Internet Engineering Task Force JM. Valin Internet-Draft Xiph.Org Foundation Updates: 6716 (if approved) J. Buethe Intended status: Standards Track Amazon Expires: 26 August 2024 23 February 2024 Deep Audio Redundancy (DRED) Extension for the Opus Codec draft-valin-opus-dred-05 Abstract This document proposes a mechanism for embedding very low bitrate deep audio redundancy (DRED) within the Opus codec (RFC6716) bitstream. 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 26 August 2024. Copyright Notice Copyright (c) 2024 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. Valin & Buethe Expires 26 August 2024 [Page 1] Internet-Draft Opus DRED February 2024 Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Requirements Language . . . . . . . . . . . . . . . . . . 2 2. DRED Description . . . . . . . . . . . . . . . . . . . . . . 2 2.1. Acoustic Features . . . . . . . . . . . . . . . . . . . . 3 2.2. Rate-Distortion-Optimized Variational Autoencoder (RDO) . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2.2.1. Encoder architecture . . . . . . . . . . . . . . . . 6 2.2.2. Decoder architecture . . . . . . . . . . . . . . . . 7 2.2.3. Statistical data . . . . . . . . . . . . . . . . . . 7 2.2.4. Vocoder . . . . . . . . . . . . . . . . . . . . . . . 16 3. DRED Extension Format . . . . . . . . . . . . . . . . . . . . 16 3.1. Latent decoding . . . . . . . . . . . . . . . . . . . . . 18 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19 4.1. Opus Media Type Update . . . . . . . . . . . . . . . . . 19 4.2. Mapping to SDP Parameters . . . . . . . . . . . . . . . . 20 5. Security Considerations . . . . . . . . . . . . . . . . . . . 20 6. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 6.1. Normative References . . . . . . . . . . . . . . . . . . 20 6.2. Informative References . . . . . . . . . . . . . . . . . 21 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 21 1. Introduction This document proposes a mechanism for embedding very low bitrate deep audio redundancy (DRED) within the Opus codec [RFC6716] bitstream. 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. DRED Description Opus already includes a low-bitrate redundancy (LBRR) mechanism to transmit redundancy in-band to improve robustness to packet loss. LBRR is however limited to a single frame of redundancy, and typically uses about 2/3 of the bitrate of the "regular" Opus packet. The DRED extension allows up to one second or more [Open question: should we set a limit?] redundancy to be included in each packet, using a bitrate about 1/50 of the regular Opus bitrate. Valin & Buethe Expires 26 August 2024 [Page 2] Internet-Draft Opus DRED February 2024 DRED works by having the encoder transmit acoustic features in the Opus bitstream. On the receiver side, if packets are lost, then the first packet to arrive will contain the acoustic features for a certain duration in the past. The decoder can then use the features to synthesize the missing speech -- either from the last received or from the last audio samples produced by packet loss concealment (PLC). Although the synthesized speech samples should be consistent with the last known samples at the point of the transition, the features do not contain waveform-specific or phase-specific information so the synthesized speech waveform will significantly deviate from the original waveform, despite sounding similar. 2.1. Acoustic Features DRED uses 20 acoustic features to synthesize speech. The first 18 are Bark-frequency cepstral coefficients (BFCC) and the last represent the pitch frequency and the voicing information. The BFCC features are based on bands that match the CELT bands, as shown in Table 1. Valin & Buethe Expires 26 August 2024 [Page 3] Internet-Draft Opus DRED February 2024 +======+======================+================+===============+ | Band | Start frequency (Hz) | Center | End frequency | | | | frequency (Hz) | (Hz) | +======+======================+================+===============+ | 0 | 0 | 0 | 200 | +------+----------------------+----------------+---------------+ | 1 | 0 | 200 | 400 | +------+----------------------+----------------+---------------+ | 2 | 200 | 400 | 600 | +------+----------------------+----------------+---------------+ | 3 | 400 | 600 | 800 | +------+----------------------+----------------+---------------+ | 4 | 600 | 800 | 1000 | +------+----------------------+----------------+---------------+ | 5 | 800 | 1000 | 1200 | +------+----------------------+----------------+---------------+ | 6 | 1000 | 1200 | 1400 | +------+----------------------+----------------+---------------+ | 7 | 1200 | 1400 | 1600 | +------+----------------------+----------------+---------------+ | 8 | 1400 | 1600 | 2000 | +------+----------------------+----------------+---------------+ | 9 | 1600 | 2000 | 2400 | +------+----------------------+----------------+---------------+ | 10 | 2000 | 2400 | 2800 | +------+----------------------+----------------+---------------+ | 11 | 2400 | 2800 | 3200 | +------+----------------------+----------------+---------------+ | 12 | 2800 | 3200 | 4000 | +------+----------------------+----------------+---------------+ | 13 | 3200 | 4000 | 4800 | +------+----------------------+----------------+---------------+ | 14 | 4000 | 4800 | 5600 | +------+----------------------+----------------+---------------+ | 15 | 4800 | 5600 | 6800 | +------+----------------------+----------------+---------------+ | 16 | 5600 | 6800 | 8000 | +------+----------------------+----------------+---------------+ | 17 | 6800 | 8000 | 8000 | +------+----------------------+----------------+---------------+ Table 1: Band definitions for DRED TODO: Specify exact computation of the cepstral features and voicing. Open question: how do we specify the neural pitch estimator? Valin & Buethe Expires 26 August 2024 [Page 4] Internet-Draft Opus DRED February 2024 2.2. Rate-Distortion-Optimized Variational Autoencoder (RDO) The features described above need to be transmitted to the decoder with the fewest number of bits possible. Although it is not acceptable to make redundancy from one packet depend on the redundancy of another packet, we can use as much prediction as we like within one packet. In practical use, the same audio feature vector is included in many different packets (50 for 1 second redundancy). For that reason, we do not want to fully re-encode acoustic features for each packet. On the decoder side, since the most recent audio is the most likely to be used, we minimize the computation time by having the audio encoded from the most recent, going backward in time. TODO: Specify the cepstral features and voicing. Open question: how do we specify the neural pitch estimator? Valin & Buethe Expires 26 August 2024 [Page 5] Internet-Draft Opus DRED February 2024 Audio | v +---------------+ | RDOVAE encoder| +---------------+ | v +---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | L | L | L | L | L | L | L | L | L | L | L | L | L | L | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | S | S | S | S | S | S | S | S | S | S | S | S | S | S | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+ | | | v | | +---+---+---+---+---+---+---+ | | decoder <--| L | | L | | L | | L | | | +---+---+---+---+---+---+---+ | | | S | | | +---+ | | v | +---+---+---+---+---+---+---+ | decoder <--| L | | L | | L | | L | | +---+---+---+---+---+---+---+ | | S | | +---+ | v +---+---+---+---+---+---+---+ decoder <--| L | | L | | L | | L | +---+---+---+---+---+---+---+ | S | +---+ Figure 1: DRED encoding/decoding 2.2.1. Encoder architecture Every 20 ms, the encoder takes in a pair of 20-dimensional acoustic feature vectors as input and produces one initial state (IS) and one latent vector. Each latent vector encodes 40 ms (their information overlaps), so only half the latent vectors need to be transmitted. Although an encoder is provided for reference, the encoder architecture is not normative. Each redundancy packet contains the latest initial state, along with latent vectors ordered from the latest (the one aligned with the initial state) to the earliest one the encoder includes. Each conponent of the IS and latent vectors are quantized and then entropy-coded following a Laplace distribution. The same procedure is used for both the latent vectors Valin & Buethe Expires 26 August 2024 [Page 6] Internet-Draft Opus DRED February 2024 and the initial state (we will describe the process for a latent variable). The quantized index X is obtained by scaling the i'th latent variable z_i by a scaling factor s_{i,q} that depends on both i and on the quantizer q. We then apply a "dead-zone" function zeta(z) = z - d*tanh(z / (d + epsilon)), where d also depends on i and q, and epsilon=0.1. The result is then rounded to the nearest integer: X = round(zeta(s_{i,q}*z_i)). The Laplace distribution used for entropy coding is parameterized with a probability that the value is zero (p0), as well as a decay factor r (0 < r < 1). Both p0 and r depend on i and q. The probability p(X) for a coefficient is given by: / | p0 , if X = 0 | P(X) = < |X| | (1 - p0) * r , if X != 0 | --------------- \ 2 * (1 - r) 2.2.2. Decoder architecture Unlike the encoder, the decoder is normative. The decoder uses the same Laplace distribution above to decode the symbols and then scales them back by 1/s_{i,q}. The initial state is used as input to initialize the decoder's gated recurrent units (GRUs). The latent vectors are used on at a time as input the DNN decoder, which produces 4 vectors of 20 acoustic features for each input latent vector. Open question: how do we specify the decoder DNN architecture and (especially) the weights? We expect about 500k to 1M weights, most of which can be represented as 8-bit integers, the others as floating-point. 