INTERNET-DRAFT                                              Ladan Gharai
<draft-ietf-avt-smpte292-video-00.txt>                           USC/ISI
                                                            Gary Goncher
                                                        David Richardson
                                                University of Washington
                                                          Allison Mankin
                                                           July 13, 2000

                     RTP Payload Format for SMPTE 292M

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
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This document specifies a packetization scheme for encapsulating
uncompressed HDTV as defined by SMPTE 292M [1] into a payload format for
the Real-Time Transport Protocol (RTP).  The RTP packet counter is
extended to 26 bits to accommodate SMPTE 292M's 1.485Gb/s data rate, and
additional positioning information is added to the payload header.

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1.  Introduction

The serial digital interface, SMPTE 292M, defines a universal medium of
interchange for uncompressed HDTV between various types of video
equipment (camera's, encoders, VTRs, ...) at data rates of 1.485Gb/s
(and 1.485/1.001 Gb/s).  Source data are 10-bit words, sampled at 4:2:2.
In this draft we specify how to transfer SMPTE 292M over RTP.

2.  A Note on Compressed HDTV over RTP

HDTV is compressed using a subset of MPEG-2 [3] as its compression
scheme.   This subset is fully described in document A/53 [4] of the
Advanced Television Standards Committee. The ATSC has also adopted the
MPEG-2 transport system (ISO/IEC 13818-1) [5]. Therefore:

1. The HDTV transport system is a compatible subset of the MPEG-2
transport system. Section 2 of RFC 2250 [7] describes the RTP payload
for  MPEG-2's transport system, where multiple fixed length (188 bytes)
MTS packets are aggregated into a single RTP packet.

2. Compressed HDTV is a subset of MPEG-2 MP@HL with some additional
restrictions. Section 3 of RFC 2250 describes a packetization scheme for
MPEG-2 elementary streams. The additional restrictions of HDTV do not
have any implications for RTP packetization.

3.  Payload Design

Each video frame of SMPTE292M in packetized into a number of variable
size RTP packets. All active, vertical blanking and timing information
is packetized. The end of a frame is marked by the M bit in the RTP
header.   A single packet may contain data for two consecutive scan
lines. The SMPTE292M decoder uses the sync info in the scan lines to
detect the start of scan lines.

A single packet may also contain information from adjacent scan lines in
two consecutive frames, or by agreement between sender and receiver the
last packet of a video frame may be padded and the the new frame start
in a new packet.

The standard 16 bit RTP sequence counter is extended to 26 bits to
accommodate HDTV's high data rates. At 1.485Gb/s, with packet sizes of

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at least 1k, 26bits allows for 5minute period before the sequence
counter wraps around.

The payload header includes the offset of the payload data in the video
frame. The offset is for 20-bit video words and accounts for active and
inactive samples.

Given SMPTE292M's 4:2:2 color subsampling, scan line fragmentation must
occur on sample-pair boundaries, such that Y and Cb and Cr values are
not split across packets.

4.  RTP Packetization

The standard RTP header is followed by a 4 byte payload header, and the
payload data.

    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 |P|X|   CC  |M|    PT       |     sequence# (low bits)      |
   |                     time stamp                                |
   |                                                               |
   |squence#(high bits)|      offset in frame                      |

4.1.  The RTP Header

The following fields of the RTP fixed header are used for SMPTE 292M

Payload Type (PT): 7bits
     A dynamically allocated  payload type field which designates the
     payload as SMPTE 292M.

Timestamp: 32 bits
     The timestamp field shall be defined from a counter at 10 MHz. The
     timestamp shall be defined as the arrival time of the first 20-bit
     video sample to be transmitted in the current packet. At an arrival
     rate of 74.25 MHz for 20-bit 292M video samples with 24/30/60 Hz
     frame rates, the timestamp will be unique for packets with more

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     than 8 video samples (20 bytes). Timestamps shall increase
     monotonically until they roll over at 32 bits.

     The 10 MHz timestamp clock may be obtained from a GPS (Global
     Positioning System) board. These boards have a disciplined
     oscillator that is synchronized to GPS time. The disciplined
     oscillator can be as accurate as 1 in 10-12, but is more typically
     1 in 10-8. Thus clocks at widely separate locations can be
     synchronized with an accuracy of 100 ns for video timing recovery.

Marker bit (M): 1bit
     The Marker bit denotes the end of a video frame, and is set to 1
     for the last packet of the video frame and is otherwise  set to 0
     for all other packets.

