Network Working Group                                          JM. Valin
Internet-Draft                                              Octasic Inc.
Intended status: Standards Track                                  K. Vos
Expires: February 25, 2011                                         Skype
                                                         August 24, 2010

                           Codec Requirements


   This document provides specific requirements for an Internet audio
   codec.  These requirements address quality, sampling rate, bit-rate,
   and packet loss robustness, as well as other desirable properties.

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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   This Internet-Draft will expire on February 25, 2011.

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Applications . . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1.  Point to point calls . . . . . . . . . . . . . . . . . . .  4
     2.2.  Conferencing . . . . . . . . . . . . . . . . . . . . . . .  4
     2.3.  Telepresence . . . . . . . . . . . . . . . . . . . . . . .  5
     2.4.  Teleoperation and Remote Software Services . . . . . . . .  5
     2.5.  In-game voice chat . . . . . . . . . . . . . . . . . . . .  6
     2.6.  Live distributed music performances / Internet music
           lessons  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.7.  Other applications . . . . . . . . . . . . . . . . . . . .  7
   3.  Constraints Imposed by the Internet on the Codec . . . . . . .  8
     3.1.  Security . . . . . . . . . . . . . . . . . . . . . . . . .  9
   4.  Detailed Basic Requirements  . . . . . . . . . . . . . . . . . 10
     4.1.  Operating space  . . . . . . . . . . . . . . . . . . . . . 10
     4.2.  Quality and bit-rate . . . . . . . . . . . . . . . . . . . 10
     4.3.  Packet loss robustness . . . . . . . . . . . . . . . . . . 11
     4.4.  Computational resources  . . . . . . . . . . . . . . . . . 11
   5.  Additional considerations  . . . . . . . . . . . . . . . . . . 14
     5.1.  Low-complexity audio mixing  . . . . . . . . . . . . . . . 14
     5.2.  Encoder side potential for improvement . . . . . . . . . . 14
     5.3.  Layered bit-stream . . . . . . . . . . . . . . . . . . . . 14
     5.4.  Partial redundancy . . . . . . . . . . . . . . . . . . . . 15
     5.5.  Stereo support . . . . . . . . . . . . . . . . . . . . . . 15
     5.6.  Bit error robustness . . . . . . . . . . . . . . . . . . . 15
     5.7.  Time stretching and shortening . . . . . . . . . . . . . . 15
     5.8.  Input robustness . . . . . . . . . . . . . . . . . . . . . 16
     5.9.  Legacy compatibility . . . . . . . . . . . . . . . . . . . 16
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  Informative References . . . . . . . . . . . . . . . . . . . . 20
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 21

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1.  Introduction

   This document provides requirements for an audio codec designed
   specifically for use over the Internet.  The requirements attempt to
   address the needs of the most common Internet interactive audio
   transmission applications and to ensure good quality when operating
   in conditions that are typical for the Internet.  These requirements
   address the quality, sampling rate, delay, bit-rate, and packet loss
   robustness.  Other desirable codec properties are considered as well.

   Throughout this document, we will use the following conventions when
   referring to the sampling rate of a signal:

      Narrowband: 8 kHz sampling rate

      Wideband: 16 kHz sampling rate

      Super-wideband: 32 kHz sampling rate

      Full-band: 44.1/48 kHz and above

   Codec bit-rates in bits per second (b/s) will be considered without
   counting any overhead (IP/UDP/RTP headers, padding, ...).  The codec
   delay is the total algorithmic delay when one adds the codec frame
   size to the "look-ahead".  It is thus the minimum theoretically
   achievable end-to-end delay of a transmission system that uses the

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2.  Applications

   The following applications should be considered for Internet audio
   codecs, along with their requirements:

   o  Point to point calls

   o  Conferencing

   o  Telepresence

   o  Teleoperation

   o  In-game voice chat

   o  Live distributed music performances / Internet music lessons

   o  Other applications

2.1.  Point to point calls

   Point to point calls are voice over IP (VoIP) calls from two
   "standard" (fixed or mobile) phones, and implemented in hardware or
   software.  For these applications, a wideband codec is required,
   along with narrowband support for compatibility with legacy telephony
   equipment (PSTN).  It is expected for the range of useful bit-rates
   to be 12 - 32 kb/s for wideband speech and 8 - 16 kb/s for narrowband
   speech.  The codec delay must be less than 40 ms, but no more than 25
   ms is desirable.  Support for encoding music is not required, but it
   is desirable for the codec not to make background (on-hold) music
   excessively unpleasant to hear.  Also, the codec should be robust to
   noise (produce intelligible speech and no annoying artifacts) even at
   lower bit-rates.

