AVT                                                            D. Singer
Internet-Draft                                       Apple Computer Inc.
Expires: December 17, 2006                                 June 15, 2006


             Associating SMPTE time-codes with RTP streams
                    draft-ietf-avt-smpte-rtp-03.txt

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document describes a mechanism for associating SMPTE time-codes
   with media streams, in a way that is independent of the RTP payload
   format of the media stream itself.









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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  Design Goals . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Signaling (setup) information  . . . . . . . . . . . . . . . .  7
   5.  Implementation Note  . . . . . . . . . . . . . . . . . . . . .  9
   6.  In-stream information  . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Compact Format of the Time-code  . . . . . . . . . . . . . 10
     6.2.  Full Format of the Time-code . . . . . . . . . . . . . . . 10
     6.3.  Associations in RTCP . . . . . . . . . . . . . . . . . . . 10
     6.4.  Associations in RTP  . . . . . . . . . . . . . . . . . . . 11
   7.  Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   10. RFC Editor Considerations  . . . . . . . . . . . . . . . . . . 16
   Appendix A.  Acknowledgments . . . . . . . . . . . . . . . . . . . 17
   Appendix B.  Change History  . . . . . . . . . . . . . . . . . . . 18
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
   Intellectual Property and Copyright Statements . . . . . . . . . . 20






























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1.  Introduction

   First a brief background on SMPTE time-codes [SMPTE-12M].

   SMPTE time-codes count frames.  There are two common forms of
   display: either a simple counter, or what looks like a normal clock
   value (hh:mm:ss.frame).  When the frame rate is truly integer, then
   this can be a normal clock value, in that seconds tick by at the same
   rate as the seconds we know and love.

   However, NTSC video infamously runs slightly slower than 30 frames/
   second.  Some people call it 29.97 (which isn't quite right) and some
   say that a frame takes 1001 ticks of a 30000 tick/second clock (which
   is closer).  Be that as it may, SMPTE time codes count 30 of these
   frames and deem that to make a second.

   This causes a SMPTE time-code display to 'run slow' compared to real-
   time.  To ameliorate this, sometimes a format called drop-frame is
   used.  Some of the frame numbers are skipped, so that the counter
   periodically 'catches up' (so some time-code-seconds actually only
   have 28 frames in them).

   It is worth noting that in neither case is the SMPTE time-code an
   accurate clock; in the first case, it runs slow, and in the second,
   the adjustments are abrupt and periodic - and still not quite
   accurate.  Hence the rest of this document tries to be clear when
   referring to a second in a time-code as a 'time-code second'.

   However, SMPTE time-codes do run in real-time when used with systems
   with integral frames/second (e.g. film content at 24 frames/second,
   or PAL video).




















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2.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].














































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3.  Design Goals

   What we desire is a system that allows us to associate a SMPTE time-
   code with some media in an RTP [RFC3550] stream.  Since in RTP all
   media has a clock already, we can often leverage that fact.  If we
   treat the media as having 'segments' of time in which the time-code
   is simply counting up, then the time-code anywhere within a segment
   can be calculated if you know:

   o  the RTP timestamp of the start of the segment;

   o  the time-code of the start of the segment;

   o  the counting rate and other parameters of the time-code;

   o  the RTP timestamp where you want to know the time-code.

   There are two cases to consider:

   1.  the time-codes are piece-wise continuous with only occasional
       discontinuities;

   2.  the continuity of the time-codes is not certain (or not known).

   The first can be handled by providing details of the time-code axis
   and an initial mapping from RTP time to time-code time, and periodic
   mappings in RTCP packets.

   The second requires in-band signaling within the RTP packets
   themselves.

   Both cases are covered by this specification.

   There are applications where the transport of all 8 bytes of the
   SMPTE 24M timecode are important (e.g. when the date of the time-code
   must be known, or when the RTP transport is used as a transparent
   pipe).  On the other hand, there are cases (e.g. when timecodes are
   used with compressed audio) when bandwidth is also important.  To
   support both use cases, provision is made for both compact and full
   forms of the time-code.

   Receivers MUST support timecodes in both RTCP and RTP, and both forms
   (compact and full) of the time-code.  Senders, of course, are free to
   choose.

   Note that the compact form allows frame numbers greater than the full
   form (a field of 6 bits vs. a full BCD digit and a 2-bit BCD digit,
   which gives a maximum transmitted value of 29).  In some cases, the



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   color frame flag (bit 11) is used to 'extend' the tens of frames
   field from 2 to 3 bits; however, such practices are outside the scope
   of this specification.
















































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4.  Signaling (setup) information

   If the recipient must ever calculate time-codes based on the RTP
   times, then some setup information is needed.  This is sent out-of-
   band.

