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Bran 5 Expires: February 03, 2014 Plantronics 6 August 02, 2013 8 WebRTC Audio Codec and Processing Requirements 9 draft-ietf-rtcweb-audio-02 11 Abstract 13 This document outlines the audio codec and processing requirements 14 for WebRTC client application and endpoint devices. 16 Status of This Memo 18 This Internet-Draft is submitted in full conformance with the 19 provisions of BCP 78 and BCP 79. 21 Internet-Drafts are working documents of the Internet Engineering 22 Task Force (IETF). Note that other groups may also distribute 23 working documents as Internet-Drafts. The list of current Internet- 24 Drafts is at http://datatracker.ietf.org/drafts/current/. 26 Internet-Drafts are draft documents valid for a maximum of six months 27 and may be updated, replaced, or obsoleted by other documents at any 28 time. It is inappropriate to use Internet-Drafts as reference 29 material or to cite them other than as "work in progress." 31 This Internet-Draft will expire on February 03, 2014. 33 Copyright Notice 35 Copyright (c) 2013 IETF Trust and the persons identified as the 36 document authors. All rights reserved. 38 This document is subject to BCP 78 and the IETF Trust's Legal 39 Provisions Relating to IETF Documents 40 (http://trustee.ietf.org/license-info) in effect on the date of 41 publication of this document. Please review these documents 42 carefully, as they describe your rights and restrictions with respect 43 to this document. Code Components extracted from this document must 44 include Simplified BSD License text as described in Section 4.e of 45 the Trust Legal Provisions and are provided without warranty as 46 described in the Simplified BSD License. 48 Table of Contents 49 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 50 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 2 51 3. Codec Requirements . . . . . . . . . . . . . . . . . . . . . 2 52 4. Audio Level . . . . . . . . . . . . . . . . . . . . . . . . . 3 53 5. Acoustic Echo Cancellation (AEC) . . . . . . . . . . . . . . 4 54 6. Legacy VoIP Interoperability . . . . . . . . . . . . . . . . 4 55 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4 56 8. Security Considerations . . . . . . . . . . . . . . . . . . . 5 57 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 5 58 10. Normative References . . . . . . . . . . . . . . . . . . . . 5 59 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 5 61 1. Introduction 63 An integral part of the success and adoption of the Web Real Time 64 Communications (WebRTC) will be the voice and video interoperability 65 between WebRTC applications. This specification will outline the 66 audio processing and codec requirements for WebRTC client 67 implementations. 69 2. Terminology 71 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", 72 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this 73 document are to be interpreted as described in RFC 2119 [RFC2119]. 75 3. Codec Requirements 77 To ensure a baseline level of interoperability between WebRTC 78 clients, a minimum set of required codecs are specified below. While 79 this section specifies the codecs that will be mandated for all 80 WebRTC client implementations, it leaves the question of supporting 81 additional codecs to the will of the implementer. 83 WebRTC clients are REQUIRED to implement the following audio codecs. 85 o Opus [RFC6716], with the payload format specified in [Opus-RTP] 86 and any ptime value up to 120 ms 88 o G.711 PCMA and PCMU with one channel, a rate of 8000 Hz and a 89 ptime of 20 - see section 4.5.14 of [RFC3551] 91 o Telephone Event - [RFC4733] 93 For all cases where the client is able to process audio at a sampling 94 rate higher than 8 kHz, it is RECOMMENDED that Opus be offered before 95 PCMA/PCMU. For Opus, all modes MUST be supported on the decoder 96 side. The choice of encoder-side modes is left to the implementer. 97 Clients MAY use the offer/answer mechanism to signal a preference for 98 a particular mode or ptime. 100 4. Audio Level 102 It is desirable to standardize the "on the wire" audio level for 103 speech transmission to avoid users having to manually adjust the 104 playback and to facilitate mixing in conferencing applications. It 105 is also desirable to be consistent with ITU-T recommendations G.169 106 and G.115, which recommend an active audio level of -19 dBm0. 107 However, unlike G.169 and G.115, the audio for WebRTC is not 108 constrained to have a passband specified by G.712 and can in fact be 109 sampled at any sampling rate from 8 kHz to 48 kHz and up. For this 110 reason, the level SHOULD be normalized by only considering 111 frequencies above 300 Hz, regardless of the sampling rate used. The 112 level SHOULD also be adapted to avoid clipping, either by lowering 113 the gain to a level below -19 dBm0, or through the use of a 114 compressor. 