aacenc_tns.c

Go to the documentation of this file.

/*
 * AAC encoder TNS
 * Copyright (C) 2015 Rostislav Pehlivanov
 *
 * This file is part of FFmpeg.
 *
 * FFmpeg is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation; either
 * version 2.1 of the License, or (at your option) any later version.
 *
 * FFmpeg is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 *
 * You should have received a copy of the GNU Lesser General Public
 * License along with FFmpeg; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
 */

/**
 * @file
 * AAC encoder temporal noise shaping
 * @author Rostislav Pehlivanov ( atomnuker gmail com )
 */

#include "libavutil/libm.h"
#include "aacenc.h"
#include "aacenc_tns.h"
#include "aactab.h"
#include "aacenc_utils.h"
#include "aacenc_quantization.h"

/* Could be set to 3 to save an additional bit at the cost of little quality */
#define TNS_Q_BITS 4

/* Coefficient resolution in short windows */
#define TNS_Q_BITS_IS8 4

/* We really need the bits we save here elsewhere */
#define TNS_ENABLE_COEF_COMPRESSION

/* TNS will only be used if the LPC gain is within these margins */
#define TNS_GAIN_THRESHOLD_LOW      1.4f
#define TNS_GAIN_THRESHOLD_HIGH     1.16f*TNS_GAIN_THRESHOLD_LOW

static inline int compress_coeffs(int *coef, int order, int c_bits)
{
    int i;
    const int low_idx   = c_bits ?  4 : 2;
    const int shift_val = c_bits ?  8 : 4;
    const int high_idx  = c_bits ? 11 : 5;
#ifndef TNS_ENABLE_COEF_COMPRESSION
    return 0;
#endif /* TNS_ENABLE_COEF_COMPRESSION */
    for (i = 0; i < order; i++)
        if (coef[i] >= low_idx && coef[i] <= high_idx)
            return 0;
    for (i = 0; i < order; i++)
        coef[i] -= (coef[i] > high_idx) ? shift_val : 0;
    return 1;
}

/**
 * Encode TNS data.
 * Coefficient compression is simply not lossless as it should be
 * on any decoder tested and as such is not active.
 */
void ff_aac_encode_tns_info(AACEncContext *s, SingleChannelElement *sce)
{
    TemporalNoiseShaping *tns = &sce->tns;
    int i, w, filt, coef_compress = 0, coef_len;
    const int is8 = sce->ics.window_sequence[0] == EIGHT_SHORT_SEQUENCE;
    const int c_bits = is8 ? TNS_Q_BITS_IS8 == 4 : TNS_Q_BITS == 4;

    if (!sce->tns.present)
        return;

    for (i = 0; i < sce->ics.num_windows; i++) {
        put_bits(&s->pb, 2 - is8, sce->tns.n_filt[i]);
        if (!tns->n_filt[i])
            continue;
        put_bits(&s->pb, 1, c_bits);
        for (filt = 0; filt < tns->n_filt[i]; filt++) {
            put_bits(&s->pb, 6 - 2 * is8, tns->length[i][filt]);
            put_bits(&s->pb, 5 - 2 * is8, tns->order[i][filt]);
            if (!tns->order[i][filt])
                continue;
            put_bits(&s->pb, 1, tns->direction[i][filt]);
            coef_compress = compress_coeffs(tns->coef_idx[i][filt],
                                            tns->order[i][filt], c_bits);
            put_bits(&s->pb, 1, coef_compress);
            coef_len = c_bits + 3 - coef_compress;
            for (w = 0; w < tns->order[i][filt]; w++)
                put_bits(&s->pb, coef_len, tns->coef_idx[i][filt][w]);
        }
    }
}

/* Apply TNS filter */
void ff_aac_apply_tns(AACEncContext *s, SingleChannelElement *sce)
{
    TemporalNoiseShaping *tns = &sce->tns;
    IndividualChannelStream *ics = &sce->ics;
    int w, filt, m, i, top, order, bottom, start, end, size, inc;
    const int mmm = FFMIN(ics->tns_max_bands, ics->max_sfb);
    float lpc[TNS_MAX_ORDER];

    for (w = 0; w < ics->num_windows; w++) {
        bottom = ics->num_swb;
        for (filt = 0; filt < tns->n_filt[w]; filt++) {
            top    = bottom;
            bottom = FFMAX(0, top - tns->length[w][filt]);
            order  = tns->order[w][filt];
            if (order == 0)
                continue;