   The setup information includes:

   1.  the duration, in the RTP timescale, timescale, of a single frame-
       count in the 'frames' portion of the time-code (frame_duration)

   2.  the number of those frames that make a time-code-second
       (frames_per_tc_second); framecounter values may be between 0 and
       (frames_per_tc_second - 1)

   3.  is-NTSC-drop-frame: should the usual 'left out numbers' of drop-
       frame be applied or not?

   Note that other information we need to do the calculation (e.g. the
   clock rate of the RTP timestamp) is supplied already and assumed to
   be available.

   For example, if associated with a video track with the common time-
   scale of 90000, then frame-duration of 3003 and frames-per-tc-second
   of 30 would yield a 'normal' SMPTE time-code for NTSC video.
   Similarly values of 3750 and 24 yield a time-code for 24 fps film
   content, and so on.

   Note also that we supply explicitly the frame duration and frames/
   second, even though they are obviously closely related.  These
   removes any ambiguity of what the counter values should be in the
   case of drop-frame counting.  These three values MUST correspond with
   each other.

   When SDP is used, these three parameters are transmitted as
   extensionattributes, with the following syntax.  The form of the
   extension attributes is 'owned' by the extension name.  These
   parameters to the extension do not need registration action beyond
   their documentation here.  Note that the parameters are supplied as
   extension attributes, suitable for in-line use in RTP, even if in a
   given stream only the RTCP mapping is used.










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     digit = "0"|"1"|"2"|"3"|"4"|"5"|"6"|"7"|"8"|"9"

     integer = digit *(digit)

     frameduration = integer

     framespersecond = integer

     drop = "/drop-frame" |

     extensionattributes = frameduration "/" framespersecond drop

   If '/drop' is specified, then the first two frame numbers are omitted
   from the count of each minute,, except for minutes 00, 10, 20, 30,
   40, 50, as documented in SMPTE specification [SMPTE-12M] section
   4.2.2.

   The extension name used for the signaling is "org.ietf.avt.smpte12M/
   082005".  This name signals the possible presence of associations in
   RTCP or RTP, as defined below.

   An example in SDP, on a stream with a timescale of 600, might be

     a=extmap:4 org.ietf.avt.smpte12M/082005 20/30



























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5.  Implementation Note

   This section contains a suggestion on how to calculate a time-code
   for a time T2, given an initial code at time T1, and the frame
   duration

   It might seem that when drop-frame is used, there is a 'fence post'
   problem: how many minutes in which frame-numbers are dropped have
   past since the initial time-code?  However, this can be avoided if
   all calculations are 'zero-based'; then the number of 'fence posts'
   is known.

     framesSinceTCzero := TimeCodeToFrameCount( initialTimeCode );
     framesSinceMapping := floor( (T2-T1)/frameDuration );
     totalFrames := framesSinceTCzero + framesSinceMapping;
     timeCode := FrameCountToTimeCode( totalFrames );



































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6.  In-stream information

6.1.  Compact Format of the Time-code

   A compact binary SMPTE time-code in this design occupies 24 bits.  It
   is NOT formatted in the BCD system, but uses binary fixed-width
   fields.  It has the following structure:

   sign(1) -- 1 for negative, 0 for positive

   hours (5 bits) -- 0 to 23; the values 24-31 are reserved

   minutes (6 bits) -- 0 to 59; 60-63 are reserved

   seconds (6 bits) -- 0 to 59; 60-63 are reserved

   frames(6 bits) -- 0 to (frames-per-tc-second - 1)

   Note that these fields are larger than the provision in SMPTE 12M
   where binary-coded decimal is used (and notably, where only two bits
   are provided for the tens digit of the frame count, so frame numbers
   above 39 cannot be represented).

6.2.  Full Format of the Time-code

   A full binary SMPTE time-code occupies 64 bits.  It is formatted
   exactly as in SMPTE 12M [SMPTE-12M].

6.3.  Associations in RTCP

   When the time-codes are piece-wise continuous, we then supply in RTCP
   packets an RTP timestamp and an SMPTE time-code, for the start of
   each run of calculable time-codes.  This establishes the time-code
   for all RTP times greater than or equal to the one given, until a
   subsequent RTCP packet reestablishes the mapping.

   Note that the RTP time-stamp in the RTCP mapping may not match the
   time-stamp of any frame in the media stream.  For video, it normally
   would; but a time-stamp transition may happen part-way through a
   decoded audio frame.  Since they share the same clock, the timing of
   that transition and the timing of the audio stream itself have the
   same accuracy.

   The association is a new RTCP Control Packet Type, using the value
   194 (to be registered).  This control packet has one of the two
   following forms, differentiated by its length:





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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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|    SC   |PT=SMPTETC=194 |             length=3          |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |                     SSRC of packet sender                     |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |                         RTP timestamp                         |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |S|  hours  |  minutes  |  seconds  |  frames   |  reserved=0   |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+


   The fields S (sign), hours, minutes, seconds, and frames, are defined
   above.