2.2.  Conferencing

   Conferencing applications (which support multi-party calls) have
   additional requirements on top of the requirements for point-to-point
   calls.  Conferencing systems often have higher-fidelity audio
   equipment and have greater network bandwidth available -- especially
   when video transmission is involved.  For that reason, support for
   super-wideband audio becomes important, with useful bit-rates in the
   32 - 64 kb/s range.  The ability to vary the bit-rate according to
   the "difficulty" of the audio signal (VBR) is a desirable feature for
   the codec.  This not only saves bandwidth "on average", but it can
   also help conference servers make more efficient use of the available
   bandwidth by using more bandwidth for important audio streams and
   less bandwidth for less important ones (e.g. background noise).

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   Conferencing end-points often operate in hands-free conditions, which
   creates acoustic echo problems.  For this reason lower delay is
   important, as it reduces the quality degradation due to any residual
   echo after acoustic echo cancellation (AEC).  For this reason, the
   codec delay must be less than 30 ms for this application.  An
   optional low-delay mode with less than 10 ms delay is desirable, but
   not required.

   Most conferencing systems operate with a bridge that mixes some (or
   all) of the audio streams and sends them back to all the
   participants.  In that case, it is important that the codec not
   produce annoying artefacts when two voices are present at the same
   time.  Also, this mixing operation should be as easy as possible to
   perform.  To make it easier to determine which streams have to be
   mixed (and which are noise/silence), it must be possible to measure
   (or estimate) the voice activity in a packet without having to fully
   decode the packet (saving most of the complexity when the packet need
   not be decoded).  Also, the ability to save on the computational
   complexity when mixing is also desirable, but not required.  For
   example, a transform codec may make it possible to mix the streams in
   the transform domain, without having to go back to time-domain.  Low-
   complexity up-sampling and down-sampling within the codec is also a
   desirable feature when mixing streams with different sampling rates.

2.3.  Telepresence

   Most telepresence applications can be considered to be essentially
   very high-quality video-conferencing environments, so all of the
   conferencing requirements also apply to telepresence.  In addition,
   telepresence applications require super-wideband and full-band audio
   capability with useful bit-rates in the 32 - 80 kb/s range.  While
   voice is still the most important signal to be encoded, it must be
   possible to obtain good quality (even if not transparent) music.

   Most telepresence applications require more than one audio channel,
   so support for stereo and multi-channel is important.  While this can
   always be accomplished by encoding multiple single-channel streams,
   it is preferable to take advantage of the redundancy that exists
   between channels.

2.4.  Teleoperation and Remote Software Services

   Teleoperation applications are similar to telepresence, with the
   exception that they involve remote physical interactions.  For
   example, the user may be controlling a robot while receiving real-
   time audio feedback from that robot.  For these applications, the
   delay has to be less than 10 ms.  The other requirements of
   telepresence (quality, bit-rate, multi-channel) apply to

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   teleoperation as well.  The only exception is that mixing is not an
   important issue for teleoperation.

   The requirements for remote software services are simiar to those of
   teleoperation.  These applications include remote desktop
   applications, remote virtualization, and interactive media
   application being rendered remotely (e.g. video games rendered on
   central servers).  For all these applications, full-band audio with
   an algorithmic delay below 10 ms are important.

2.5.  In-game voice chat

   An increasing number of computer/console games make use of VoIP to
   allow players to communicate in real-time.  The requirements for
   gaming are similar to those of conferencing, with the main difference
   being that narrowband compatibility is not necessary.  While for most
   applications a codec delay up to 30 ms is acceptable, a low-delay (<
   10 ms) option is highly desirable, especially for games with rapid
   interactions.  The ability to use VBR (with a maximum allowed
   bitrate) is also highly desirable because it can significantly reduce
   the bandwidth requirement for a game server.