116 AUTHORS' NOTE: The idea of using the same level as what the ITU-T 117 recommends is that it should improve inter-operability while at the 118 same time maintaining sufficient dynamic range and reducing the risk 119 of clipping. The main drawbacks are that the resulting level is 120 about 12 dB lower than typical "commercial music" levels and it 121 leaves room for ill-behaved clients to be much louder than a normal 122 client. While using music-type levels is not really an option (it 123 would require using the same compressor-limitors that studios use), 124 it would be possible to have a level slightly higher (e.g. 3 dB) 125 than what is recommended above without causing interoperability 126 problems. 128 Assuming 16-bit PCM with a value of +/-32767, -19 dBm0 corresponds to 129 a root mean square (RMS) level of 2600. Only active speech should be 130 considered in the RMS calculation. If the client has control over 131 the entire audio capture path, as is typically the case for a regular 132 phone, then it is RECOMMENDED that the gain be adjusted in such a way 133 that active speech have a level of 2600 (-19 dBm0) for an average 134 speaker. If the client does not have control over the entire audio 135 capture, as is typically the case for a software client, then the 136 client SHOULD use automatic gain control (AGC) to dynamically adjust 137 the level to 2600 (-19 dBm0) +/- 6 dB. For music or desktop sharing 138 applications, the level SHOULD NOT be automatically adjusted and the 139 client SHOULD allow the user to set the gain manually. 141 The RECOMMENDED filter for normalizing the signal energy is a second- 142 order Butterworth filter with a 300 Hz cutoff frequency. 144 It is common for the audio output on some devices to be "calibrated" 145 for playing back pre-recorded "commercial" music, which is typically 146 around 12 dB louder than the level recommended in this section. 147 Because of this, clients MAY increase the gain before playback. 149 5. Acoustic Echo Cancellation (AEC) 151 It is plausible that the dominant near to mid-term WebRTC usage model 152 will be people using the interactive audio and video capabilities to 153 communicate with each other via web browsers running on a notebook 154 computer that has built-in microphone and speakers. The notebook-as- 155 communication-device paradigm presents challenging echo cancellation 156 problems, the specific remedy of which will not be mandated here. 157 However, while no specific algorithm or standard will be required by 158 WebRTC compatible clients, echo cancellation will improve the user 159 experience and should be implemented by the endpoint device. 161 WebRTC clients SHOULD include an AEC and if that is not possible, the 162 clients SHOULD ensure that the speaker-to-microphone gain is below 163 unity at all frequencies to avoid instability when none of the client 164 has echo cancellation. For clients that do not control the audio 165 capture and playback devices directly, it is RECOMMENDED to support 166 echo cancellation between devices running at slight different 167 sampling rates, such as when a webcam is used for microphone. 169 The client SHOULD allow either the entire AEC or the non-linear 170 processing (NLP) to be turned off for applications, such as music, 171 that do not behave well with the spectral attenuation methods 172 typically used in NLPs. It SHOULD have the ability to detect the 173 presence of a headset and disable echo cancellation. 175 For some applications where the remote client may not have an echo 176 canceller, the local client MAY include a far-end echo canceller, but 177 if that is the case, it SHOULD be disabled by default. 179 6. Legacy VoIP Interoperability 181 The codec requirements above will ensure, at a minimum, voice 182 interoperability capabilities between WebRTC client applications and 183 legacy phone systems. 185 7. IANA Considerations 187 This document makes no request of IANA. 189 Note to RFC Editor: this section may be removed on publication as an 190 RFC. 192 8. Security Considerations 194 The codec requirements have no additional security considerations 195 other than those captured in 196 [I-D.ekr-security-considerations-for-rtc-web]. 198 9. Acknowledgements 200 This draft incorporates ideas and text from various other drafts. In 201 particularly we would like to acknowledge, and say thanks for, work 202 we incorporated from Harald Alvestrand and Cullen Jennings. 204 10. Normative References 206 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate 207 Requirement Levels", BCP 14, RFC 2119, March 1997. 209 [RFC3551] Schulzrinne, H. and S. Casner, "RTP Profile for Audio and 210 Video Conferences with Minimal Control", STD 65, RFC 3551, 211 July 2003. 213 [RFC4733] Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF 214 Digits, Telephony Tones, and Telephony Signals", RFC 4733, 215 December 2006. 217 [RFC6716] Valin, JM., Vos, K., and T. Terriberry, "Definition of the 218 Opus Audio Codec", RFC 6716, September 2012. 220 [Opus-RTP] 221 Spittka, J., Vos, K., and JM. Valin, "RTP Payload Format 222 for Opus Codec", August 2013. 224 [I-D.ekr-security-considerations-for-rtc-web] 225 Rescorla, E.K., "Security Considerations for RTC-Web", May 226 2011. 228 Authors' Addresses 230 Jean-Marc Valin 231 Mozilla 232 650 Castro Street 233 Mountain View, CA 94041 234 USA 236 Email: jmvalin@jmvalin.ca 237 Cary Bran 238 Plantronics 239 345 Encinial Street 240 Santa Cruz, CA 95060 241 USA 243 Phone: +1 206 661-2398 244 Email: cary.bran@plantronics.com