6  Functional description of the Decoder  Word‑p. 453

6.1  LP-based Decoding

6.1.1  General LP-based decoding

6.1.1.1  LSF decoding

6.1.1.1.1  General LSF decoding

6.1.1.1.2  LSF decoding for voiced coding mode at 16 kHz internal sampling frequency  Word‑p. 456

6.1.1.2  Reconstruction of the excitation  Word‑p. 458

6.1.1.2.1  Reconstruction of the excitation in GC and VC modes and high rate IC/UC modes

6.1.1.2.2  Reconstruction of the excitation in TC mode  Word‑p. 465

6.1.1.2.3  Reconstruction of the excitation in UC mode at low rates

6.1.1.2.4  Reconstruction of the excitation in IC/UC mode at 9.6 kbps  Word‑p. 467

6.1.1.2.5  Reconstruction of the excitation in GSC  Word‑p. 468

6.1.1.3  Excitation post-processing  Word‑p. 469

6.1.1.3.1  Anti-sparseness processing

6.1.1.3.2  Gain smoothing for noise enhancement

6.1.1.3.3  Pitch enhancer  Word‑p. 470

6.1.1.3.4  Music post processing  Word‑p. 471

6.1.2  Source Controlled VBR decoding  Word‑p. 478

6.1.3  Synthesis

6.1.4  Post-processing  Word‑p. 479

6.1.4.1  Adaptive post-filtering

6.1.4.1.1  Long-term post-filter

6.1.4.1.2  Short-term post-filter  Word‑p. 480

6.1.4.1.3  Post-filter NB parameters

6.1.4.1.4  Post-filter WB and SWB parameters  Word‑p. 481

6.1.4.1.5  Tilt compensation

6.1.4.1.6  Adaptive gain control  Word‑p. 482

6.1.4.2  Bass post-filter

6.1.5  Decoding of upper band for LP-based Coding Modes  Word‑p. 485

6.1.5.1  Decoding Time-domain Bandwidth Extension

6.1.5.1.1  Generation of the upsampled version of the lowband excitation

6.1.5.1.2  Non-Linear Excitation Generation  Word‑p. 486

6.1.5.1.3  De-quantization of high band parameters  Word‑p. 487

6.1.5.1.4  LSP interpolation

6.1.5.1.5  Spectral flip in time domain

6.1.5.1.6  Down-sample using all-pass filters

6.1.5.1.7  Adaptive spectral whitening  Word‑p. 488

6.1.5.1.8  Envelope modulated noise mixing

6.1.5.1.9  Spectral shaping of the noise added excitation  Word‑p. 490

6.1.5.1.10  Post processing of the shaped excitation

6.1.5.1.11  Gain shape update

6.1.5.1.12  SHB synthesis  Word‑p. 491

6.1.5.1.13  Core-switching and high-band memory updates  Word‑p. 492

6.1.5.1.14  TEC/TFA post processing  Word‑p. 493

6.1.5.1.15  Full-band synthesis  Word‑p. 495

6.1.5.2  Multi-mode FD Bandwidth Extension decoding

6.1.5.2.1  SWB multi-mode FD BWE decoding

6.1.5.2.2  WB multi-mode FD BWE decoding  Word‑p. 503

6.1.5.3  Decoding of upper band at 64 kb/s  Word‑p. 511

6.1.5.3.1  Decoding in normal mode

6.1.5.3.2  Decoding in transient mode  Word‑p. 516

6.1.5.3.3  Windowing and frequency-to-time transformation  Word‑p. 518

6.1.5.3.4  Post-processing in temporal domain

6.2  MDCT Coding mode decoding  Word‑p. 521

6.2.1  General MDCT decoding

6.2.2  MDCT based TCX

6.2.2.1  Rate dependent configurations

6.2.2.2  Init module parameters

6.2.2.2.1  TCX block configuration

6.2.2.2.2  LPC parameter

6.2.2.2.3  PLC Wavefrom adjustment  Word‑p. 522

6.2.2.2.4  Global Gain

6.2.2.2.5  Noise fill parameter  Word‑p. 523

6.2.2.2.6  LTP

6.2.2.2.7  TNS parameter

6.2.2.2.8  Harmonic model

6.2.2.2.9  IGF bit stream reader

6.2.2.2.10  Spectral data  Word‑p. 526

6.2.2.2.11  Residual bits

6.2.2.3  Decoding process  Word‑p. 527

6.2.2.3.1  Arithmetic decoder

6.2.2.3.2  Adaptive low frequency de-emphasis  Word‑p. 528

6.2.2.3.3  Global gain decoding  Word‑p. 529