2.2.3. Statistical data We define 16 different quantization settings, ranging from q=0 (higher bitrate) to q=15 (lower bitrate). For each quantizer and for each latent variable or initial state coefficient, we have a normative scale (s), decay (r), and p0 value. Note that the dead- zone parameters d are not normative. Valin & Buethe Expires 26 August 2024 [Page 7] Internet-Draft Opus DRED February 2024 +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |k |Q0 |Q1 |Q2 |Q3 |Q4 |Q5 |Q6 |Q7|Q8|Q9|Q10|Q11|Q12|Q13|Q14| Q15 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |0 |255|208|168|134|106|82 |64 |48|36|26|17 |10 |3 |3 |2 | 2 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |1 |255|219|187|160|137|117|101|81|70|50|23 |6 |6 |5 |3 | 2 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |2 |255|218|187|160|138|118|102|84|71|63|31 |7 |7 |5 |3 | 2 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |3 |255|217|186|159|137|118|102|87|76|66|53 |25 |11 |5 |2 | 1 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |4 |255|216|183|155|131|111|95 |79|67|57|48 |42 |35 |29 |24 | 21 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |5 |255|219|189|163|141|122|107|90|87|31|11 |3 |3 |2 |1 | 1 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |6 |255|218|187|160|138|119|103|87|72|45|18 |6 |5 |3 |2 | 2 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |7 |255|217|184|157|133|113|96 |78|67|53|34 |17 |6 |5 |4 | 3 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |8 |255|222|192|167|146|128|114|87|78|63|40 |9 |8 |6 |4 | 3 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |9 |255|217|184|157|135|115|99 |84|73|65|56 |48 |18 |11 |6 | 2 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |10|255|219|189|163|141|122|107|90|74|40|15 |5 |4 |3 |2 | 1 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |11|255|214|180|151|127|108|91 |76|65|56|47 |41 |35 |31 |27 | 24 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |12|255|215|181|152|129|109|93 |78|67|57|49 |43 |38 |33 |29 | 27 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |13|255|218|187|160|138|119|102|87|75|56|34 |19 |7 |4 |2 | 2 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |14|255|219|188|162|139|120|103|80|69|34|12 |3 |3 |2 |1 | 1 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |15|255|219|189|164|143|124|108|69|20|5 |0 |1 |1 |1 |1 | 0 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |16|255|217|185|158|136|117|101|86|76|67|58 |47 |15 |11 |7 | 6 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |17|255|217|184|157|135|115|99 |84|74|63|54 |47 |16 |10 |7 | 5 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |18|255|213|178|149|124|104|87 |72|60|50|42 |35 |29 |25 |21 | 18 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |19|255|215|181|152|127|105|86 |58|46|21|10 |2 |0 |0 |0 | 0 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |20|255|214|179|149|125|104|87 |72|61|51|43 |36 |31 |27 |23 | 20 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ Table 2: Scale values for latent Valin & Buethe Expires 26 August 2024 [Page 8] Internet-Draft Opus DRED February 2024 +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |k |Q0|Q1|Q2|Q3|Q4|Q5|Q6 |Q7 |Q8 |Q9 |Q10|Q11|Q12| Q13 | Q14 | Q15 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |0 |1 |1 |0 |0 |0 |1 |1 |2 |3 |12 |27 |44 |178| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |1 |0 |0 |7 |17|29|45|70 |107|160|255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |2 |10|13|16|20|24|29|35 |41 |53 |255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |3 |0 |1 |5 |9 |14|20|26 |37 |51 |81 |124|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |4 |0 |0 |0 |1 |4 |6 |9 |11 |16 |24 |37 |53 |87 | 108 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |5 |6 |12|17|24|31|41|56 |85 |255|255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |6 |11|15|18|22|27|33|41 |48 |53 |255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |7 |0 |0 |0 |5 |11|17|27 |46 |75 |124|220|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |8 |0 |8 |25|43|66|94|133|168|231|255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |9 |0 |0 |2 |6 |11|16|23 |31 |44 |71 |104|158|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |10|7 |12|17|22|28|36|47 |59 |81 |255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |11|0 |0 |0 |1 |2 |4 |5 |7 |9 |12 |15 |19 |23 | 27 | 30 | 38 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |12|0 |0 |1 |2 |4 |6 |9 |11 |14 |20 |28 |37 |57 | 65 | 75 | 96 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |13|0 |3 |7 |11|16|21|28 |39 |54 |67 |255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |14|13|18|22|28|34|43|56 |72 |255|255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |15|0 |0 |4 |13|23|37|56 |255|255|255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |16|4 |7 |11|14|19|24|30 |39 |49 |70 |96 |123|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |17|0 |0 |3 |7 |11|16|21 |28 |38 |54 |73 |108|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |18|0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |2 |3 |5 | 7 | 9 | 11 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |19|5 |12|18|26|34|43|56 |84 |255|255|255|255|255| 255 | 255 | 255 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ |20|0 |0 |0 |0 |0 |0 |1 |2 |3 |5 |8 |11 |14 | 16 | 18 | 21 | +==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+ Table 3: Dead zone values for latent Valin & Buethe Expires 26 August 2024 [Page 9] Internet-Draft Opus DRED February 2024 +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |k |Q0 |Q1 |Q2 |Q3 |Q4 |Q5 |Q6 |Q7 |Q8 |Q9 |Q10|Q11|Q12|Q13|Q14|Q15| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |0 |233|228|222|214|204|191|176|155|135|106|66 |32 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |1 |94 |85 |72 |59 |45 |32 |21 |10 |4 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |2 |91 |75 |58 |43 |29 |17 |9 |4 |2 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |3 |112|96 |81 |65 |51 |38 |26 |16 |10 |4 |1 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |4 |149|138|125|109|93 |77 |61 |45 |32 |21 |12 |7 |3 |1 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |5 |65 |50 |36 |24 |14 |8 |4 |2 |0 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |6 |92 |75 |59 |43 |29 |18 |10 |5 |2 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |7 |118|107|97 |74 |60 |48 |38 |29 |17 |6 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |8 |55 |47 |36 |27 |19 |13 |8 |3 |2 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |9 |122|107|92 |76 |60 |46 |34 |22 |15 |9 |4 |2 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |10|82 |67 |53 |40 |29 |20 |14 |8 |4 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |11|190|181|171|160|149|135|120|101|85 |68 |52 |38 |26 |17 |10 |6 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |12|175|165|154|143|128|113|98 |81 |67 |53 |41 |31 |23 |15 |9 |5 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |13|100|85 |70 |56 |42 |31 |21 |12 |6 |1 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |14|80 |64 |49 |35 |23 |14 |7 |2 |0 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |15|62 |47 |33 |21 |12 |6 |3 |0 |0 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |16|125|109|92 |75 |59 |43 |30 |18 |10 |5 |1 |1 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |17|130|114|98 |82 |66 |50 |37 |24 |15 |7 |2 |1 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |18|236|233|229|224|219|213|206|198|189|180|169|158|146|132|118|104| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |19|90 |72 |54 |37 |24 |15 |9 |3 |0 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |20|219|213|207|199|190|181|172|160|148|133|118|103|88 |74 |62 |51 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ Table 4: Decay (r) values for latent Valin & Buethe Expires 26 August 2024 [Page 10] Internet-Draft Opus DRED February 2024 +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |k |Q0 |Q1 |Q2 |Q3 |Q4 |Q5 |Q6 |Q7 |Q8 |Q9 |Q10|Q11|Q12|Q13|Q14|Q15| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |0 |12 |14 |18 |22 |27 |35 |44 |57 |78 |106|152|201|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |1 |162|171|184|197|211|224|235|246|252|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |2 |137|147|158|171|184|198|212|228|241|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |3 |134|142|152|163|175|188|201|216|228|242|253|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |4 |107|118|126|135|144|155|166|179|192|207|223|235|248|253|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |5 |138|152|167|183|199|215|231|246|255|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |6 |118|130|144|158|174|190|206|223|237|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |7 |138|149|159|167|180|194|208|227|239|250|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |8 |201|209|220|229|237|243|248|253|254|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |9 |114|123|133|145|158|172|186|204|218|234|246|253|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |10|145|157|169|182|196|209|223|237|248|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |11|66 |75 |85 |96 |107|115|122|132|140|151|163|175|189|201|213|224| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |12|81 |91 |102|113|122|131|140|153|164|177|192|205|220|230|238|244| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |13|143|153|163|175|187|199|211|226|237|249|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |14|146|157|170|183|198|213|228|245|255|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |15|159|168|179|193|208|222|237|255|255|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |16|122|130|140|150|161|174|187|203|216|232|245|253|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |17|121|128|137|147|159|170|183|198|212|228|241|250|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |18|20 |23 |27 |32 |37 |43 |50 |58 |67 |76 |87 |98 |108|116|125|134| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |19|104|120|139|159|182|205|227|251|255|255|255|255|255|255|255|255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |20|37 |43 |49 |57 |66 |75 |84 |96 |106|115|126|137|148|159|169|180| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ Table 5: P(0) values for latent Valin & Buethe Expires 26 August 2024 [Page 11] Internet-Draft Opus DRED February 2024 +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |k |Q0 |Q1 |Q2 |Q3 |Q4 |Q5 |Q6 |Q7|Q8|Q9|Q10|Q11|Q12|Q13|Q14| Q15 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |0 |255|215|181|153|129|109|93 |78|67|58|51 |45 |40 |35 |31 | 27 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |1 |255|215|181|153|128|108|91 |77|65|55|47 |41 |36 |31 |27 | 24 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |2 |255|233|205|175|146|120|97 |77|62|49|40 |33 |27 |23 |19 | 15 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |3 |255|215|181|152|127|107|89 |74|62|53|44 |37 |32 |28 |24 | 21 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |4 |255|216|182|154|131|111|95 |81|70|63|57 |51 |47 |41 |36 | 31 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |5 |255|215|181|152|128|108|91 |76|64|55|46 |39 |34 |29 |25 | 21 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |6 |255|216|182|155|131|111|95 |81|71|65|60 |53 |47 |41 |36 | 32 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |7 |255|216|183|155|132|113|98 |87|79|79|78 |69 |62 |53 |46 | 40 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |8 |255|215|181|152|128|108|91 |77|65|56|47 |41 |36 |31 |27 | 24 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |9 |255|216|183|155|131|112|96 |82|71|62|54 |47 |41 |37 |34 | 42 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |10|121|114|102|84 |61 |43 |31 |1 |0 |2 |131|188|255|216|181| 151 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |11|255|215|182|153|129|108|91 |77|65|55|47 |40 |34 |28 |24 | 20 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |12|255|217|184|155|130|110|92 |77|64|54|45 |38 |32 |27 |23 | 19 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |13|255|227|196|166|140|118|98 |82|69|57|48 |40 |34 |29 |24 | 20 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |14|255|216|182|154|130|110|93 |80|69|60|53 |47 |42 |37 |32 | 28 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |15|255|216|184|156|133|114|98 |87|77|72|66 |59 |52 |46 |40 | 36 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |16|255|216|184|156|134|115|100|91|82|77|67 |59 |52 |46 |40 | 36 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |17|255|216|183|155|131|110|93 |78|66|57|49 |42 |37 |32 |28 | 25 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ |18|71 |65 |60 |54 |49 |45 |42 |45|49|92|189|235|255|213|177| 146 | +==+===+===+===+===+===+===+===+==+==+==+===+===+===+===+===+=====+ Table 6: Scale values for state Valin & Buethe Expires 26 August 2024 [Page 12] Internet-Draft Opus DRED February 2024 +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |k |Q0 |Q1 |Q2 |Q3 |Q4 |Q5 |Q6 |Q7 |Q8 |Q9 |Q10|Q11|Q12|Q13|Q14|Q15| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |0 |13 |12 |11 |11 |11 |11 |11 |11 |11 |13 |12 |9 |7 |13 |19 |26 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |1 |16 |14 |12 |11 |9 |8 |7 |4 |4 |4 |4 |5 |7 |5 |3 |7 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |2 |9 |8 |7 |6 |6 |4 |3 |3 |2 |3 |2 |0 |3 |2 |4 |4 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |3 |6 |8 |8 |9 |9 |9 |10 |8 |8 |11 |11 |10 |15 |22 |28 |37 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |4 |20 |18 |17 |16 |15 |15 |15 |14 |13 |14 |13 |9 |9 |14 |21 |30 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |5 |10 |8 |7 |5 |4 |4 |3 |3 |2 |3 |4 |6 |8 |9 |10 |10 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |6 |13 |13 |13 |13 |13 |13 |14 |12 |12 |11 |2 |1 |10 |17 |24 |34 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |7 |35 |30 |25 |22 |19 |17 |16 |18 |15 |22 |0 |1 |0 |4 |7 |12 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |8 |13 |11 |9 |8 |6 |5 |4 |3 |2 |3 |3 |4 |9 |6 |2 |5 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |9 |15 |15 |15 |15 |15 |16 |17 |17 |18 |16 |20 |26 |34 |46 |75 |255| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |10|255|255|255|255|255|255|255|255|255|2 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |11|9 |7 |6 |5 |4 |3 |2 |1 |1 |0 |0 |1 |2 |2 |3 |3 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |12|11 |9 |6 |5 |3 |2 |2 |2 |2 |3 |4 |4 |3 |3 |3 |2 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |13|10 |8 |6 |5 |4 |3 |2 |2 |1 |2 |2 |2 |4 |3 |4 |1 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |14|23 |19 |17 |14 |12 |11 |9 |8 |8 |11 |9 |4 |4 |7 |9 |13 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |15|14 |14 |14 |15 |16 |17 |18 |20 |18 |0 |8 |13 |14 |23 |33 |50 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |16|26 |24 |21 |19 |17 |16 |12 |7 |0 |11 |14 |14 |17 |24 |32 |46 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |17|43 |38 |32 |27 |22 |18 |14 |7 |1 |0 |0 |0 |0 |0 |0 |0 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |18|255|255|255|255|255|255|255|255|255|121|29 |4 |1 |0 |1 |4 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ Table 7: Dead zone values for state Valin & Buethe Expires 26 August 2024 [Page 13] Internet-Draft Opus DRED February 2024 +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |k |Q0 |Q1 |Q2 |Q3 |Q4 |Q5 |Q6 |Q7 |Q8 |Q9 |Q10|Q11|Q12|Q13|Q14|Q15| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |0 |207|199|190|181|169|158|145|130|116|103|90 |77 |66 |52 |39 |27 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |1 |224|218|212|205|196|187|177|165|152|139|126|112|101|87 |74 |60 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |2 |253|253|252|252|251|250|249|247|245|242|239|235|231|226|220|213| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |3 |207|199|190|180|169|157|144|128|113|99 |82 |68 |56 |46 |37 |30 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |4 |197|187|177|165|152|139|124|109|95 |84 |74 |64 |56 |42 |30 |19 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |5 |233|229|224|218|212|205|197|187|177|166|154|140|127|112|97 |81 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |6 |190|181|170|158|144|130|115|100|86 |78 |70 |60 |48 |36 |25 |16 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |7 |198|189|178|167|154|141|127|115|106|107|107|96 |86 |71 |57 |43 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |8 |232|227|223|217|210|203|194|183|173|161|149|136|124|111|99 |84 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |9 |180|168|156|143|128|112|97 |79 |64 |50 |37 |25 |17 |10 |7 |5 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |10|4 |3 |1 |0 |0 |0 |0 |0 |0 |0 |19 |104|132|117|100|83 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |11|245|243|240|237|234|230|226|220|214|208|200|191|182|171|160|147| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |12|251|251|250|249|247|246|244|241|239|235|232|227|222|216|210|202| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |13|254|253|253|253|252|251|250|249|248|246|244|242|239|236|233|229| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |14|210|203|194|185|174|162|149|136|122|109|98 |88 |78 |64 |51 |38 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |15|173|162|149|135|120|105|91 |78 |67 |63 |53 |43 |32 |22 |15 |9 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |16|169|156|142|128|112|98 |85 |77 |71 |61 |48 |37 |28 |18 |10 |5 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |17|223|218|212|205|197|188|179|166|155|143|131|120|110|99 |89 |79 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |18|22 |17 |12 |7 |4 |2 |1 |2 |11 |90 |166|183|188|178|164|150| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ Table 8: Decay (r) values for state Valin & Buethe Expires 26 August 2024 [Page 14] Internet-Draft Opus DRED February 2024 +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |k |Q0 |Q1 |Q2 |Q3 |Q4 |Q5 |Q6 |Q7 |Q8 |Q9 |Q10|Q11|Q12|Q13|Q14|Q15| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |0 |40 |45 |52 |59 |67 |75 |84 |95 |105|115|124|132|139|153|167|182| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |1 |24 |28 |32 |37 |43 |49 |56 |63 |72 |80 |90 |100|110|119|128|142| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |2 |1 |2 |2 |2 |2 |3 |4 |5 |6 |7 |9 |11 |13 |16 |19 |23 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |3 |35 |41 |48 |56 |65 |75 |85 |97 |109|124|139|153|168|183|197|210| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |4 |45 |50 |56 |64 |72 |81 |90 |101|110|118|125|132|139|155|171|188| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |5 |15 |18 |21 |24 |29 |33 |39 |45 |52 |60 |69 |78 |88 |98 |108|119| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |6 |47 |54 |62 |70 |79 |89 |99 |110|119|126|127|136|150|167|183|200| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |7 |44 |49 |54 |60 |67 |74 |82 |90 |95 |97 |91 |99 |107|121|135|151| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |8 |15 |17 |20 |23 |27 |31 |35 |40 |46 |53 |61 |70 |78 |88 |96 |109| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |9 |58 |65 |73 |82 |92 |102|112|125|136|146|160|176|193|209|226|251| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |10|252|253|255|255|255|255|255|255|255|255|189|93 |72 |83 |96 |110| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |11|7 |8 |9 |11 |13 |15 |18 |21 |24 |29 |33 |39 |46 |53 |60 |69 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |12|2 |3 |3 |4 |4 |5 |6 |7 |9 |11 |13 |15 |17 |21 |24 |29 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |13|1 |1 |1 |2 |2 |2 |3 |4 |4 |5 |6 |7 |8 |10 |12 |14 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |14|25 |28 |33 |39 |45 |52 |60 |70 |79 |89 |98 |106|114|128|142|157| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |15|56 |64 |73 |83 |93 |105|116|128|135|131|142|155|168|185|201|218| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |16|53 |61 |69 |78 |88 |98 |109|116|121|131|145|159|172|188|204|220| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |17|17 |21 |25 |31 |39 |45 |52 |58 |65 |74 |84 |94 |105|116|128|139| +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ |18|230|235|240|246|250|252|254|251|235|129|50 |39 |36 |43 |51 |60 | +==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+ Table 9: P(0) values for state Valin & Buethe Expires 26 August 2024 [Page 15] Internet-Draft Opus DRED February 2024 2.2.4. Vocoder A vocoder is needed to turn the acoustic features into actual speech to fill in the audio for any missing packets. Although the decoder is not normative, certain properties are needed for DRED to function adequately. First, the vocoder SHOULD be able to start synthesizing speech by continuing an existing waveform, reducing the artifacts caused at the beginning of a lost packet. If such property cannot be achieved, then the implementation SHOULD at least make an attempt to synchronize the phase of the synthesized speech with the last received speech, and attempt some form of blending, e.g. by splicing the signals in the LPC residual domain. A second important property of the vocoder is to not rely on more than one feature vector of look-ahead. To synthesize speech between time t-10ms and t, the vocoder SHOULD NOT rely on acoustic features centered beyond t+5ms (i.e. covering t-5ms to t+15ms). The vocoder MAY use more look-ahead when it is available, but there are cases (e.g. last lost packet) where the amount of acoustic feature vectors will be limited. For frames sizes less than 20 ms, the decoder SHOULD be prelated to deal with having less than one feature vector of look-ahead. 3. DRED Extension Format We use the Opus extension mechanism [opus-extension] 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 126 to avoid causing interoperability problems] The principles behind the DRED mechanism defined in this extension are explained in [dred-paper]. All the data in the extension payload is encoded using the Opus entropy coder defined in Section 4.1 of [RFC6716]. Since some of the fields at the beginning of the payload are encoded with flat binary probabilities, they can still be interpreted as bits. Valin & Buethe Expires 26 August 2024 [Page 16] Internet-Draft Opus DRED February 2024 The extension starts with 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: q=min(Qmax, round(Q0 + dQ_table[dQ] * k)), where Qmax is the maximum quantizer allowed. For example, using Q0=5 and dQ=2 (3/16), frame k=20 would use a quantizer of round(5 + 3/16 * k) = 9. We then have one bit (X) that flags whether an extended offset is used. If X=0, then a 5-bit offset indicator follows. The offset is a positive integer in units of 2.5 ms. It indicates the time of the last sample analysed for the transmitted features in the packet, measured from 40ms after the first sample in the Opus frame that contains the extension data. If X=1, then we have an extended offset field, with an additional 8 bits to signal the offset. This makes it possible to signal a maximum offset of (2^13-1)*2.5ms, or approximately 20.5 seconds. If Q0<14 and dQ!=0, then the offset is followed by the range-coded Qmax parameter. The probability of Qmax=15 is set to 1/2 (one bit is used), whereas other possible values (Q0 < Qmax < 15) are coded with a flat probability distribution. The pdf for Qmax is {nval, 1, 1, ...}/(2*nval), where there are nval=14-Q0 ones. The Qmax=15 symbol is first, followed by other values in ascending order, starting from Qmax=Q0+1. The compressed redundancy information consists of an initial state coded, followed by a sequence of 40-ms latent vectors. Both the initial state and the latent vectors are the entropy-coded using a Laplace distribution. The number of 40-ms DRED latent vectors is not coded explicitly. Instead, the decoder keeps decoding them until it runs out of bits. More specifically, the decoder MUST NOT decode blocks when fewer than 8 bits remain in the DRED payload. There is no arbitrary limit on the number of vectors that can be coded in a packet, but the authors do not believe that using more than a few seconds of redundancy is likely to be useful. Also, decoders MAY ignore any redundancy data beyond a certain amount. Valin & Buethe Expires 26 August 2024 [Page 17] Internet-Draft Opus DRED February 2024 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Q0 | dQ |X| (Ext. offset) | Offset |Qmax| Initial state | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- + : : + ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | Latent vectors 0, | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ + | latent vector 1, ... | : : + +-+-+-+-+-| | Latent vector n-1 | unused | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: Extension framing 3.1. Latent decoding Since the DRED decoder is normative, we describe DRED from the decoder perspective, but the encoder is