Content for TS 26.253 Word version: 18.1.1
6 Functional description of the decoder
6.1 Decoder overview
6.2 Common processing and decoding tools
6.3 Stereo audio decoding
6.4 Scene-based audio (SBA) decoding
6.5 Metadata-assisted spatial audio (MASA) decoding
6.6 Object-based audio (ISM) decoding
6.7 Multi-channel audio (MC) decoding
6.8 Combined Object-based audio and SBA (OSBA) decoding
6.9 Combined Object-based audio and MASA (OMASA) decoding
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6 Functional description of the decoder p. 444
6.1 Decoder overview p. 444
6.2 Common processing and decoding tools p. 445
6.2.1 Common decoder processing overview p. 445
6.2.1.1 High-pass filtering p. 445
6.2.1.2 Limiter p. 445
6.2.1.2.1 Overview p. 445
6.2.1.2.2 Detailed algorithmic description p. 445
6.2.1.2.3 Handling of strong saturations p. 446
6.2.1.3 Time-domain decorrelator p. 448
6.2.2 Core-decoder processing p. 449
6.2.2.1 Overview p. 449
6.2.2.2 LP based decoding p. 450
6.2.2.2.1 Variable bitrate ACELP decoding p. 450
6.2.2.2.2 Fast algebraic codebook decoding p. 450
6.2.2.2.3 AVQ Bit Savings Decoder p. 450
6.2.2.2.4 Bass Post-Filter p. 454
6.2.2.2.5 GSC core decoding p. 454
6.2.2.2.6 Bandwidth extension in time domain p. 457
6.2.2.2.7 Multimode FD bandwidth extension coding p. 457
6.2.2.3 MDCT based decoding p. 457
6.2.2.3.1 General p. 457
6.2.2.3.2 Variable bitrate MDCT decoding p. 457
6.2.2.3.3 TCX entropy decoding p. 457
6.2.2.3.4 PLC method selection p. 459
6.2.2.3.5 Phase ECU enhancements p. 460
6.2.2.3.6 Long term prediction processing p. 461
6.2.2.4 Background noise estimation p. 463
6.2.2.4.1 General p. 463
6.2.2.4.2 Power spectrum compression p. 463
6.2.2.4.3 Compensation for the loss of variance p. 465
6.2.2.4.4 Spectral smoothing p. 465
6.2.2.4.5 Initialization period p. 466
6.2.2.4.6 Power spectrum expansion p. 468
6.2.2.5 Switching coding modes p. 468
6.2.2.6 DTX/CNG operation p. 469
6.2.3 Common audio decoding tools p. 470
6.2.3.1 General p. 470
6.2.3.2 Single Channel Element (SCE) decoder p. 470
6.2.3.3 Channel Pair Element (CPE) decoder p. 471
6.2.3.4 Multichannel coding tool (MCT) decoder p. 473
6.2.3.4.1 General overview p. 473
6.2.3.4.2 Core and SNS parameter decoding p. 473
6.2.3.4.3 MCT parameter decoding p. 473
6.2.3.4.4 Bitrate distribution p. 473
6.2.3.4.5 Spectral data decoding and noise filling p. 474
6.2.3.4.6 Application of IGF and stereo IGF p. 474
6.2.3.4.7 MCT decoding p. 474
6.2.3.4.8 Noise shaping p. 474
6.2.3.4.9 Inverse MDCT transform to time domain p. 475
6.2.3.4.10 MCT PLC p. 475
6.2.4 Common spatial metadata decoding tools p. 475
6.2.4.1 Direction metadata decoding tools p. 475
6.2.4.1.1 Overview p. 475
6.2.4.1.2 Direction dequantization tools p. 475
6.2.4.1.3 Direction metadata raw decoding p. 476
6.2.4.1.4 Direction metadata entropy decoding tools p. 476
6.2.4.2 Diffuseness and energy ratio decoding methods p. 477
6.2.4.2.1 Diffuseness decoding p. 477
6.2.4.2.2 Diffuseness and energy ratio decoding with two concurrent directions p. 477