6.2.2.3.4  Residual bits decoding

6.2.2.3.5  TCX formant enhancement  Word‑p. 530

6.2.2.3.6  Noise Filling  Word‑p. 531

6.2.2.3.7  Apply global gain and LPC shaping in MDCT domain  Word‑p. 532

6.2.2.3.8  IGF apply  Word‑p. 533

6.2.2.3.9  Inverse window grouping (TCX5 separation)  Word‑p. 538

6.2.2.3.10  Temporal Noise Shaping  Word‑p. 539

6.2.2.3.11  IGF temporal flattening

6.2.3  High Quality MDCT decoder (HQ)  Word‑p. 540

6.2.3.1  Low-rate HQ decoder

6.2.3.1.1  Mode decoding

6.2.3.1.2  Energy Envelope decoding

6.2.3.1.3  Spectral coefficients decoding  Word‑p. 542

6.2.3.2  High-rate HQ decoder  Word‑p. 552

6.2.3.2.1  Normal Mode

6.2.3.2.2  Transient Mode  Word‑p. 561

6.2.3.2.3  Harmonic Mode

6.2.3.2.4  HVQ  Word‑p. 564

6.2.3.2.5  Generic Mode  Word‑p. 565

6.2.3.2.6  PVQ decoding and de-indexing  Word‑p. 570

6.2.4  Frequency-to-time transformation  Word‑p. 572

6.2.4.1  Long block transformation (ALDO window)

6.2.4.1.1  eDCT

6.2.4.1.2  Unfolding and windowing

6.2.4.1.3  Overlap-add  Word‑p. 573

6.2.4.1.4  Pre-echo attenuation  Word‑p. 574

6.2.4.2  Transient location dependent overlap and transform length  Word‑p. 580

6.2.4.3  Short block transformation

6.2.4.3.1  Short window transform in TDA domain

6.2.4.3.2  Short window transform for MDCT based TCX  Word‑p. 581

6.2.4.4  Special window transitions

6.2.4.4.1  ALDO to short transition

6.2.4.4.2  Short to ALDO transition  Word‑p. 582

6.2.4.5  Low Rate MDCT Synthesis

6.3  Switching coding modes in decoding

6.3.1  General description

6.3.2  MDCT coding mode to CELP coding mode  Word‑p. 583

6.3.2.1  MDCT to CELP transition 1 (MC1)

6.3.2.2  MDCT to CELP transition 2 (MC2)  Word‑p. 584

6.3.2.3  MDCT to CELP transition 3 (MC3)

6.3.3  CELP coding mode to MDCT coding mode

6.3.3.1  CELP coding mode to MDCT based TCX coding mode  Word‑p. 585

6.3.3.2  CELP coding mode to HQ MDCT coding mode  Word‑p. 586

6.3.3.2.1  Constrained CELP decoding and simplified BWE decoding  Word‑p. 587

6.3.3.2.1.1  Optimized cubic interpolation

6.3.3.2.2  HQ MDCT decoding with a modified synthesis window  Word‑p. 588

6.3.3.2.3  Cross-fading

6.3.4  Internal sampling rate switching

6.3.4.1  Reset of LPC memory

6.3.4.2  Conversion of LP filter between 12.8 and 16 kHz internal sampling rates  Word‑p. 589

6.3.4.3  Extrapolation of LP filter

6.3.4.4  Update of CELP synthesis memories

6.3.4.5  Update of CELP decoded past signal

6.3.4.6  Post-processing

6.3.4.6.1  Adaptive post-filtering

6.3.4.6.2  Bass post filter

6.3.4.7  CLDFB

6.3.5  EVS primary modes and AMR-WB IO  Word‑p. 590

6.3.5.1  Switching from primary modes to AMR-WB IO

6.3.5.2  Switching from AMR-WB IO mode to primary modes

6.3.6  Rate switching

6.3.6.1  Rate switching along with internal sampling rate switching

6.3.6.2  Rate switching along with coding mode switching

6.3.6.3  Adaptive post-filter reset and smoothing

6.3.7  Bandwidth switching

6.3.7.1  Bandwidth switching detector

6.3.7.2  Super wideband switching to wideband  Word‑p. 591

6.3.7.2.1  TBE mode

6.3.7.2.2  Multi-mode FD BWE mode  Word‑p. 593

6.3.7.2.3  MDCT core  Word‑p. 594

6.3.7.3  Wideband switching to super wideband

6.4  De-emphasis

6.5  Resampling to the output sampling frequency

6.6  Decoding of frame erasure concealment side information

6.7  Decoding in DTX/CNG operation  Word‑p. 595

