Deep Audio Redundancy (DRED) Extension for the Opus Codec
draft-valin-opus-dred-05
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Authors | Jean-Marc Valin , Jan Buethe | ||
| Last updated | 2024-02-23 | ||
| RFC stream | (None) | ||
| Intended RFC status | (None) | ||
| Formats | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-valin-opus-dred-05
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.
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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.
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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.
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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.
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+======+======================+================+===============+
| 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?
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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?
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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
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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.
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|3 |255|217|186|159|137|118|102|87|76|66|53 |25 |11 |5 |2 | 1 |
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|4 |255|216|183|155|131|111|95 |79|67|57|48 |42 |35 |29 |24 | 21 |
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|5 |255|219|189|163|141|122|107|90|87|31|11 |3 |3 |2 |1 | 1 |
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|6 |255|218|187|160|138|119|103|87|72|45|18 |6 |5 |3 |2 | 2 |
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|7 |255|217|184|157|133|113|96 |78|67|53|34 |17 |6 |5 |4 | 3 |
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|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
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+==+==+==+==+==+==+==+===+===+===+===+===+===+===+=====+=====+=====+
|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
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+==+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+===+
|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
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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.
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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.
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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:
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/ 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.
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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
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[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>.
[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>.
[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>.
[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,
<https://www.rfc-editor.org/info/rfc6562>.
[dred-paper]
Valin, J.-M., Buethe, J., and A. Mustafa, "Low-Bitrate
Redundancy Coding of Speech Using a Rate-Distortion-
Optimized Variational Autoencoder", 2023,
<https://arxiv.org/abs/2212.04453>.
Authors' Addresses
Jean-Marc Valin
Xiph.Org Foundation
Canada
Email: jmvalin@jmvalin.ca
Jan Buethe
Amazon
Germany
Email: jbuethe@amazon.com
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