Internet Engineering Task Force                           J. Buethe, Ed.Internet-Draft                                       Meta Platforms Inc.Updates: 6716 (if approved)                                  J.-M. ValinIntended status: Standards Track                                  GoogleExpires: 29 July 2026                                    25 January 2026  Integration of Speech Codec Enhancement Methods into the Opus Codec          draft-ietf-mlcodec-opus-speech-coding-enhancement-03Abstract   This document proposes a set of requirements for integrating a speech   codec enhancement method into the Opus codec [RFC6716]Status of This Memo   This Internet-Draft is submitted in full conformance with the   provisions of BCP 78 and BCP 79.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF).  Note that other groups may also distribute   working documents as Internet-Drafts.  The list of current Internet-   Drafts is at https://datatracker.ietf.org/drafts/current/.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on 29 July 2026.Copyright Notice   Copyright (c) 2026 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents (https://trustee.ietf.org/   license-info) in effect on the date of publication of this document.   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.  Code Components   extracted from this document must include Revised BSD License text as   described in Section 4.e of the Trust Legal Provisions and are   provided without warranty as described in the Revised BSD License.Buethe & Valin            Expires 29 July 2026                  [Page 1]Internet-Draft       Opus Speech Coding Enhancement         January 2026Table of Contents   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3   2.  An Illustrative Example . . . . . . . . . . . . . . . . . . .   3   3.  Definition of a SILK enhancement algorithm  . . . . . . . . .   5   4.  Qualification of a SILK enhancement algorithm . . . . . . . .   5     4.1.  General requirements  . . . . . . . . . . . . . . . . . .   5       4.1.1.  Subjective Evaluation . . . . . . . . . . . . . . . .   5       4.1.2.  Delay and Phase considerations  . . . . . . . . . . .   6       4.1.3.  Encoder Requirements  . . . . . . . . . . . . . . . .   6     4.2.  Requirements specific to non-extending SILK enhancement           algorithms for wideband speech  . . . . . . . . . . . . .   6       4.2.1.  Objective Evaluation  . . . . . . . . . . . . . . . .   6       4.2.2.  Requirements specific to extending SILK enhancement               algorithms for wideband speech  . . . . . . . . . . .   8   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8     7.1.  Normative References  . . . . . . . . . . . . . . . . . .   8     7.2.  Informative References  . . . . . . . . . . . . . . . . .   9   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   91.  Introduction   Since the specification of the original Opus codec [RFC6716], new   data-driven speech codec enhancement methods emerged which outperform   classical enhancement methods by a large margin.  Using such   enhancement methods to improve the quality of the Opus speech codec   SILK requires an update of [RFC6716] since SILK is an embedded coding   mode and changing the output of the SILK decoder will lead to a   violation of the Opus conformance criteria.  The purpose of this   document is hence to update [RFC6716] to enable the use of a speech   codec enhancement algorihm.  Specifically, this document defines the   notion of a SILK enhancement algorithm and sets forth a list of   requirements, some mandatory, some optional, that aim to ensure   (1)  consistent performance of the enhancement method itself,   (2)  preservation of decoder performance (e.g. seamless mode        switching), and   (3)  preservation of basic interoperability when tuning the Opus        encoder for use with an enhanced decoder.   While the first two objectives target the Opus decoder alone, the   third objective introduces new restrictions on the Opus encoder.   However, these are not expected to interfer with any existingBuethe & Valin            Expires 29 July 2026                  [Page 2]Internet-Draft       Opus Speech Coding Enhancement         January 2026   implementation of an Opus encoder since they target potential   interoperability issues arising from new incentives connected to the   possibility to enhance the Opus decoder.   The approach of specifying requirements instead of specifying the   enhancement algorithm itself has the advantage of allowing the Opus   decoder to benefit from future improvements in a field that currently   sees rapid development.  Still, a description of the linear-adaptive   coding enhancer (LACE) and its integration into the Opus decoder is   included as an illustrative example for a SILK enhancement method.1.1.  Requirements Language   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and   "OPTIONAL" in this document are to be interpreted as described in BCP   14 [RFC2119] [RFC8174] when, and only when, they appear in all   capitals, as shown here.2.  