6.2.4.3 Direction metadata decoding methods p. 478
6.2.4.3.1 Direction metadata decoding overview p. 478
6.2.4.3.2 Decoding EC1 p. 478
6.2.4.3.3 Decoding EC2 p. 479
6.2.4.3.4 Decoding EC3 p. 479
6.2.4.4 Coherence decoding p. 481
6.2.4.5 DTX decoding p. 482
6.2.5 Common filter bank operation tools p. 483
6.2.5.1 Complex Low-delay Filter Bank (CLDFB) analysis p. 483
6.2.5.2 Complex Low-delay Filter Bank (CLDFB) synthesis p. 483
6.2.6 CLDFB domain decorrelator p. 484
6.2.6.1 Overview p. 484
6.2.6.2 Onset detection and removal p. 484
6.2.6.3 De-correlation p. 485
6.2.6.3.1 Overview p. 485
6.2.6.3.2 Derivation of filter coefficients from lattice coefficients p. 485
6.2.6.3.3 Decorrelator IIR Filtering p. 488
6.2.6.4 Energy adjustment p. 489
6.2.6.5 Final output p. 489
6.2.7 Common tools for decoding with time scale modification (JBM) p. 490
6.2.7.1 Overview p. 490
6.2.7.2 Transport channel buffer management p. 490
6.2.7.3 Time scale modification p. 491
6.2.7.4 Metadata and processing parameter adaption p. 492
6.2.7.4.1 Overview p. 492
6.2.7.4.2 Metadata adaption p. 492
6.2.7.4.3 Processing parameter adaption p. 494
6.2.7.5 Bitrate switching p. 496
6.3 Stereo audio decoding p. 497
6.3.1 Common stereo decoding tools p. 497
6.3.1.1 Common Stereo postprocessing p. 497
6.3.1.2 ITD compensation p. 497
6.3.2 Unified stereo decoding p. 498
6.3.2.1 Inter-channel BWE (IC-BWE) decoder p. 498
6.3.2.1.1 Inter-Channel BWE Decoder overview p. 498
6.3.2.1.2 Inter-Channel BWE Decoder Block p. 499
6.3.2.2 TD-based stereo decoding p. 500
6.3.2.2.1 TD stereo decoder overview p. 500
6.3.2.2.2 Reusing close-loop pitch and adaptive cobebook of the primary channel p. 501
6.3.2.2.3 Reusing LP filter coefficients of the primary channel and dequantization of LSF parameters for the secondary channel p. 502
6.3.2.2.4 Estimation of spatial parameters of the background noise p. 503
6.3.2.3 DFT-based stereo decoding p. 503
6.3.2.3.1 General p. 503
6.3.2.3.2 STFT analysis p. 503
6.3.2.3.3 Stereo parameter decoding p. 504
6.3.2.3.4 Residual signal decoding p. 505
6.3.2.3.5 Bass post filter application p. 506
6.3.2.3.6 Residual predicted signal generation p. 508
6.3.2.3.7 Stereo upmix p. 515
6.3.2.3.8 Stereo comfort noise injection p. 515
6.3.2.3.9 STFT synthesis p. 517
6.3.2.3.10 DFT-based stereo PLC p. 518
6.3.3 MDCT-based stereo decoding p. 521
6.3.3.1 General Overview p. 521
6.3.3.2 Decoding of parameters p. 521
6.3.3.2.1 TCX block configuration p. 521
6.3.3.2.2 Core parameters decoding p. 521
6.3.3.2.3 SNS parameters decoding p. 521
6.3.3.2.4 Stereo parameters decoding p. 523
6.3.3.3 Decoding process p. 524
6.3.3.3.1 Spectral data decoding and noise filling p. 524
6.3.3.3.2 Application of stereo IGF p. 524
6.3.3.4 Stereo decoding process p. 525
6.3.3.5 Post-stereo decoding process p. 527
6.3.3.5.1 General p. 527
6.3.3.5.2 Spectral noise shaping p. 527
6.3.3.5.3 Temporal noise shaping p. 528
6.3.3.6 Frequency-to-time domain transformations p. 528
6.3.3.6.1 General p. 528
6.3.3.6.2 Kernel switching p. 528