expected to have the corresponding behavior. DRED uses the same range coder as the rest of Opus, as described in Section 4.1 of [RFC6716]. Because the non- entropy-coded bits (Q0, dQ, ...) do not amount to an integer number of bytes, it is simpler to code them using the range coder. The result is the same for those bits, but it ensures that the complete DRED payload is an integer number of bytes (which is important to handle the end condition). The initial state and latent vectors are handled in the same way, both coded one dimension at a time. For each dimension, the decoder uses the quantization tables to determine the _r_ and _p0_ parameters. If r=0 or p0=255 for the current symbols and quantizer, then no symbol is decoded and the decoded quantized value is 0. Otherwise, decoding proceeds as follows. The first symbol decoded determines whether the quantized index is zero, positive, or negative (in that order). The decoder uses the pdf {2*p0_{i,q}, 256-p0_{i,q}, 256-p0_{i,q}}/512. If the value is non-zero, a second symbol is decoded. We start by generating an "inverse cdf" in Q15: Valin & Buethe Expires 26 August 2024 [Page 18] Internet-Draft Opus DRED February 2024 / 32768 , if i < 0 | | MAX(7, 128*r_{i,q}) , if i = 0 icdf(i) = < | MAX(7-i, (icdf[i-1]*r_{i,q})//32768) , if 0 < i < 7 | \ 0 , i>= 7 where // denotes the truncating integer division. The pdf is then given by pdf[i] = icdf[i-1]-icdf[i]. If the decoded symbol equals 7, then another symbol is decoded and added to the 7 already decoded. The process is repeated until the decoded symbol is different from 7. At that point, the sign is applied and the decoded value is equal to quantized_index*256/s_{i,q}. 4. IANA Considerations [Note: Until the IANA performs the actions described below, implementers should use 126 instead of 32 as the extension number. Moreover, the DRED payload temporarily uses a two-byte prefix for compatibility: a 'D' character, followed by a version number (currently 10).] This document assigns ID 32 to the "Opus Extension IDs" registry created in [opus-extension] to implement the proposed DRED extension. 4.1. Opus Media Type Update This document updates the audio/opus media type registration [RFC7587] to add the following two optional parameters: ext32-dred-duration: Specifies the maximum amount of DRED information (in milliseconds) that the receiver can use. The receiver MUST be able to handle any valid DRED duration even if it does not make use of it. The sender MUST NOT send more than the specified amount of redundancy to avoid leaking information beyond what the receiver expects. sprop-ext32-dred-duration: Maximum amount of DRED information (in milliseconds) that the sender is likely to use. The received MUST be able to handle any valid DRED duration even if it does not make use of it. The sender MUST NOT send more than the specified amount of redundancy to avoid leaking information beyond what the receiver expects. Valin & Buethe Expires 26 August 2024 [Page 19] Internet-Draft Opus DRED February 2024 4.2. Mapping to SDP Parameters The media type parameters described above map to declarative SDP and SDP offer-answer in the same way as other optional parameters in [RFC7587]. Regardless of any a=fmtp SDP attribute specified, the receiver MUST be capable of receiving any signal. 5. Security Considerations When using a Selective Forwarding Unit (SFU), it is possible for the DRED payload to include speech that would not otherwise have been transmitted. For example, a new user joining may receive audio that was transmitted before them joining. If such behavior is a security or confidentiality concern, then the SFU SHOULD use the ext32-dred- duration and sprop-ext32-dred-duration parameters to limit the amount of redundancy and/or temporarily drop DRED payloads when that could leak information. 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 computationally 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 [RFC6562], 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. 6. References 6.1. Normative References Valin & Buethe Expires 26 August 2024 [Page 20] Internet-Draft Opus DRED February 2024 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the Opus Audio Codec", RFC 6716, DOI 10.17487/RFC6716, September 2012, . [opus-extension] Terriberry, T.B. and J.-M. Valin, "Extension Formatting for the Opus Codec (draft-ietf-mlcodec-opus-extension)", October 2023. 6.2. Informative References [RFC6562] Perkins, C. and JM. Valin, "Guidelines for the Use of Variable Bit Rate Audio with Secure RTP", RFC 6562, DOI 10.17487/RFC6562, March 2012, . [dred-paper] Valin, J.-M., Buethe, J., and A. Mustafa, "Low-Bitrate Redundancy Coding of Speech Using a Rate-Distortion- Optimized Variational Autoencoder", 2023, . Authors' Addresses Jean-Marc Valin Xiph.Org Foundation Canada Email: jmvalin@jmvalin.ca Jan Buethe Amazon Germany Email: jbuethe@amazon.com Valin & Buethe Expires 26 August 2024 [Page 21]