An Illustrative Example   We use the linear-adaptive coding enhancer (LACE) [lace-paper] as an   illustrative example to highlight the specific challenges of   integrating a speech codec enhancement method into the Opus decoder.   LACE is trained to enhance the output signal of the SILK decoder, the   speech coding mode of Opus, and Figure 1 depicts a high-level   overview of the Opus decoder with LACE added as an enhancement   algorithm.   The first requirement for a speech coding enhancement method concerns   the performance of the method itself.  In this example it relates to   the question how the SILK decoder output compares to the LACE output.   In [lace-paper] this has been evaluated on clean speech samples using   a P.808 listening test [p.808] as well as the objective method PESQ,   which showed consistent improvement for all tested bitrates.  For a   general enhancement method it will be necessary to specify testing   material and performance criteria to prevent unintended quality   degradation of the Opus codec.   The second requirement concerns performance of the Opus decoder as a   whole.  Depending on the bitstream the decoder may have to perform   mode switching, e.g. between SILK and CELT, or it may combine the   SILK and CELT outputs when the codec operates in hybrid mode.   Changes to the SILK output signal by an enhancement method, such as   added delay, phase shifts, or level alterations can therefore   negatively impact the performance of the Opus decoder even if the   first requirement is met.  LACE solves this problem by adding no   delay and by being approximately phase and level preserving.Buethe & Valin            Expires 29 July 2026                  [Page 3]Internet-Draft       Opus Speech Coding Enhancement         January 2026   However, since many enhancement methods are non causal and non phase   preserving, these requirements may be too strict for a general   enhancement method.   The third requirement concerns interoperability.  The Opus   specification provides significant freedom for tuning the encoder and   the presence of an enhancement method in the decoder may change the   optimal encoding choices significantly.  In the present example   encoding e.g. wideband content at 6 kb/s still leads to fair-to-good   quality when using then LACE-enhanced decoder while the quality of a   legacy decoder is significantly worse.  To make full use of these new   enhancement methods, such encoder tunings should be allowed but basic   interoperability with legacy decoders or other enhanced decoders   needs to be ensured.                    ┌──────────────────────────────┐                    │           Bitstream          │                    └─────┬──────────────────┬─────┘                          │                  │                          ▼                  ▼                    ┌───────────┐      ┌───────────┐                    │   CELT    │      │   SILK    │                    │  decoder  │      │  decoder  │                    └─────┬─────┘      └─────┬─────┘                          │                  │                          │                  ▼                          │            ┌───────────┐                          │            │   LACE    │                          │            └─────┬─────┘                          │                  │                          │                  ▼                          │            ┌───────────┐                          │            │ Resampler │                          │            └─────┬─────┘                          │                  │                          ▼                  ▼                    ┌──────────────────────────────┐                    │        Mode Handling         │                    └──────────────┬───────────────┘                                   │                                   ▼                            decoded  signal       Figure 1: A simplified Opus decoder diagram including LACE as                             enhancement moduleBuethe & Valin            Expires 29 July 2026                  [Page 4]Internet-Draft       Opus Speech Coding Enhancement         January 2026   LACE has meanwhile been superceded by the Non-Linear adaptive coding   enhancer (NoLACE) [nolace-paper] which shares all basic properties of   LACE outlined above but provides higher quality.  This stresses the   advantage of specifying requirements for an enhancement method over   specifying the method itself.3.  Definition of a SILK enhancement algorithm   A SILK enhancement algorithm denotes any algorithm that modifies or   replaces the output of the SILK decoder an example of which is   depicted in Figure 1.  If the decoder sampling rate allows for a   higher bandwidth than the encoded bandwidth, a SILK enhancement   algorithm may also increase the bandwidth of the output signal,   replacing the resampler in Figure 1, or it may modify the combination   of a SILK-decoded wideband signal with a CELT decoded highband signal   in hybrid mode.  However, it may not modify the output of pure CELT   frames.  A SILK enhancement algorithm that extends the bandwidth of   the input signal will be referred to as extending, whereas a SILK   enhancement algorithm preserving the bandwidth of the input signal   will be referred to as non-extending.  