Sequence Number (low bits): 16 bits
     The low order bits for RTP sequence counter. The standard 16 bit
     RTP  sequence number is augmented by 10 bits in the payload header
     in order to accommodate the 1.485Gb/s data rate of SMPTE292M.

4.2.  Payload Header

Sequence Number (high bits):  10bits
    The high order bits for the 26bit RTP sequence counter.

Offset in Frame: 22bits
     An offset for the position of 20-bit video words in the video
     frame. The offset includes all information in the video frame and
     scan lines. Range of values are:
       1.  0 to 1125*2750-1 for  1080/24p.
       2.  0 to 1125*2200-1 for  1080/30i.
       3.  0 to  750*1650-1 for  720/60p.

5.  Payload Format

For 4:2:2 color subsampling Cb and Cr values are subsampled by a factor
of two horizontally and are co-sited with even numbered Y samples.
Therefore, Cb, Cr and Y samples must be arranged and transmitted in the
following order:
                        Cb, Y, Cr, Y, Cb, Y, Cr, ...
where  the first Cb, Y, Cr  sequence refers to co-sited luminance and
color-difference samples, and the next Y belongs to the next luminance

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Therefore, as set forth in RFC2431, for 10-bit words, each group of four
samples must be encoded into a 40-bit word (five octets) prior to
transmission.  The following is a representation of a 720 sample packet
with 10-bit quantization:

               0         1         2         3
               0 2 4 6 8 0 2 4 6 8 0 2 4 6 8 0 2 4 6 8
              |   Cb0   |   Y0    |   Cr0   |   Y1    |
              |   Cb1   |   Y2    |   Cr1   |   Y3    |
              |  Cb359  |  Y718   |  Cr359  |  Y719   |
                (Note that the word width is 40 bits)
      Octets: |   0   |   1   |   2   |   3   |   4   |

The octets shown in these diagrams are transmitted in network bit and
byte order, that is, left-to-right as shown.

6.  Security Considerations

RTP packets using the payload format defined in this specification are
subject to the security considerations discussed in the RTP
specification [4], and any appropriate RTP profile.  This implies that
confidentiality of the media streams is achieved by encryption.  Because
the data compression used with this payload format is applied end-to-
end, encryption may be performed after compression so there is no
conflict between the two operations.

This payload type does not exhibit any significant non-uniformity in the
receiver side computational complexity for packet processing to cause a
potential denial-of-service threat.

It is perhaps to be noted that the bandwidth of this payload is high
enough (1.5 Gbps without the RTP overhead) to cause potential for
denial-of-service if transmitted onto most currently available Internet
paths.  In the absence from the standards track of a suitable congestion
control mechanism for flows of this sort, use of the payload should be
narrowly limited to suitably connected network endpoints and great care

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taken with the scope of multicast transmissions.  This potential threat
is common to all high bit rate applications.

7.  IANA Considerations

[To be done]

8.  Full Copyright Statement

Copyright (C) The Internet Society (1999). All Rights Reserved.

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it or
assist in its implementation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are included
on all such copies and derivative works.

However, this document itself may not be modified in any way, such as by
removing the copyright notice or references to the Internet Soci- ety or
other Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be fol- lowed,
or as required to translate it into languages other than English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an "AS

9.  Authors' Addresses

 Ladan Gharai

 Gary Goncher

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INTERNET-DRAFT                                             July 13, 2000

 Allison Mankin

 David Richardson

10.  Bibliography

[1] Society of Motion Picture and Television Engineers,
    Bit-Serial Digital Interface for High-Definition Television
    Systems, SMPTE292M, 1998.

[2] Society of Motion Picture and Television Engineers,
    1280*720 Scanning, Analog and Digital Representation and Analog
    Interfaces, SMPTE 296M, 1998.

[3] ISO/IEC International Standard 13818-2; "Generic coding of
    moving pictures and associated audio information: Video", 1996.

[4] ATSC Digital Television Standard Document A/53, September 1995,

[5] ISO/IEC International Standard 13818-1; "Generic coding of
    moving pictures and associated audio information: Systems",1996.

[6] Schulzrinne, Casner, Frederick, Jacobson, "RTP: A transport
    protocol for real time Applications", RFC 1889, IETF,
    January 1996.

[7] Hoffman, Fernando, Goyal, Civanlar, "RTP Payload Format for
    MPEG1/MPEG2 Video", RFC 2250, IETF, January 1998.

[8] Schulzrinne, "RTP Profile for Audio and Video Conferences with
    Minimal Control", RFC 1890, IETF, January 1996.

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