Internet Engineering Task Force INTERNET-DRAFT D. Singer draft-singer-smpte-rtp-00 Apple Computer Feb 13 2005 Expires: Aug 13 2005 Associating SMPTE time-codes with RTP streams IPR Notice By submitting this Internet-Draft, I certify that any applicable patent or other IPR claims of which I am aware have been disclosed, or will be disclosed, and any of which I become aware will be disclosed, in accordance with RFC 3668. Status of This Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than a "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html 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 RFC 2119. Distribution of this document is unlimited. Copyright Notice Copyright (C) The Internet Society (2005). All Rights Reserved. D. Singer [Page 1] Internet Draft draft-singer-smpte-rtp-00.doc Feb 13 2005 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. 1 Introduction First a brief background on SMPTE time-codes [SMPTE]. 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 or 29 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 in the rest of this document I try 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). The 'drift' issue is (I believe) unique to NTSC video. 2 Design Goals What we desire is a system that allows us to associate a SMPTE time- code with some media in an RTP [RTP] stream. Since in RTP all media has a clock already, we can 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: D. Singer [Page 2] Internet Draft draft-singer-smpte-rtp-00.doc Feb 13 2005 1. the RTP timestamp of the start of the segment; 2. the time-code of the start of the segment; 3. the counting rate and other parameters of the time-code; 4. the RTP timestamp where you want to know the time-code. My proposal is that we put periodic mappings between (1) and (2) into RTCP packets, and provide (3) the 'setup' information out-of-band, for example in SDP. Then given a timestamp (4), we can calculate exactly what the time-code must be. The setup information includes: (the timescale of the RTP stream, already provided); the duration, in that timescale, of a single frame-count in the 'frames' portion of the time-code the number of those frames that make a time-code-second the following booleans: is-NTSC-drop-frame: should the usual 'left out numbers' of drop-frame be applied or not? wrap-at-24-hours: should the hours portion wrap from 23 to 0, or keep counting up? allow-negative-time-codes: are negative time-codes used in this stream? display-time-code-as-counter: should the display be an integer frame-count, or hh:mm:ss.fr format? time-code-displayed: is it intended that this time-code be displayed somehow? 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. Now, we put into an RTCP APP packet (or a new RTCP packet), a mapping between an RTP time-stamp value and the time-code. The RTP timestamp and the that time-code are 32 bits; the time-code is either a signed counter value (if we're in counter format), or it is the format: hours(8) -- 0 to 255 sign(1) -- 1 for negative, 0 for positive minutes(7) -- 0 to 59 seconds(8) -- 0 to 59 frames(8) -- 0 to (frames-per-tc-second - 1) This establishes the time-code for all RTP times greater than or equal to the one given, until a subsequent APP packet reestablishes the mapping. It's unfortunate that the sign is in the middle, but that allows the hours to use the full range, and the minutes don't need to. D. Singer [Page 3] Internet Draft draft-singer-smpte-rtp-00.doc Feb 13 2005 Note that the RTP time-stamp in the 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. 3 Discussion This design has the advantage of introducing no new IP packets into the sessions, 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 APP packets need not use the same RTP timestamp as the sender report 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, If time-codes change more rapidly than RTCP packets are normally sent, then this might force a more rapid transmission. However, it should be possible to send multiple mappings in one RTCP packet. 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 likelihood of this happening can be reduced, however, by permitting a pair of RTP times in the mapping, and defining that the mapping is only valid between those times. This only works for stored media, when look-ahead is possible, of course. D. Singer [Page 4] Internet Draft draft-singer-smpte-rtp-00.doc Feb 13 2005 This design has the advantage of being independent of both the RTP profile and the media-specific RTP payload design. However, it does this at the expense of certainty. It is also possible to imagine some payloads with explicit provision for this mapping. However, such provision need not go as far as a full SMPTE time-code in every packet. If the RTCP APP packets have a small 'time-code epoch' indicator, then that same epoch indicator can be in each packet. The current design assumes that clients will hold mappings until they are superseded, and that a client may need to buffer some number of upcoming mappings. It may be necessary to introduce explicit statements about the amount of buffering needed. For trick modes, it may be desirable to signal that a given section of media has the time-code running in reverse; this would require a new sign bit in the mapping record. 4 Security Considerations SMPTE time-codes are only informative and it is hard to see security considerations from associating them with media streams. 5 IANA Considerations None. 6 RFC Editor Considerations None. 7 Full Copyright Statement Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. This document and the information contained herein are provided on an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. 8 Intellectual Property Notice The IETF takes no position regarding the validity or scope of any intellectual property or other rights that might be claimed to D. Singer [Page 5] Internet Draft draft-singer-smpte-rtp-00.doc Feb 13 2005 pertain to the implementation or use of the technology described in this document or the extent to which any license under such rights might or might not be available; neither does it represent that it has made any effort to identify any such rights. Information on the IETF's procedures with respect to rights in standards-track and standards-related documentation can be found in BCP-11. Copies of claims of rights made available for publication and any assurances of licenses to be made available, or the result of an attempt made to obtain a general license or permission for the use of such proprietary rights by implementers or users of this specification can be obtained from the IETF Secretariat. The IETF invites any interested party to bring to its attention any copyrights, patents or patent applications, or other proprietary rights which may cover technology that may be required to practice this standard. Please address the information to the IETF Executive Director. Acknowledgments Both Brian Link and John Lazzaro provided helpful comments on an initial draft (not all of which are yet addressed). D. Singer [Page 6] Internet Draft draft-singer-smpte-rtp-00.doc Feb 13 2005 Authors' Contact Information David Singer Apple Computer, Inc. One Infinite Loop, MS:302-3MT Cupertino CA 95014 USA Email: singer@apple.com Tel: +1 408 974 3162 6. References [RTP] RFC3550, STD0064, RTP: A Transport Protocol for Real-Time Applications, H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, July 2003 [SMPTE-12M] SMPTE 12M-1999, Television, Audio and Film - Time and Control Code Dates Written: Feb 13 2005 Expires: Aug 13 2005 D. Singer [Page 7]