Furthermore, an Opus decoder   including a SILK enhancement algorithm will be referred to as   enhanced decoder.  Note however, that simply resampling the signal to   a higher sampling rate is neither considered enhancement nor   extending.4.  Qualification of a SILK enhancement algorithm4.1.  General requirements4.1.1.  Subjective Evaluation   Objective metrics for quality evaluation have often proved unreliable   especially for evaluating completely new algorithms for processing   speech or audio signals.  Therefore, any SILK enhancement algorithm   SHOULD undergo subjective evaluation before integration into the Opus   decoder.  For genuinely new algorithms, it is RECOMMENDED to perform   either an absolute category rating (ACR) or degradation category   rating (DCR) listening test according to [p.800] or [p.808], where   the test conditions SHOULD cover a relevant range of bitrates.  For   modifications of previously tested algorithms, e.g. changing the size   of a LACE model or adding small tunings for quality improvement or   complexity reduction, at least an informal subjective evaluation   SHOULD be carried out.  Any enhancement method SHOULD significantly   improve quality for at least one encoder operating point while   showing no significant degradation for other operating points.Buethe & Valin            Expires 29 July 2026                  [Page 5]Internet-Draft       Opus Speech Coding Enhancement         January 20264.1.2.  Delay and Phase considerations   SILK is approximately phase preserving and to avoid additional delay   and maintain usability for applications relying on phase information,   any SILK enhancement algorithm SHOULD also be approximately phase   preserving.4.1.3.  Encoder Requirements   The Opus specification [RFC6716] provides much freedom for encoding   an audio signal and the presence of a powerful enhancement method can   provide an incentive to use that freedom to produce bitstreams that,   when decoded with a legacy Opus decoder, do not result in a   reproduction of the input signal anymore.  To prevent this, the   following requirement is added for an Opus encoder that is designed   to be used with an enhanced Opus decoder: if an Opus encoder produces   a bitstream that can be decoded into a human-recognizable   reproduction of the encoded signal with an enhanced Opus decoder,   then that bitstream MUST also result in a human-recognizable   reproduction of the encoded signal when decoded with a legacy Opus   decoder.4.2.  Requirements specific to non-extending SILK enhancement algorithms      for wideband speech4.2.1.  Objective Evaluation   Every non-extending SILK enhancement algorithm for SILK decoded   wideband speech signals MUST pass all objective tests put forth in   this section.  This collection of tests is designed to uncover major   failure points of the tested algorithm that could be due to improper   design or training data, or due to improper integration into the opus   decoder.  It is not designed to (and cannot) assess the quality of a   particular enhancement method.   The tests are based on comparing a degradation score for audio   samples decoded from a list of bitstreams contained in   https://media.xiph.org/opus/ietf/osce_testvectors_v0.zip (FIXME: find   final location) to a reference degradation score computed from audio   decoded with a reference decoder.  The exact reference decoder is   TBD.  Each test corresponds to an encoder operating point and the   test names follow the scheme   osce_test_BITRATE_BITRATEMODE_FRAMESIZEms_BANDWIDTH_cCOMPLEXITY_MODE   whereBuethe & Valin            Expires 29 July 2026                  [Page 6]Internet-Draft       Opus Speech Coding Enhancement         January 2026   (1)  BITRATE is either a number specifying the encoder bitrate in        bits per second or the string "SWITCHING" indicating the bitrate        has been switched during encoding,   (2)  BITRATEMODE is either vbr or cbr indicating variable bitrate or        constant bitrate encoding,   (3)  FRAMESIZE is either 10 or 20,   (4)  BANDWIDTH specifies the maximal bandwidth and is always WB for        this test (note however that the actual bandwidth can be lower),   (5)  COMPLEXITY is a number from 0 to 10 and specifies the encoder        complexity,   (6)  MODE refers to the coding mode and is either "native" or        "celtswitching".  In "native" mode, the encoder decision whether        to use SILK or CELT is based on signal classification whereas in        "celtswitching" mode the encoder has been forced to switch        between SILK and CELT at a fix rate.   The testvectors are further divided into groups, where each group   contains either speech samples from the same language or dialect, or   music content.  Each group GROUP is tested separately and the test is   passed if it is passed for every group.  The bitstreams in TESTNAME/   GROUP follow the naming pattern CLIPNAME_TESTNAME which associates   each bitstream uniquely with a reference signal reference_clips/   CLIPNAME.s16.  