            // tns_decode_coef
            compute_lpc_coefs(tns->coef[w][filt], order, lpc, 0, 0, 0);

            start = ics->swb_offset[FFMIN(bottom, mmm)];
            end   = ics->swb_offset[FFMIN(   top, mmm)];
            if ((size = end - start) <= 0)
                continue;
            if (tns->direction[w][filt]) {
                inc = -1;
                start = end - 1;
            } else {
                inc = 1;
            }
            start += w * 128;

            /* AR filter */
            for (m = 0; m < size; m++, start += inc) {
                for (i = 1; i <= FFMIN(m, order); i++) {
                    sce->coeffs[start] += lpc[i-1]*sce->pcoeffs[start - i*inc];
                }
            }
        }
    }
}

/*
 * c_bits - 1 if 4 bit coefficients, 0 if 3 bit coefficients
 */
static inline void quantize_coefs(double *coef, int *idx, float *lpc, int order,
                                  int c_bits)
{
    int i;
    const float *quant_arr = tns_tmp2_map[c_bits];
    for (i = 0; i < order; i++) {
        idx[i] = quant_array_idx(coef[i], quant_arr, c_bits ? 16 : 8);
        lpc[i] = quant_arr[idx[i]];
    }
}

/*
 * 3 bits per coefficient with 8 short windows
 */
void ff_aac_search_for_tns(AACEncContext *s, SingleChannelElement *sce)
{
    TemporalNoiseShaping *tns = &sce->tns;
    int w, g, count = 0;
    double gain, coefs[MAX_LPC_ORDER];
    const int mmm = FFMIN(sce->ics.tns_max_bands, sce->ics.max_sfb);
    const int is8 = sce->ics.window_sequence[0] == EIGHT_SHORT_SEQUENCE;
    const int c_bits = is8 ? TNS_Q_BITS_IS8 == 4 : TNS_Q_BITS == 4;
    const int sfb_start = av_clip(tns_min_sfb[is8][s->samplerate_index], 0, mmm);
    const int sfb_end   = av_clip(sce->ics.num_swb, 0, mmm);
    const int order = is8 ? 7 : s->profile == FF_PROFILE_AAC_LOW ? 12 : TNS_MAX_ORDER;
    const int slant = sce->ics.window_sequence[0] == LONG_STOP_SEQUENCE  ? 1 :
                      sce->ics.window_sequence[0] == LONG_START_SEQUENCE ? 0 : 2;
    const int sfb_len = sfb_end - sfb_start;
    const int coef_len = sce->ics.swb_offset[sfb_end] - sce->ics.swb_offset[sfb_start];

    if (coef_len <= 0 || sfb_len <= 0) {
        sce->tns.present = 0;
        return;
    }

    for (w = 0; w < sce->ics.num_windows; w++) {
        float en[2] = {0.0f, 0.0f};
        int oc_start = 0, os_start = 0;
        int coef_start = sce->ics.swb_offset[sfb_start];

        for (g = sfb_start; g < sce->ics.num_swb && g <= sfb_end; g++) {
            FFPsyBand *band = &s->psy.ch[s->cur_channel].psy_bands[w*16+g];
            if (g > sfb_start + (sfb_len/2))
                en[1] += band->energy;
            else
                en[0] += band->energy;
        }

        /* LPC */
        gain = ff_lpc_calc_ref_coefs_f(&s->lpc, &sce->coeffs[w*128 + coef_start],
                                       coef_len, order, coefs);

        if (!order || !isfinite(gain) || gain < TNS_GAIN_THRESHOLD_LOW || gain > TNS_GAIN_THRESHOLD_HIGH)
            continue;

        tns->n_filt[w] = is8 ? 1 : order != TNS_MAX_ORDER ? 2 : 3;
        for (g = 0; g < tns->n_filt[w]; g++) {
            tns->direction[w][g] = slant != 2 ? slant : en[g] < en[!g];
            tns->order[w][g] = g < tns->n_filt[w] ? order/tns->n_filt[w] : order - oc_start;
            tns->length[w][g] = g < tns->n_filt[w] ? sfb_len/tns->n_filt[w] : sfb_len - os_start;
            quantize_coefs(&coefs[oc_start], tns->coef_idx[w][g], tns->coef[w][g],
                            tns->order[w][g], c_bits);
            oc_start += tns->order[w][g];
            os_start += tns->length[w][g];
        }
        count++;
    }
    sce->tns.present = !!count;
}