Internet-Draft Opus Speech Coding Enhancement October 2023
Buethe & Valin Expires 25 April 2024 [Page]
Internet Engineering Task Force
6716 (if approved)
Intended Status:
Standards Track
J. Buethe, Ed.
J.-M. Valin

Integration of Speech Codec Enhancement Methods into the Opus Codec


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 25 April 2024.

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

consistent performance of the enhancement method itself,
preservation of decoder performance (e.g. seamless mode switching), and
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.

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.

                 │           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.

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.

update audio/opus media type registration [RFC7587] to include a parameter speech_enhancement with possible values 0 and 1
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.
update [opus-extension] to include extension IDs beyond 127 for data-less extensions

8. References

8.1. Normative References

Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Valin, JM., Vos, K., and T. Terriberry, "Definition of the Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716, , <>.
Spittka, J., Vos, K., and JM. Valin, "RTP Payload Format for the Opus Speech and Audio Codec", RFC 7587, DOI 10.17487/RFC7587, , <>.
Valin, J.-M., "Extension Formatting for the Opus Codec (draft-valin-opus-extension)", .

8.2. Informative References

Buethe, J., Valin, J.-M., and A. Mustafa, "LACE: A light-weight, causal Model for enhancing coded Speech through Adaptive Convolutions", .
Buethe, J., Mustafa, A., Valin, J.-M., Helwani, K., and M. Goodwin, "NoLACE: Improving Low-Complexity Speech Codec Enhancement Through Adaptive Temporal Shaping", .

Authors' Addresses

Jan (editor)