
		lastbin = min16(bndtab[k] + bndsz[k], end); 
		bndpsd[k] = psd[j]; 
		j++; 

		for (i = j; i < lastbin; i++) 
		{ 
			bndpsd[k] = logadd(bndpsd[k], psd[j]);
			j++; 
		} 
		
		k++; 
	} while (end > lastbin);
}

static void ba_compute_excitation(sint_16 start, sint_16 end,sint_16 fgain,
		sint_16 fastleak, sint_16 slowleak, sint_16 is_lfe, sint_16 bndpsd[],
		sint_16 excite[])
{
	int bin;
	sint_16 bndstrt;
	sint_16 bndend;
	sint_16 lowcomp = 0;
	sint_16 begin = 0;

	/* Compute excitation function */
	bndstrt = masktab[start]; 
	bndend = masktab[end - 1] + 1; 
	
	if (bndstrt == 0) /* For fbw and lfe channels */ 
	{ 
		lowcomp = calc_lowcomp(lowcomp, bndpsd[0], bndpsd[1], 0); 
		excite[0] = bndpsd[0] - fgain - lowcomp; 
		lowcomp = calc_lowcomp(lowcomp, bndpsd[1], bndpsd[2], 1);
		excite[1] = bndpsd[1] - fgain - lowcomp; 
		begin = 7 ; 
		
		/* Note: Do not call calc_lowcomp() for the last band of the lfe channel, (bin = 6) */ 
		for (bin = 2; bin < 7; bin++) 
		{ 
			if (!(is_lfe && (bin == 6)))
				lowcomp = calc_lowcomp(lowcomp, bndpsd[bin], bndpsd[bin+1], bin); 
			fastleak = bndpsd[bin] - fgain; 
			slowleak = bndpsd[bin] - sgain; 
			excite[bin] = fastleak - lowcomp; 
			
			if (!(is_lfe && (bin == 6)))
			{
				if (bndpsd[bin] <= bndpsd[bin+1]) 
				{
					begin = bin + 1 ; 
					break; 
				} 
			}
		} 
		
		for (bin = begin; bin < min16(bndend, 22); bin++) 
		{ 
			if (!(is_lfe && (bin == 6)))
				lowcomp = calc_lowcomp(lowcomp, bndpsd[bin], bndpsd[bin+1], bin); 
			fastleak -= fdecay ; 
			fastleak = max16(fastleak, bndpsd[bin] - fgain); 
			slowleak -= sdecay ; 
			slowleak = max16(slowleak, bndpsd[bin] - sgain); 
			excite[bin] = max16(fastleak - lowcomp, slowleak); 
		} 
		begin = 22; 
	} 
	else /* For coupling channel */ 
	{ 
		begin = bndstrt; 
	} 

	for (bin = begin; bin < bndend; bin++) 
	{ 
		fastleak -= fdecay; 
		fastleak = max16(fastleak, bndpsd[bin] - fgain); 
		slowleak -= sdecay; 
		slowleak = max16(slowleak, bndpsd[bin] - sgain); 
		excite[bin] = max16(fastleak, slowleak) ; 
	} 
}

static void ba_compute_mask(sint_16 start, sint_16 end, uint_16 fscod,
		uint_16 deltbae, uint_16 deltnseg, uint_16 deltoffst[], uint_16 deltba[],
		uint_16 deltlen[], sint_16 excite[], sint_16 mask[])
{
	int bin,k;
	sint_16 bndstrt;
	sint_16 bndend;
	sint_16 delta;

	bndstrt = masktab[start]; 
	bndend = masktab[end - 1] + 1; 

	/* Compute the masking curve */

	for (bin = bndstrt; bin < bndend; bin++) 
	{ 
		if (bndpsd[bin] < dbknee) 
		{ 
			excite[bin] += ((dbknee - bndpsd[bin]) >> 2); 
		} 
		mask[bin] = max16(excite[bin], hth[fscod][bin]);
	}
	
	/* Perform delta bit modulation if necessary */
	if ((deltbae == 0) || (deltbae == 1)) 
	{ 
		sint_16 band = 0; 
		sint_16 seg = 0; 
		
		for (seg = 0; seg < deltnseg+1; seg++) 
		{ 
			band += deltoffst[seg]; 
			if (deltba[seg] >= 4) 
			{ 
				delta = (deltba[seg] - 3) << 7;
			} 
			else 
			{ 
				delta = (deltba[seg] - 4) << 7;
			} 

			for (k = 0; k < deltlen[seg]; k++) 
			{ 
				mask[band&0xff] += delta; 
				band++; 
			} 
		} 
	}
}

static void ba_compute_bap(sint_16 start, sint_16 end, sint_16 snroffset,
		sint_16 psd[], sint_16 mask[], sint_16 bap[])
{
	int i,j,k;
	sint_16 lastbin = 0;
	sint_16 address = 0;

	/* Compute the bit allocation pointer for each bin */
	i = start; 
	j = masktab[start]; 

	do 
	{ 
		lastbin = min16(bndtab[j] + bndsz[j], end); 
		mask[j] -= snroffset; 
		mask[j] -= floor; 
		
		if (mask[j] < 0) 
			mask[j] = 0; 

		mask[j] &= 0x1fe0;
		mask[j] += floor; 
		for (k = i; k < lastbin; k++) 
		{ 
			address = (psd[i] - mask[j]) >> 5; 
			address = min16(63, max16(0, address)); 
			bap[i] = baptab[address]; 
			i++; 
		} 
		j++; 
	} while (end > lastbin);
}

static sint_16 
calc_lowcomp(sint_16 a,sint_16 b0,sint_16 b1,sint_16 bin) 
{ 

	if (bin < 7) 
	{ 
		if ((b0 + 256) == b1)
			a = 384; 
	 	else if (b0 > b1) 
			a = max16(0, a - 64); 
	} 
	else if (bin < 20) 
	{ 
		if ((b0 + 256) == b1) 
			a = 320; 
		else if (b0 > b1) 
			a = max16(0, a - 64) ; 
	}
	else  
		a = max16(0, a - 128); 
	
	return(a);
}