6.3.3.6.3 Transition between long and short blocks with double overlap p. 528
6.3.3.7 PLC in MDCT-based stereo p. 530
6.3.4 Switching between stereo modes p. 535
6.3.4.1 Overview p. 535
6.3.4.2 General p. 535
6.3.4.3 Memory handling p. 538
6.3.4.4 Switching between DFT and TD stereo modes p. 539
6.3.4.4.1 Switching from TD stereo to DFT stereo p. 539
6.3.4.4.2 Switching from DFT stereo to TD stereo p. 542
6.3.4.5 Switching between DFT and MDCT stereo modes p. 545
6.3.4.5.1 Switching from DFT stereo to MDCT stereo p. 545
6.3.4.5.2 Switching from MDCT stereo to DFT stereo p. 545
6.3.4.6 Switching between TD and MDCT stereo modes p. 545
6.3.5 DTX operation p. 546
6.3.5.1 DTX in Unified stereo p. 546
6.3.5.1.1 General p. 546
6.3.5.1.2 Stereo CNG spectral shape extraction p. 546
6.3.5.1.3 Stereo CNG side gain, ITD and IPD decoding p. 549
6.3.5.1.4 Stereo CNG coherence decoding p. 550
6.3.5.1.5 Stereo CNG synthesis p. 551
6.3.5.2 DTX in MDCT-based stereo p. 551
6.3.6 Stereo output format conversion p. 553
6.3.6.1 Overview p. 553
6.3.6.2 Stereo-to-Mono output p. 553
6.3.6.2.1 DFT stereo mono output p. 553
6.3.6.2.2 TD stereo mono output p. 554
6.3.6.2.3 MDCT stereo mono output p. 554
6.3.6.3 Stereo-to-MC output p. 558
6.3.7 Stereo audio decoding with TSM p. 558
6.4 Scene-based audio (SBA) decoding p. 558
6.4.1 Combined DirAC and SPAR based SBA decoding overview p. 558
6.4.1.1 Overview p. 558
6.4.1.2 FOA signal decoding at all bitrates and HOA2, HOA3 signal decoding at bitrates below 256 kbps p. 559
6.4.1.2.1 Overview p. 559
6.4.1.2.2 Metadata decoding p. 560
6.4.1.2.3 Core-coder decoding p. 560
6.4.1.2.4 SBA upmix and rendering p. 560
6.4.1.3 HOA2 and HOA3 signal decoding at bitrate 256 kbps p. 561
6.4.1.3.1 Overview p. 561
6.4.1.3.2 Metadata decoding p. 561
6.4.1.3.3 Core-coder decoding p. 561
6.4.1.3.4 SBA upmix and rendering p. 562
6.4.1.4 HOA2 and HOA3 signal decoding at bitrates 384 kbps p. 562
6.4.1.4.1 Overview p. 562
6.4.1.4.2 Metadata decoding p. 563
6.4.1.4.3 Core-coder decoding p. 563
6.4.1.4.4 SBA upmix and rendering p. 563
6.4.1.5 HOA2 and HOA3 signal coding at bitrates 512 kbps p. 564
6.4.1.5.1 Overview p. 564
6.4.1.5.2 Metadata decoding p. 564
6.4.1.5.3 Core-coder decoding p. 564
6.4.1.5.4 SBA upmix and rendering p. 564
6.4.2 DirAC parameter decoding p. 565
6.4.3 SPAR parameter decoding p. 565
6.4.3.1 General p. 565
6.4.3.2 Set active W prediction flag p. 565
6.4.3.3 Decode quantization and coding strategy bits p. 565
6.4.3.4 Time differential decoding p. 566
6.4.3.4.1 General p. 566
6.4.3.4.2 Time differential decoding across same quantization strategies p. 566
6.4.3.4.3 Time differential decoding across different quantization strategies p. 567
6.4.3.5 Band interleaved decoding at 13.2 kbps and 16.4 kbps p. 567
6.4.3.6 Arithmetic decoder p. 567
6.4.3.7 Huffman decoder p. 567
6.4.4 SPAR and DirAC parameter merge p. 567
6.4.4.1 SBA mono handling p. 567
6.4.4.2 SPAR to DirAC parameter conversion p. 568
6.4.4.3 DirAC model for the SPAR parameters p. 569
6.4.5 Upmix matrix calculation p. 570
6.4.5.1 Overview p. 570