2.6.  Live distributed music performances / Internet music lessons

   Live music over the Internet requires extremely low end-to-end delay
   and is one of the most demanding application for interactive audio
   transmission.  It has been observed that for most scenarios, total
   end-to-end delays up to 25 ms could be tolerated by musicians, with
   the absolute limit (where none of the scenarios are possible) being
   around 50 ms [carot09].  In order to achieve this low delay on the
   Internet -- either in the same city or a nearby city -- the network
   propagation time must be taken into account.  When also subtracting
   the delay of the audio buffer, jitter buffer, and acoustic path, that
   leaves around 2 ms to 10 ms for the total delay of the codec.
   Considering the speed of light in fiber, every 1 ms reduction in the
   codec delay increases the range over which synchronization is
   possible by approximately 200 km.

   Acoustic echo is expected to be an even more important issue for
   network music than it is in conferencing, especially considering that
   the music quality requirements essentially forbid the use of a
   "nonlinear processor" (NLP) with the AEC.  This is another reason why
   very low delay is essential.

   Considering that the application is music, the full audio bandwidth
   (44.1 or 48 kHz sampling rate) must be transmitted with a bit-rate
   that is sufficient to provide near-transparent to transparent
   quality.  With the current audio coding technology, this corresponds

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   to approximately 64 kb/s to 128 kb/s per channel.  As for
   telepresence, support for two or more channels is often desired, so
   it would be useful for a codec to be able to take advantage of the
   redundancy that is often present between audio channels.

2.7.  Other applications

   The above list is by no means a complete list of all applications
   involving interactive audio transmission on the Internet.  However,
   it is believed that meeting the needs of all these different
   applications should be sufficient to ensure that most applications
   not listed will also be met.

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3.  Constraints Imposed by the Internet on the Codec

   Packet losses are inevitable on the Internet and dealing with those
   is one of the most fundamental requirements for an Internet audio
   codec.  While any audio codec can be combined with a good packet loss
   concealment (PLC) algorithm, the important aspect is what happens on
   the first packets received _after_ the loss.  More specifically, this
   means that:

   o  it should be possible to interpret the contents of any received
      packet, irrespective of previous losses as specified in BCP 36
      [PAYLOADS]; and

   o  the decoder should re-synchronize as quickly as possible (i.e. the
      output should quickly converge to the output that would have been
      obtained if no-loss had occurred).

   The constraint of being able to decode any packet implies the
   following considerations for an audio codec:

   o  The size of a compressed frame must be kept smaller than the MTU
      to avoid fragmentation;

   o  The interpretation of any parameter encoded in the bit-stream must
      not depend on information contained in other packets.  For
      example, it is not acceptable for a codec to allow signaling a
      mode change in one packet and assume that subsequent frames will
      be decoded according to that mode.

   Although the interpretation of parameters cannot depend on other
   packets, it is still reasonable to use some amount of prediction
   across frames, provided that the predictors can resynchronize quickly
   in case of a lost packet.  In this case, it is important to use the
   best compromise between the gain in coding efficiency and the loss in
   packet loss robustness due to the use of inter-frame prediction.  It
   is a desirable property for the codec to allow some real-time control
   of that trade-off so that it can take advantage of more prediction
   when the loss rate is small, while being more robust to losses when
   the loss rate is high.

   To improve the robustness to packet loss, it would be desirable for
   the codec to allow an adaptive (data- and network-dependent) amount
   of side information to help improve audio quality when losses occur.
   For example, this side information may include the retransmission of
   certain parameters encoded in the previous frame(s).

   Another important property of the Internet is that it is mostly a
   best-effort network, with no guaranteed bandwidth.  This means that

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   the codec has to be able to vary its output bit-rate dynamically (in
   real-time), without requiring an out-of-band signaling mechanism, and
   without causing audible artifacts at the bit-rate change boundaries.
   Additional desirable features are:

   o  Having the possibility to use smooth bit-rate changes with one
      byte/frame resolution;

   o  Making it possible for a codec to adapt its bit-rate based on the
      source signal being encoded (source-controlled VBR) to maximize
      the quality for a certain _average_ bit-rate.

   Because the Internet transmits data in bytes, a codec should produce
   compressed data in integer numbers of bytes.  In general, the codec
   design should take into consideration explicit congestion
   notification (ECN) and may include features that would improve the
   quality of an ECN implementation.

   The IETF has defined a set of application-layer protocols to be used
   for transmitting real-time transport of multimedia data, including
   voice.  It is thus important for the resulting codec to be easy to
   use with these protocols.  For example, it must be possible to create
   an [RTP] payload format that conforms to BCP 36 [PAYLOADS].  If any
   codec parameters need to be negotiated between end-points, the
   negotiation should be as easy as possible to carry over SIP/SDP or
   alternatively over XMPP/Jingle.

