Integration of Speech Codec Enhancement Methods into the Opus Codec
draft-buethe-opus-speech-coding-enhancement-02
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Jan Buethe , Jean-Marc Valin | ||
| Last updated | 2024-04-21 | ||
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| Intended RFC status | (None) | ||
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draft-buethe-opus-speech-coding-enhancement-02
Internet Engineering Task Force J. Buethe, Ed.
Internet-Draft Amazon
Updates: 6716 (if approved) J.-M. Valin
Intended status: Standards Track Xiph.Org Foundation
Expires: 23 October 2024 21 April 2024
Integration of Speech Codec Enhancement Methods into the Opus Codec
draft-buethe-opus-speech-coding-enhancement-02
Abstract
This document proposes a method for integrating a speech codec
enhancement method into the Opus codec [RFC6716]
Status of This Memo
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This Internet-Draft will expire on 23 October 2024.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2
2. An Illustrative Example . . . . . . . . . . . . . . . . . . . 3
3. Requirements for the Enhancement Method . . . . . . . . . . . 4
4. Requirements for Opus Decoder Integration . . . . . . . . . . 4
5. Interoperability . . . . . . . . . . . . . . . . . . . . . . 5
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 5
7. Security Considerations . . . . . . . . . . . . . . . . . . . 5
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 5
8.1. Normative References . . . . . . . . . . . . . . . . . . 5
8.2. Informative References . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 6
1. 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. This document
proposes a method to integrate such enhancement methods into the Opus
decoder including a set of requirements that 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 the enhanced decoder.
The document furthermore contains a description of the linear-
adaptive coding enhancer (LACE) and its integration into the Opus
decoder as an illustrative example.
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.
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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 model.
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 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.
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.
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┌──────────────────────────────┐
│ Bitstream │
└─────┬──────────────────┬─────┘
│ │
▼ ▼
┌───────────┐ ┌───────────┐
│ CELT │ │ SILK │
│ decoder │ │ decoder │
└─────┬─────┘ └─────┬─────┘
│ │
│ ▼
│ ┌───────────┐
│ │ LACE │
│ └─────┬─────┘
│ │
│ ▼
│ ┌───────────┐
│ │ Resampler │
│ └─────┬─────┘
│ │
▼ ▼
┌──────────────────────────────┐
│ Mode Handling │
└──────────────┬───────────────┘
│
▼
decoded signal
Figure 1: A simplified Opus decoder diagram including LACE as
enhancement module
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. Requirements for the Enhancement Method
TBD
4. Requirements for Opus Decoder Integration
TBD
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5. Interoperability
TBD
6. 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 enhancment. 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 extensions
7. Security Considerations
TBD
8. References
8.1. Normative References
[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>.
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[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>.
[opus-extension]
Valin, J.-M., "Extension Formatting for the Opus Codec
(draft-valin-opus-extension)", April 2023.
8.2. 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", 2023.
Authors' Addresses
Jan (editor)
Amazon
Germany
Email: jbuethe@amazon.com
Jean-Marc
Xiph.Org Foundation
Canada
Email: jmvalin@jmvalin.ca
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