6.4.5.2 SPAR matrix generation (P and C matrices) p. 570
6.4.5.3 Upmix matrix generation p. 571
6.4.6 Decoded audio processing p. 571
6.4.6.1 Downmix signal decoding (core-decoding) p. 571
6.4.6.2 AGC p. 571
6.4.6.3 PCA p. 572
6.4.6.4 SPAR upmix processing p. 574
6.4.6.4.1 General p. 574
6.4.6.4.2 Time domain decorrelator p. 574
6.4.6.4.3 CLDFB analysis p. 574
6.4.6.4.4 CLDFB upmixing p. 574
6.4.6.4.5 MDFT filterbank banded upmix matrix to CLDFB banded upmix matrix conversion p. 576
6.4.6.4.6 Crossfading in CLDFB p. 577
6.4.6.4.7 Additional VLBR smoothing p. 577
6.4.6.5 DirAC synthesis and rendering p. 577
6.4.6.5.1 Overview p. 577
6.4.6.5.2 Mode selection p. 578
6.4.6.5.3 High-order operation mode p. 578
6.4.6.5.4 Low-order operation mode p. 581
6.4.6.5.5 Bitrate switching p. 584
6.4.6.5.6 Binaural rendering p. 584
6.4.6.5.7 Loudspeaker rendering p. 585
6.4.6.5.8 Stereo and mono output p. 587
6.4.6.6 CLDFB synthesis p. 591
6.4.7 DTX operation p. 592
6.4.8 SBA PLC p. 593
6.4.8.1 PLC in DirAC p. 593
6.4.8.2 PLC in SPAR p. 593
6.4.8.2.1 SPAR processing of erroneous frames p. 593
6.4.8.2.2 Recovery after one or more lost frames p. 594
6.4.8.2.3 SPAR parameter recovery for 24.4 kbps and above p. 594
6.4.8.2.4 SPAR parameter recovery for 13.2 kbps and 16.4 kbps p. 594
6.4.8.2.5 Interpolation of missing SPAR parameters from current existing SPAR parameters p. 594
6.4.9 SBA bitrate switching p. 595
6.4.10 SBA output format conversion p. 595
6.4.11 SBA decoding with TSM p. 596
6.5 Metadata-assisted spatial audio (MASA) decoding p. 596
6.5.1 Overview p. 596
6.5.2 Decoding MASA configuration p. 597
6.5.3 MASA metadata decoding p. 598
6.5.3.1 Overview p. 598
6.5.3.2 Energy ratio decoding p. 599
6.5.3.2.1 Energy ratio decoding for bitrates up to 256 kbps p. 599
6.5.3.2.2 Energy ratio decoding for 384 kbps and 512 kbps p. 599
6.5.3.3 Direction decoding p. 599
6.5.3.3.1 Direction decoding for bitrates up to 256 kbps p. 599
6.5.3.3.2 Direction decoding for 384 kbps and 512 kbps p. 599
6.5.3.4 Spread coherence decoding p. 600
6.5.3.4.1 Spread coherence decoding for bitrates up to 256 kbps p. 600
6.5.3.4.2 Spread coherence decoding for 384 kbps and 512 kbps p. 600
6.5.3.5 Surround coherence decoding p. 600
6.5.3.5.1 Surround coherence decoding for bitrates up to 256 kbps p. 600
6.5.3.5.2 Surround coherence decoding for 384 kbps and 512 kbps p. 600
6.5.3.6 Decoding of the second direction parameters p. 600
6.5.3.6.1 Joint decoding of parameters associated with first and second sound source directions p. 600
6.5.3.6.2 Independent decoding of parameters associated with first and second sound source directions p. 600
6.5.3.7 Reconstruction of MASA metadata from low-rate reduction p. 600
6.5.3.8 Determining encoding metric based on the quality of the spatial metadata representation in encoding p. 601
6.5.3.9 Reconstruction of full MASA format metadata p. 602
6.5.4 MASA DTX operation p. 603
6.5.5 MASA PLC p. 603
6.5.6 MASA bitrate switching p. 603
6.5.7 MASA rendering p. 603
6.5.7.1 Binaural and stereo rendering p. 603