3.1.  Security

   Just like for any protocol to be used over the Internet, security is
   a very important aspect to consider.  This goes beyond the obvious
   considerations of preventing buffer overflows and similar attacks
   that can lead to denial-of-service or remote code execution.  One
   very important security aspect is to make sure that the decoders have
   a bounded and reasonable worst-case complexity.  This prevents an
   attacker from causing a DoS by sending packets that are specially
   crafted to take a very long (or infinite) time to decode.

   A more subtle aspect is the information leak that can occur when the
   codec is used over an encrypted channel (e.g.  [SRTP]).  For example,
   it was suggested [wright08] that use of source-controlled VBR may
   reveal some information about a conversation through the size of the
   compressed packets.  This would have to be investigated when
   standardizing a codec.

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4.  Detailed Basic Requirements

   This section summarizes all the constraints imposed by the target
   applications and by the Internet into a set of actual requirements
   for codec development.

4.1.  Operating space

   The operating space for the target applications can be divided in
   terms of delay: most applications require a "medium delay" (20-30
   ms), while a few require a "very low delay" (< 10 ms).  It makes
   sense to divide the space based on delay because lowering the delay
   has a cost in terms of quality vs bit-rate.

   For medium delay, the resulting codec must be able to efficiently
   operate within the following range of bit-rates (per channel):

   o  Narrowband: 8 kb/s to 16 kb/s

   o  Wideband: 12 to 32 kb/s

   o  Super-wideband: 24 to 64 kb/s

   o  Full-band: 32 to 80 kb/s

   Obviously, a lower-delay codec that can operate in the above range is
   also acceptable.

   For very low delay, the resulting codec will need to operate within
   the following range of bit-rates (per channel):

   o  Super-wideband: 32 to 80 kb/s

   o  Full-band: 48 to 128 kb/s

   o  (Narrowband and wideband not required)

4.2.  Quality and bit-rate

   The quality of a codec is directly linked to the bit-rate, so these
   two must be considered jointly.  When comparing the bit-rate of
   codecs, the overhead of IP/UDP/RTP headers should not be considered,
   but any additional bits required in the RTP payload format after the
   header (e.g. required signalling) should be considered.  In terms of
   quality vs bit-rate, the codec to be developed must be better than
   the currently available codecs that satisfy the IPR requirements in
   the guidelines document, which are:

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   o  For narrowband: Speex (NB), GSM-FR, and iLBC(*)

   o  For wideband: Speex (WB), G.722, G.722.1(*)

   o  For super-wideband: Speex (UWB), G.722.1C(*)

   The codecs marked with (*) do not meet all the licensing guidelines,
   but the codecs to be developed should still not perform significantly
   worse.  Quality should be measured for multiple languages, including
   tonal languages.  The case of multiple simultaneous voices (as
   sometimes happens in conferencing) should be evaluated as well.

   The comparison with the above codecs assumes that the codecs being
   compared have similar delay characteristics.  The bit-rate required
   for a certain level of quality may be higher than the referenced
   codecs in cases where a much lower delay is required.  In that case,
   the increase in bit-rate must be less than the ratio between the

   It is desirable for the codecs to support source-controlled variable
   bit-rate (VBR) to take advantage from the fact that different inputs
   require a different bitrate to achieve the same quality.  However, it
   should still be possible to use the codec at truely constant bit-rate
   to ensure that no information leak is possible when using an
   encrypted channel.

4.3.  Packet loss robustness

   Robustness to packet loss is a very important aspect of any codec to
   be used on the Internet.  Codecs must maintain acceptable quality at
   loss rates up to 5% and maintain good intelligibility up to 15% loss
   rate.  At any sampling rate, bit-rate, and packet loss rate, the
   quality must be no less than the quality obtained with the Speex
   codec or the GSM-FR codec in the same conditions.  The actual packet
   loss "patterns" to be used in testing must be obtained from real
   packet loss traces collected on the Internet, rather than from loss
   models.  These traces should be representative of the typical
   environments in which the applications of Section 2 operate.  For
   example, traces related to VoIP calls should consider the loss
   patterns observed for typical home broadband and corporate

4.4.  Computational resources

   The resulting codec should be implementable on a wide range of
   devices, so there should be a fixed-point implementation or at least
   assurance that a reasonable fixed-point is possible.  The
   computational resources figures listed below are meant to be upper

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   bounds.  Even below these bounds, resources should still be
   minimized.  Any proposed increase in computational resources
   consumption (e.g. to increase quality) should be carefully evaluated
   even if the resulting resource consumption is below the upper bound.
   Having variable complexity would be useful (but not required) in
   achieving that goal as it would allow trading quality/bit-rate for
   lower complexity.