   For this short form, the length takes the fixed value 3, indicating a
   control packet of 4 32-bit words.


       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
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |V=2|P|    SC   |PT=SMPTETC=194 |             length=4          |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |                     SSRC of packet sender                     |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |                         RTP timestamp                         |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
      |                          Full 8-byte                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      SMPTE 12M timecode                       |
      +=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+


   For this full time-code, long form, the length takes the fixed value
   4, indicating a control packet of 5 32-bit words.

6.4.  Associations in RTP

   When the time-codes are not known to be piece-wise continuous, or
   absolute surety of mapping is desired, then the mapping can be placed
   into some or all of the RTP packets.  This is a less desirable route;
   it uses the RTP header extension, which some terminals may find
   problematic.  And clearly placing mapping information in every packet
   uses more bandwidth.

   In as many RTP packets as needed (possibly all), a named header



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   extension is used to associate an RTP time to a SMPTE time-code.
   (See related specification of named header extensions for RTP).

   There are two forms of this header extension, again differentiated by
   their length.  The short form associates a compact time-code with the
   RTP timestamp of the packet.  The long form allows associates a full
   time-code with a timestamp offset from the RTP timestamp of the
   packet.

   The short form has a length of 3 bytes (24 bits).  The long form has
   a length of 12 bytes (96 bits), and consists of a full SMPTE 12M
   time-code, followed by a signed 32-bit offset D from the RTP
   timestamp.  If the packet has timestamp T, this establishes an RTP to
   time-code association for the RTP time T+D.





































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7.  Discussion

   This design has the advantage of not requiring the introduction of
   new IP packets into the sessions or new data into the main data
   channel, using low-bandwidth (vanishingly low in the case of streams
   with no discontinuities), and is independent of the design of the RTP
   packets themselves: the RTP profile (including possibly encryption)
   and the RTP payload format.  SMPTE time-codes can be associated with
   any RTP stream, including those with existing payload formats.

   It might be argued that we could set the initial mapping also in the
   SDP, since RTCP packets might get lost.  But this means that the SDP
   now has to have knowledge of the RTP random offset, which is nasty;
   and if one puts this APP packet into all sender reports, it's
   probably good enough.  Then if you don't have time-codes, you don't
   have audio-video-sync either.

   This associates the time-code with a particular media stream.  An
   alternative would be to make it an RTP stream in its own right; but
   the data rate is so low, this seems egregious.  And by packing it
   inline, we can do this backwards-compatible for gateways etc. that
   already handle dual-stream.

   The RTCP packets (or the in-band codes) need not use the same RTP
   timestamp as the sender report (or transmission time) in the same
   RTCP packet.  They can be sent 'ahead of need' if possible (e.g. for
   stored content, when the server can look-ahead) or just-in-time -
   send an RTCP immediately a discontinuity in the time-code is
   detected, and allow media-buffering in the client the chance to
   'catch' the RTCP before the matching RTP packet is processed and
   displayed.

   There is no way in this draft to detect that an RTCP packet has been
   lost, and that a mapping may be being used outside its intended
   range.

   The design assumes that clients will hold mappings until they are
   superseded, and that a client may need to buffer some number of
   upcoming mappings.












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8.  Security Considerations

   SMPTE time-codes are only informative and it is hard to see security
   considerations from associating them with media streams.















































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9.  IANA Considerations

   The RTCP packet type used for SMPTE time-code needs to be registered.
   The abbreviation is "SMPTETC", the full name is "SMPTE time-code
   mapping", the suggested value is 194, and the specification is this
   document.

   The name used for the EXTMAP attribute needs to be registered.  This
   names is "org.ietf.avt.smpte12M/082005", and it should be associated
   with this document.









































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10.  RFC Editor Considerations

   None.
















































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Appendix A.  Acknowledgments

   Both Brian Link and John Lazzaro provided helpful comments on an
   initial draft.  Colin Perkins was helpful in reviewing and dealing
   with the details.














































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Appendix B.  Change History

   o  August 2005: 00 Draft-avt-smpte-rtp made from
      draft-singer-smpte-rtp; added question on full time-code option

   o  January 2006: 01 Updated to XML2RFC; inserted BNF, documented SDP
      usage, changed the name to org.ietf..., other cleanup

   o  June 2006: 02 Allowed both compact and full time-codes in both
      RTCP and inline

   o  June 2006: 03 Minor clarifications, provided signalling example,
      clarified that the same signalling is used for both RTCP and/or
      RTP use


11.  References

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", RFC 3550, STD 0064, July 2003.

   [SMPTE-12M]
              Society of Motion Picture and Television Engineers,
              "Television, Audio and Film - Time and Control Code",
              SMPTE 12M-1999.






















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Author's Address

   David Singer
   Apple Computer Inc.
   1 Infinite Loop
   Cupertino, CA  95014
   US

   Phone: +1 408 996 1010
   Email: singer@apple.com









































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