For every CLIPNAME in GROUP let REFMOC(CLIPNAME)   denote the reference degradation score stored in the YAML [RFC9512]   file TESTNAME/reference_scores_TESTNAME.yml under GROUP as primary   key and CLIPNAME as secondary key.  Furthermore, let   CLIPNAME_test.s16 denote the signal decoded with the enhanced decoder   under test at a sampling frequency of 16 kHz after delay   compensation.  The degradation for the test signal CLIPNAME_test.s16   is calculated using the moc.py tool https://gitlab.xiph.org/xiph/   opus/-/blob/osce-testing/dnn/torch/osce/stndrd/qualification/moc.py   (FIXME: moc should be implemented in C and PLC will require masking)   with reference signal path as first argument and test signal path as   second argument.  The resulting degradation score will be referred to   as TESTMOC(CLIPNAME).   From the reference degradation score REFMOC(CLIPNAME) and the test   degradation score TESTMOC(CLIPNAME) a difference score is calculated   according toBuethe & Valin            Expires 29 July 2026                  [Page 7]Internet-Draft       Opus Speech Coding Enhancement         January 2026                       REFMOC(CLIPNAME) - TESTMOC(CLIPNAME)         D(CLIPNAME) = ------------------------------------                                                  0.5                           0.1 + REFMOC(CLIPNAME)   To pass the test for group GROUP, the following two criteria MUST be   met:   (1)  D(CLIPNAME) is larger than A for every CLIPNAME in GROUP,   (2)  The average of D(CLIPNAME) over GROUP is larger than B.   The exact thresholds A and B are TBD.  A test is passed if it is   passed for all groups in that test.4.2.2.  Requirements specific to extending SILK enhancement algorithms        for wideband speech   Requirements for SILK enhancement algorithms extending the bandwidth   of wideband speech are TBD.5.  IANA Considerations   The decoder should be able to signal the presence of an enhancement   method to the encoder over SDP.  The exact mechanism is TBD and the   following options are open for discussion.   (1)  update audio/opus media type registration [RFC7587] to include a        parameter speech_enhancement with possible values 0 and 1   (2)  assign an extension ID, e.g. 33, from the registry defined in        [opus-extension] to implement speech coding enhancement.  This        has the advantage of a double use, meaning the extension ID can        both be used to signal the decoder capability to the encoder and        for transmitting side information to guide a speech enhancment        method from the encoder to the decoder.  However, it needs to be        proven that side information is useful.   (3)  update [opus-extension] to include extension IDs beyond 127 for        data-less extensions6.  Security Considerations   TBD7.  References7.1.  Normative ReferencesBuethe & Valin            Expires 29 July 2026                  [Page 8]Internet-Draft       Opus Speech Coding Enhancement         January 2026   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <https://www.rfc-editor.org/info/rfc2119>.   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,              May 2017, <https://www.rfc-editor.org/info/rfc8174>.   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the              Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716,              September 2012, <https://www.rfc-editor.org/info/rfc6716>.   [RFC7587]  Spittka, J., Vos, K., and JM. Valin, "RTP Payload Format              for the Opus Speech and Audio Codec", RFC 7587,              DOI 10.17487/RFC7587, June 2015,              <https://www.rfc-editor.org/info/rfc7587>.   [RFC9512]  Polli, R., Wilde, E., and E. Aro, "YAML Media Type",              RFC 9512, DOI 10.17487/RFC9512, February 2024,              <https://www.rfc-editor.org/info/rfc9512>.   [opus-extension]              Valin, J.-M., "Extension Formatting for the Opus Codec              (draft-valin-opus-extension)", April 2023.7.2.  Informative References   [lace-paper]              Buethe, J., Valin, J.-M., and A. Mustafa, "LACE: A light-              weight, causal Model for enhancing coded Speech through              Adaptive Convolutions", 2023.   [nolace-paper]              Buethe, J., Mustafa, A., Valin, J.-M., Helwani, K., and M.              Goodwin, "NoLACE: Improving Low-Complexity Speech Codec              Enhancement Through Adaptive Temporal Shaping", 2024.   [p.800]    ITU-T, "P.800 : Methods for subjective determination of              transmission quality", August 1996,              <https://www.itu.int/rec/T-REC-P.800-199608-I>.   [p.808]    ITU-T, "P.808 : Subjective evaluation of speech quality              with a crowdsourcing approach", June 2021,              <https://www.itu.int/rec/T-REC-P.808-202106-I/en>.Authors' AddressesBuethe & Valin            Expires 29 July 2026                  [Page 9]Internet-Draft       Opus Speech Coding Enhancement         January 2026   Jan (editor)   Meta Platforms Inc.   United States of America   Email: jan.buethe@googlemail.com   Jean-Marc   Google   Canada   Email: jmvalin@jmvalin.caBuethe & Valin            Expires 29 July 2026                 [Page 10]