   The computational requirements for real-time encoding and decoding

   o  Narrowband should require little CPU resources and be
      implementable on most DSPs with a 16x16 multiplier (e.g. < 40

   o  Wideband can have a bit more complexity than narrowband, but
      should still be implementable on a cheap DSP (e.g. < 80 MIPS)

   o  Super-wideband/full-band may require higher complexity, but should
      be implementable on higher-end DSP (e.g. < 200 MIPS), and if
      possible also on cheaper DSPs as well.

   The MIPS values are approximate clock frequencies required for real-
   time encoding+decoding on a DSP capable of single-cycle MAC
   operations (16x16 multiplication accumulated into 32 bits).  Similar
   computational requirements apply to floating-point processors.  For
   example Narrowband encoding and decoding should be possible using 40
   MHz on a modern CPU (e.g. 2% of a 2 GHz x86 CPU).  For applications
   that require mixing (e.g. conferencing), it must be possible to
   estimate the energy of the decoded signal with less than 10% of the
   complexity figures listed above.

   It is the intent to maximize the range of devices on which a codec
   can be implemented.  For this reasons, the reference implementation
   must not depend on special hardware features or instructions to be
   present in order to meet the complexity requirement.  However, it may
   be desirable to take advantage of such hardware when available,
   (e.g., hardware accelerators for operations like FFTs and
   convolutions).  A codec should also minimize the use of saturating
   arithmetic so as to be implementable on architectures that do not
   provide hardware saturation (e.g.  ARMv4).

   The combined codec size and data ROM should be small enough not to
   cause significant implementation problems on typical embedded
   devices.  The codec context/state size required should be no more
   than 2*R*C bytes in floating-point, where R is the sampling rate and
   C is the number of channels.  For fixed-point, that size should be
   less than R*C. The scratch space required should also be less than

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   2*R*C bytes for floating point or less than R*C bytes for fixed-

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5.  Additional considerations

   There are additional features or characteristics that may be
   desirable under some circumstances, but should not be part of the
   strict requirements.  The benefit of meeting these considerations
   should be weighted against the associated cost.

5.1.  Low-complexity audio mixing

   In many applications that require a mixing server (e.g. conferencing,
   games), it is important to minimize the computational cost of the
   mixing.  As much as possible, it should be possible to perform the
   mixing with fewer computations than it would take to decode all the
   streams, mix them, and re-encode the result.  Properties that reduce
   the complexity of the mixing process include:

   o  the ability to derive sufficient parameters, such as loudness
      and/or spectral envelope, for estimating voice activity of a
      compressed frame without fully decoding that frame;

   o  the ability to mix the streams in an intermediate representation
      (e.g. transform domain), rather than having to fully decode the
      signals before the mixing;

   o  the use of bit-stream layers (Section 5.3) by aggregating a small
      number of active streams at lower quality.

   For conferencing applications, the total complexity of the decoding,
   VAD and mixing should be considered when evaluating proposals.

5.2.  Encoder side potential for improvement

   In many codecs, it is possible to improve the quality by improving
   the encoder without breaking compatibility (i.e. without changing the
   decoder).  Potential for improvement varies from one codec to
   another.  It is generally low for PCM or ADPCM codecs and higher for
   perceptual transform codecs.  All things being equal, being able to
   improve a codec after the bit-stream is a desirable property.
   However, this should not be done at the expense of quality in the
   reference encoder.

5.3.  Layered bit-stream

   A layered codec makes it possible to transmit only a certain subset
   of the bits and still obtain a valid bit-stream with a quality that
   is equivalent to the quality that would be obtained from encoding at
   the corresponding rate.  While this is not a necessary feature for
   most applications, it can be desirable for cases where a "mixing

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   server" needs to handle a large number of streams with limited
   computational resources.