6.7.1  Overview

6.7.2  Decoding for LP-CNG

6.7.2.1  LP-CNG decoding Overview

6.7.2.1.1  CNG parameter updates in active periods

6.7.2.1.2  DTX-hangover based parameter analysis in LP-CNG mode  Word‑p. 596

6.7.2.1.3  LP-CNG low-band energy decoding  Word‑p. 598

6.7.2.1.4  LP-CNG low-band filter parameters decoding

6.7.2.1.5  LP-CNG low-band excitation generation

6.7.2.1.6  LP-CNG low-band synthesis  Word‑p. 600

6.7.2.1.7  LP-CNG high-band decoding and synthesis

6.7.2.2  Memory update  Word‑p. 602

6.7.3  Decoding for FD-CNG

6.7.3.1  Decoding SID frames in FD-CNG

6.7.3.1.1  SID parameters decoding  Word‑p. 603

6.7.3.1.2  SID parameters interpolation

6.7.3.1.3  LPC estimation from the interpolated SID parameters  Word‑p. 604

6.7.3.2  Noise tracking during active frames in FD-CNG

6.7.3.2.1  Spectral partition energies

6.7.3.2.2  FD-CNG noise estimation  Word‑p. 605

6.7.3.2.3  Noise shaping in FD-CNG

6.7.3.3  Noise generation for SID or zero frames in FD-CNG  Word‑p. 607

6.7.3.3.1  Update of the noise levels for FD-CNG

6.7.3.3.2  Comfort noise generation in the frequency domain

6.7.3.3.3  Comfort noise generation in the time domain

6.7.3.3.4  FD-CNG decoder memory update  Word‑p. 609

6.8  AMR-WB-interoperable modes  Word‑p. 610

6.8.1  Decoding and speech synthesis

6.8.1.1  Excitation decoding

6.8.1.2  Excitation post-processing  Word‑p. 611

6.8.1.2.1  Anti-sparseness processing

6.8.1.2.2  Gain smoothing for noise enhancement

6.8.1.2.3  Pitch enhancer

6.8.1.3  Synthesis filtering

6.8.1.4  Music and Unvoiced/inactive Post-processing

6.8.1.4.1  Music post processing

6.8.1.4.2  Unvoiced and inactive post processing  Word‑p. 612

6.8.1.5  Synthesis filtering and overwriting the current CELP synthesis  Word‑p. 615

6.8.1.6  Formant post-filter

6.8.1.7  Comfort noise addition

6.8.1.8  Bass post-filter

6.8.2  Resampling  Word‑p. 616

6.8.3  High frequency band

6.8.3.1  Preliminary estimation steps

6.8.3.1.1  Estimation of tilt, figure of merit and voice factors  Word‑p. 617

6.8.3.1.2  Estimation of sub-frame gains based on LP spectral envelopes  Word‑p. 618

6.8.3.2  Generation of high-band excitation  Word‑p. 621

6.8.3.2.1  DCT

6.8.3.2.2  High band generation

6.8.3.2.3  Extraction of tonal and ambiance components  Word‑p. 623

6.8.3.2.4  Recombination  Word‑p. 624

6.8.3.2.5  Filtering in DCT domain  Word‑p. 625

6.8.3.2.6  Inverse DCT  Word‑p. 626

6.8.3.2.7  Gain computation and scaling of excitation

6.8.3.3  LP filter for the high frequency band  Word‑p. 628

6.8.3.4  High band synthesis

6.8.4  CNG decoding

6.9  Common post-processing  Word‑p. 629

6.9.1  Comfort noise addition

6.9.1.1  Noisy speech detection

6.9.1.2  Noise estimation for CNA  Word‑p. 630

6.9.1.2.1  CNA noise estimation in DTX-on mode when FD-CNG is triggered

6.9.1.2.2  CNA noise estimation in DTX-on mode when LP-CNG is triggered

6.9.1.2.3  CNA noise estimation in DTX-off mode

6.9.1.3  Noise generation in the FFT domain and addition in the time domain

6.9.1.4  Noise generation and addition in the MDCT domain  Word‑p. 631

6.9.2  Long term prediction processing

6.9.2.1  Decoding LTP parameters

6.9.2.2  LTP post filtering  Word‑p. 632

6.9.3  Complex low delay filter bank synthesis  Word‑p. 634

6.9.4  High pass filtering  Word‑p. 635

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