Network Working Group                                          JM. Valin
Internet-Draft                                                   Mozilla
Intended status: Standards Track                                 C. Bran
Expires: February 03, 2014                                   Plantronics
                                                         August 02, 2013

             WebRTC Audio Codec and Processing Requirements


   This document outlines the audio codec and processing requirements
   for WebRTC client application and endpoint devices.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on February 03, 2014.

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Table of Contents

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   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   2
   3.  Codec Requirements  . . . . . . . . . . . . . . . . . . . . .   2
   4.  Audio Level . . . . . . . . . . . . . . . . . . . . . . . . .   3
   5.  Acoustic Echo Cancellation (AEC)  . . . . . . . . . . . . . .   4
   6.  Legacy VoIP Interoperability  . . . . . . . . . . . . . . . .   4
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   4
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   5
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   5
   10. Normative References  . . . . . . . . . . . . . . . . . . . .   5
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   5

1.  Introduction

   An integral part of the success and adoption of the Web Real Time
   Communications (WebRTC) will be the voice and video interoperability
   between WebRTC applications.  This specification will outline the
   audio processing and codec requirements for WebRTC client

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

3.  Codec Requirements

   To ensure a baseline level of interoperability between WebRTC
   clients, a minimum set of required codecs are specified below.  While
   this section specifies the codecs that will be mandated for all
   WebRTC client implementations, it leaves the question of supporting
   additional codecs to the will of the implementer.

   WebRTC clients are REQUIRED to implement the following audio codecs.

   o  Opus [RFC6716], with the payload format specified in [Opus-RTP]
      and any ptime value up to 120 ms

   o  G.711 PCMA and PCMU with one channel, a rate of 8000 Hz and a
      ptime of 20 - see section 4.5.14 of [RFC3551]

   o  Telephone Event - [RFC4733]

   For all cases where the client is able to process audio at a sampling
   rate higher than 8 kHz, it is RECOMMENDED that Opus be offered before
   PCMA/PCMU.  For Opus, all modes MUST be supported on the decoder

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   side.  The choice of encoder-side modes is left to the implementer.
   Clients MAY use the offer/answer mechanism to signal a preference for
   a particular mode or ptime.

4.  Audio Level

   It is desirable to standardize the "on the wire" audio level for
   speech transmission to avoid users having to manually adjust the
   playback and to facilitate mixing in conferencing applications.  It
   is also desirable to be consistent with ITU-T recommendations G.169
   and G.115, which recommend an active audio level of -19 dBm0.
   However, unlike G.169 and G.115, the audio for WebRTC is not
   constrained to have a passband specified by G.712 and can in fact be
   sampled at any sampling rate from 8 kHz to 48 kHz and up.  For this
   reason, the level SHOULD be normalized by only considering
   frequencies above 300 Hz, regardless of the sampling rate used.  The
   level SHOULD also be adapted to avoid clipping, either by lowering
   the gain to a level below -19 dBm0, or through the use of a

   AUTHORS' NOTE: The idea of using the same level as what the ITU-T
   recommends is that it should improve inter-operability while at the
   same time maintaining sufficient dynamic range and reducing the risk
   of clipping.  The main drawbacks are that the resulting level is
   about 12 dB lower than typical "commercial music" levels and it
   leaves room for ill-behaved clients to be much louder than a normal
   client.  While using music-type levels is not really an option (it
   would require using the same compressor-limitors that studios use),
   it would be possible to have a level slightly higher (e.g.  3 dB)
   than what is recommended above without causing interoperability

   Assuming 16-bit PCM with a value of +/-32767, -19 dBm0 corresponds to
   a root mean square (RMS) level of 2600.  Only active speech should be
   considered in the RMS calculation.  If the client has control over
   the entire audio capture path, as is typically the case for a regular
   phone, then it is RECOMMENDED that the gain be adjusted in such a way
   that active speech have a level of 2600 (-19 dBm0) for an average
   speaker.  If the client does not have control over the entire audio
   capture, as is typically the case for a software client, then the
   client SHOULD use automatic gain control (AGC) to dynamically adjust
   the level to 2600 (-19 dBm0) +/- 6 dB.  For music or desktop sharing
   applications, the level SHOULD NOT be automatically adjusted and the
   client SHOULD allow the user to set the gain manually.

   The RECOMMENDED filter for normalizing the signal energy is a second-
   order Butterworth filter with a 300 Hz cutoff frequency.

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   It is common for the audio output on some devices to be "calibrated"
   for playing back pre-recorded "commercial" music, which is typically
   around 12 dB louder than the level recommended in this section.
   Because of this, clients MAY increase the gain before playback.

5.  Acoustic Echo Cancellation (AEC)

   It is plausible that the dominant near to mid-term WebRTC usage model
   will be people using the interactive audio and video capabilities to
   communicate with each other via web browsers running on a notebook
   computer that has built-in microphone and speakers.  The notebook-as-
   communication-device paradigm presents challenging echo cancellation
   problems, the specific remedy of which will not be mandated here.
   However, while no specific algorithm or standard will be required by
   WebRTC compatible clients, echo cancellation will improve the user
   experience and should be implemented by the endpoint device.

   WebRTC clients SHOULD include an AEC and if that is not possible, the
   clients SHOULD ensure that the speaker-to-microphone gain is below
   unity at all frequencies to avoid instability when none of the client
   has echo cancellation.  For clients that do not control the audio
   capture and playback devices directly, it is RECOMMENDED to support
   echo cancellation between devices running at slight different
   sampling rates, such as when a webcam is used for microphone.

   The client SHOULD allow either the entire AEC or the non-linear
   processing (NLP) to be turned off for applications, such as music,
   that do not behave well with the spectral attenuation methods
   typically used in NLPs.  It SHOULD have the ability to detect the
   presence of a headset and disable echo cancellation.

   For some applications where the remote client may not have an echo
   canceller, the local client MAY include a far-end echo canceller, but
   if that is the case, it SHOULD be disabled by default.

6.  Legacy VoIP Interoperability

   The codec requirements above will ensure, at a minimum, voice
   interoperability capabilities between WebRTC client applications and
   legacy phone systems.

7.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

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8.  Security Considerations

   The codec requirements have no additional security considerations
   other than those captured in

9.  Acknowledgements

   This draft incorporates ideas and text from various other drafts.  In
   particularly we would like to acknowledge, and say thanks for, work
   we incorporated from Harald Alvestrand and Cullen Jennings.

10.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3551]  Schulzrinne, H. and S. Casner, "RTP Profile for Audio and
              Video Conferences with Minimal Control", STD 65, RFC 3551,
              July 2003.

   [RFC4733]  Schulzrinne, H. and T. Taylor, "RTP Payload for DTMF
              Digits, Telephony Tones, and Telephony Signals", RFC 4733,
              December 2006.

   [RFC6716]  Valin, JM., Vos, K., and T. Terriberry, "Definition of the
              Opus Audio Codec", RFC 6716, September 2012.

              Spittka, J., Vos, K., and JM. Valin, "RTP Payload Format
              for Opus Codec", August 2013.

              Rescorla, E.K., "Security Considerations for RTC-Web", May

Authors' Addresses

   Jean-Marc Valin
   650 Castro Street
   Mountain View, CA  94041


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   Cary Bran
   345 Encinial Street
   Santa Cruz, CA  95060

   Phone: +1 206 661-2398

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