5.4.  Partial redundancy

   One possible way of increasing robustness to packet loss is to
   include partial redundancy within packets.  This can be achieved
   either by including the base layer of the previous frame (for a
   layered codec) or by transmitting other parameters from the previous
   frame(s) to assist the PLC algorithm in case of loss.  The ability to
   include partial redundancy for high-loss scenarios is desirable,
   provided that the feature can be dynamically turned on or off (so
   that no bandwidth is wasted in case of loss-free transmission).

5.5.  Stereo support

   It is highly desirable for the codec to have stereo support.  At a
   minimum, the codec should be able to encode two channels
   independently without causing significant stereo image artefacts.  It
   is also desirable for the codec to take advantage of the inter-
   channel redundancy in stereo audio to reduce the bitrate (for an
   equivalent quality) of stereo audio compared to coding channels

5.6.  Bit error robustness

   The vast majority of Internet-based applications do not need to be
   robust to bit errors because packets either arrive unaltered, or do
   not arrive at all.  Considering that, the emphasis should be on
   packet loss robustness and packet loss concealment.  That being said,
   it is often the case that extra robustness to bit errors can be
   achieved at no cost at all (i.e. no increase in size, complexity or
   bit-rate, no decrease in quality or packet loss robustness, ...).  In
   those cases then it is useful to make a change that increases the
   robustness to bit errors.  This can be useful for applications that
   use UDP Lite transmission (e.g. over a wireless LAN).  Robustness to
   packet loss should *never* be sacrificed to achieve higher bit error

5.7.  Time stretching and shortening

   When adaptive jitter buffers are used it is often necessary to
   stretch or shorten the audio signal to allow changes in buffering.
   While this operation can be performed directly on the decoder's
   output, it is often more computationally efficient to stretch or
   shorten the signal directly within the decoder.  It is desirable for
   the reference implementation to provide a time stretching/shortening
   implementation, although it should not be normative.

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5.8.  Input robustness

   The systems providing input to the encoder and receiving output from
   the decoder may be far from ideal in actual use.  Input and output
   audio streams may be corrupted by compounding non-linear artifacts
   from analog hardware and digital processing.  The codecs to be
   developed should be tested to ensure that they degrade gracefully
   under adverse audio input conditions.  Types of digital corruption
   that may be tested include tandeming, transcoding, low-quality
   resampling, and digital clipping.  Types of analog corruption that
   may be tested include microphones with substantial background noise,
   analog clipping, and loudspeaker distortion.  No specific end-to-end
   quality requirements are mandated for use with the proposed codec.
   It is advisable, however, that several typical in-situ environments/
   processing chains be specified for the purpose of benchmarking end-
   to-end quality with the proposed codec.

5.9.  Legacy compatibility

   In order to create the best possible codec for the Internet, there is
   no requirement for compatibility with legacy Internet codecs.

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6.  Security Considerations

   The codec requirements themselves do not have security
   considerations.  However, codec security issues are discussed in
   Section 3.1.

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7.  IANA Considerations

   This document has no actions for IANA.

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8.  Acknowledgments

   The original authors of this document are: Jean-Marc Valin, Slava
   Borilin, Koen Vos, Christopher Montgomery and Raymond (Juin-Hwey)
   Chen.  We would like to thank all the other people who contributed
   directly or indirectly to this document, including Jason Fischl,
   Gregory Maxwell, Alan Duric, Jonathan Christensen, Julian Spittka,
   Michael Knappe, Christian Hoene, and Henry Sinnreich.  We also like
   to thank Cullen Jennings and Gregory Lebovitz for their advice.

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9.  Informative References

   [carot09]  Carot, A., Werner, C., and T. Fischinger, "Towards a
              Comprehensive Cognitive Analysis of Delay-Influenced
              Rhythmical Interaction",  2009.

              Handley, M. and C. Perkins, "Guidelines for Writers of RTP
              Payload Format Specifications", RFC 2736, BCP 36.

   [RTP]      Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for real-time
              applications", RFC 3550.

   [SRTP]     Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
              Norrman, "The Secure Real-time Transport Protocol (SRTP)",
              RFC 3711, March 2004.

              Wright, C., Ballard, L., Coull, S., Monrose, F., and G.
              Masson, "Spot me if you can: Uncovering spoken phrases in
              encrypted VoIP conversations",  2008.

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Authors' Addresses

   Jean-Marc Valin
   Octasic Inc.
   4101, Molson Street
   Montreal, Quebec


   Koen Vos


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