Integration of Speech Codec Enhancement Methods into the Opus Codec

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Table of Contents

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

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¶

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¶

Authors' Addresses