6.5.7.2 Multi-channel loudspeaker and Ambisonics rendering p. 603
6.5.7.2.1 Overview p. 603
6.5.7.2.2 Determining direct and diffuse power factors and surround coherence ratio p. 604
6.5.7.2.3 Determining directional responses p. 604
6.5.7.2.4 Determining diffuse responses p. 608
6.5.7.2.5 Determining prototype audio signals for multi-channel loudspeaker output p. 608
6.5.7.2.6 Determining prototype audio signals for Ambisonics output p. 609
6.5.7.2.7 Decorrelation p. 612
6.5.7.2.8 Spatial synthesis p. 612
6.5.7.3 Mono rendering p. 615
6.5.7.4 Output for external processing p. 615
6.5.8 MASA decoding with TSM p. 615
6.6 Object-based audio (ISM) decoding p. 615
6.6.1 Discrete ISM decoding mode p. 615
6.6.2 Parametric ISM decoding mode p. 616
6.6.2.1 General p. 616
6.6.2.2 Parameters p. 617
6.6.2.3 Downmix p. 617
6.6.2.4 ParamISM Decoding and Rendering p. 618
6.6.3 ISM metadata decoding p. 622
6.6.4 DTX operation decoding p. 622
6.6.4.1 Overview p. 622
6.6.4.2 CNG control parameter decoding p. 622
6.6.4.3 Metadata Dequantization and Decoding p. 623
6.6.4.4 ISM DTX core decoding and CNG p. 623
6.6.5 ISM PLC p. 625
6.6.6 ISM bitrate switching p. 625
6.6.6.1 Metadata handling in bitrate switching p. 625
6.6.6.2 Codec reconfiguration in bitrate switching p. 626
6.6.7 ISM output format conversion p. 626
6.6.7.1 Overview p. 626
6.6.7.2 Mono downmix p. 627
6.6.8 ISM decoding with TSM p. 628
6.7 Multi-channel audio (MC) decoding p. 628
6.7.1 LFE channel decoding p. 628
6.7.1.1 Overview p. 628
6.7.1.2 Bitstream decoding p. 628
6.7.1.3 MDCT coefficient reconstruction p. 629
6.7.1.4 Inverse MDCT p. 629
6.7.1.5 Windowing and overlap-add p. 630
6.7.1.6 Output LPF and delay adjustment p. 630
6.7.1.7 LFE PLC p. 630
6.7.2 Multi-channel MASA (McMASA) decoding mode p. 633
6.7.2.1 McMASA decoding mode overview p. 633
6.7.2.2 McMASA metadata decoding p. 633
6.7.2.2.1 Decoding McMASA configuration p. 633
6.7.2.2.2 Spatial metadata decoding p. 633
6.7.2.2.3 LFE-to-total energy ratio decoding p. 633
6.7.2.3 McMASA transport audio signal decoding p. 634
6.7.2.4 McMASA rendering p. 634
6.7.2.4.1 Binaural rendering p. 634
6.7.2.4.2 Multi-channel loudspeaker rendering p. 634
6.7.2.4.3 Ambisonics rendering p. 639
6.7.2.4.4 Stereo rendering p. 639
6.7.2.4.5 Mono rendering p. 639
6.7.2.5 McMASA PLC p. 639
6.7.2.6 McMASA decoding with TSM p. 639
6.7.3 Parametric MC decoding mode p. 640
6.7.3.1 ParamMC Overview p. 640
6.7.3.2 ParamMC configuration p. 640
6.7.3.3 ParamMC Parameter Decoding p. 640
6.7.3.4 ParamMC Transport Audio Signal Decoding p. 643
6.7.3.5 ParamMC Synthesis p. 643
6.7.3.5.1 ParamMC Synthesis Overview p. 643
6.7.3.5.2 ParamMC Target Covariance Calculation p. 643
6.7.3.5.3 ParamMC Mixing Matrix Calculation p. 645
6.7.3.5.4 Decorrelation p. 648
6.7.3.5.5 Upmix p. 648
6.7.3.6 ParamMC output processing p. 649
6.7.3.6.1 ParamMC output processing selection p. 649
6.7.3.6.2 ParamMC mono/stereo output processing p. 649
6.7.3.6.3 ParamMC loudspeaker output processing p. 650
