Deep Audio Redundancy (DRED) Extension for the Opus Codec

Introduction This document proposes a mechanism for embedding very low bitrate deep audio redundancy (DRED) within the Opus codec bitstream.

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 when, and only when, they appear in all capitals, as shown here.

DRED Extension Format We use the Opus extension mechanism to add deep redundancy within the padding of an Opus packet. We use the extension ID 32, which means that the L flag signals whether a length code is included. In this document, we define only the extension payload. [Note: until adoption by the IETF, experimental implementations of DRED MUST use experiment extension ID 127 to avoid causing interoperability problems] The principles behind the DRED mechanism defined in this extension are explained in . All the data in the extension payload is encoded using the Opus entropy coder defined in Section 4.1 of . Since some of the fields at the beginning of the payload are encoded with flat binary probabilities, they can still be interpreted as bits. The extension starts with an offset indicator, encoded as a signed 5-bit integer (two's complement) in units of 2.5 ms. The offset indicates the time of the last sample analysed for the transmitted features in the packet, measured from the time of the first sample in the Opus frame that contains the extension data. The offset is followed by a 4-bit initial quantizer field (Q0) ranging from 0 to 15. That quantizer is used on the most recent frame encoded and is followed by the 3-bit quantizer slope dQ. The 3-bit dQ index selects from the following values: [0, 1/8, 3/16, 1/4, 3/8, 1/2, 3/4, 1] quantizer step per frame. The quantizer for frame k is thus given by: min(15, round(Q0 + dQ_table[dQ] * k)). For example, using Q0=5 and dQ=2 (3/16), frame k=20 would use a quantizer of round(5 + 3/16 * k) = 9. The compressed redundancy information consists of an initial state coded with a pyramid vector quantizer (PVQ), followed by the entropy-coded latent representation. The number of 40-ms DRED blocks is not coded explicitly. Instead, the decoder MUST NOT decode blocks when fewer than 8 bits remain in the DRED payload.

Extension framing

IANA Considerations This document assigns ID 32 to the "Opus Extension IDs" registry to implement the proposed DRED extension.

Security Considerations As is the case for any media codec, the decoder must be robust against malicious payloads. Similarly, the encoder must also be robust to malicious audio input since the encoder input can often be controlled by an attacker. That can happen through browser JS, echo, or when the encoder is on a gateway. DRED is designed to have a complexity that is independent of the signal characteristics. However, there exist implementation details that can cause signal-dependent complexity changes. One example is CPU treatement of denormals that can sometimes cause increased CPU load and could be triggered by malicious input. For that reason, it is important to minimize such impact to reduce the impact of DOS attacks. Similarly, since the encoding and decoding process can be cputationally costly, devices must manage the complexity to avoid attacks that could trigger too much DRED encoding or decoding to be performed. The use of variable-bitrate (VBR) encoding in DRED poses a theoretical information leak threat , but that threat is believed to be significantly lower than that posed by VBR encoding in the main Opus payload. Since this document provides a way to dymanically vary the amount of redundancy transmitted, it is also possible to reduce the overall VBR risk of Opus by using DRED as a way of making the total Opus payload constant (CBR) or nearly constant.