6.7.3.6.4 ParamMC Ambisonics output processing p. 651
6.7.3.6.5 ParamMC binaural output processing p. 651
6.7.3.7 ParamMC PLC p. 652
6.7.3.8 ParamMC bit rate switching p. 652
6.7.3.9 ParamMC decoding with TSM p. 653
6.7.4 Parametric upmix MC decoding mode p. 653
6.7.4.1 General p. 653
6.7.4.2 Special case of Binaural, Stereo and Mono rendering configurations p. 654
6.7.4.2.1 Binaural rendering configuration p. 654
6.7.4.2.2 Stereo and Mono rendering configuration p. 654
6.7.4.3 Metadata Huffman Decoding p. 654
6.7.4.4 Dequantization p. 654
6.7.4.5 Decorrelation p. 654
6.7.4.6 Upmixing operation p. 655
6.7.4.7 Output p. 655
6.7.4.8 Param Upmix PLC p. 655
6.7.4.9 Param Upmix MC decoding with TSM p. 656
6.7.5 Discrete MC decoding mode p. 656
6.7.5.1 Overview p. 656
6.7.5.2 Discrete MC PLC p. 656
6.7.6 MC bitrate switching p. 656
6.7.7 MC output format conversion p. 657
6.7.7.1 Overview p. 657
6.7.7.2 Common processing tools p. 657
6.7.7.2.1 Conversion of transport signals to TCX20 resolution p. 657
6.7.7.2.2 Computation of Equalization gain p. 657
6.7.7.3 Equalization of TCX Spectra p. 657
6.7.7.4 Rendering to a supported IVAS format p. 658
6.7.7.5 Rendering to a custom loudspeaker layout p. 663
6.7.7.6 Rendering to binaural output with head tracking p. 663
6.7.8 MC decoding with TSM p. 664
6.8 Combined Object-based audio and SBA (OSBA) decoding p. 664
6.8.1 Low-bitrate pre-rendering OSBA decoding mode p. 664
6.8.2 High-bitrate discrete OSBA decoding mode p. 664
6.8.3 OSBA PLC p. 665
6.8.4 OSBA bitrate switching p. 665
6.8.5 OSBA output format conversion p. 665
6.8.6 OSBA decoding with TSM p. 665
6.9 Combined Object-based audio and MASA (OMASA) decoding p. 667
6.9.1 OMASA format decoder overview p. 667
6.9.2 Low-bitrate pre-rendering (Rend OMASA) decoding mode p. 668
6.9.3 One object with MASA (One OMASA) decoding mode p. 668
6.9.4 Parametric one object (Param OMASA) decoding mode p. 668
6.9.4.1 Overview p. 668
6.9.4.2 MASA-to-total ratios decoding p. 669
6.9.4.3 ISM energy ratios decoding p. 670
6.9.4.4 ISM metadata decoding p. 671
6.9.5 Discrete (Disc OMASA) decoding mode p. 672
6.9.6 OMASA rendering metadata generation p. 672
6.9.7 OMASA rendering p. 672
6.9.7.1 Binaural rendering p. 672
6.9.7.1.1 Discrete coding mode p. 672
6.9.7.1.2 Other coding modes p. 673
6.9.7.2 Stereo rendering p. 673
6.9.7.3 Multi-channel loudspeaker and Ambisonics rendering p. 673
6.9.7.3.1 Overview p. 673
6.9.7.3.2 Rendering of the MASA part p. 674
6.9.7.4 Mono rendering p. 674
6.9.8 OMASA PLC p. 674
6.9.9 OMASA bitrate switching p. 675
6.9.10 OMASA decoding with Time Scale Modification (TSM) p. 675
6.9.11 OMASA decoding to original combined input format p. 675
6.9.11.1 Overview p. 675
6.9.11.2 Decoding to original combined input format in pre-rendering mode p. 675
6.9.11.3 Decoding to original combined input format in one object with MASA representation mode p. 675
6.9.11.4 Decoding to original combined input format in parametric one object mode p. 677
6.9.11.5 Decoding to original combined input format in discrete mode p. 679
