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rfc:rfc2431

Network Working Group D. Tynan Request for Comments: 2431 Claddagh Films Category: Standards Track October 1998

            RTP Payload Format for BT.656 Video Encoding

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (1998).  All Rights Reserved.

Abstract

 This document specifies the RTP payload format for encapsulating ITU
 Recommendation BT.656-3 video streams in the Real-Time Transport
 Protocol (RTP).  Each RTP packet contains all or a portion of one
 scan line as defined by ITU Recommendation BT.601-5, and includes
 fragmentation, decoding and positioning information.

1. Introduction

 This document describes a scheme to packetize uncompressed, studio-
 quality video streams as defined by BT.656 for transport using RTP
 [1].  A BT.656 video stream is defined by ITU-R Recommendation
 BT.656-3 [2], as a means of interconnecting digital television
 equipment operating on the 525-line or 625-line standards, and
 complying with the 4:2:2 encoding parameters as defined in ITU-R
 Recommendation BT.601-5 (formerly CCIR-601) [3], Part A.
 RTP is defined by the Internet Engineering Task Force (IETF) to
 provide end-to-end network transport functions suitable for
 applications transmitting real-time data over multicast or unicast
 network services.  The complete specification of RTP for a particular
 application requires the RTP protocol document [1], a profile
 specification document [4], and a payload format specification.  This
 document is intended to serve as the payload format specification for
 studio-quality video streams.

Tynan Standards Track [Page 1] RFC 2431 RTP Payload Format for BT.656 October 1998

 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 [5].

2. Definitions

 For the purposes of this document, the following definitions apply:
 Y: An 8-bit or 10-bit coded "luminance" sample.  Luminance in this
 context refers to the BT.601-5 [3] definition which is not the same
 as a true CIE luminance value.  The value of "luminance" refers
 specifically to video luma. However, in order to avoid confusion with
 the BT.656 and BT.601 standards, the video luma value is referenced
 in this document as luminance.  Each value has 220 quantization
 levels with the black level corresponding to level 16 and the peak
 white level corresponding to 235.
 Cb, Cr: An 8-bit or 10-bit coded color-difference sample (as per
 BT.601-5).  Each color-difference value has 225 quantization levels
 in the centre part of the quantization scale with a color-difference
 of zero having an encoded value of 128.
 True Black: BT.601-5 defines a true black level as the quad-sample
 sequence 0x80, 0x10, 0x80, 0x10, representing color-difference values
 of 128 (0x80) and a luminance value of 16 (0x10).
 SAV, EAV: Video timing reference codes which appear at the start and
 end of a BT.656 scan line.

3. Payload Design

 ITU Recommendation BT.656-3 defines a schema for the digital
 interconnection of television video signals in conjunction with
 BT.601-5 which defines the digital representation of the original
 analog signal.  While BT.601-5 refers to images with or without color
 subsampling, the interconnection standard (BT.656-3) specifically
 requires 4:2:2 subsampling. This specification also requires 4:2:2
 subsampling such that the luminance stream occupies twice the
 bandwidth of each of the two color-difference streams.  For normal
 4:3 aspect ratio images, this results in 720 luminance samples per
 scan line, and 360 samples of each of the two chrominance channels.
 The total number of samples per scan line in this case is 1440.
 While this payload format specification can accomodate various image
 sizes and frame rates, only those in accordance with BT.601-5 are
 currently supported.

Tynan Standards Track [Page 2] RFC 2431 RTP Payload Format for BT.656 October 1998

 Due to the lack of any form of video compression within the payload
 and sampling-rate compliance with BT.601-5, the resultant video
 stream can be considered "studio quality".  However, such a stream
 can require approximately 20 megabytes per second of network
 bandwidth.  In order to maximize packet size within a given MTU, and
 to optimize scan line decoding, each video scan line is encoded
 within one or more RTP packets.
 To allow for scan line synchronization, each packet includes certain
 flag bits (as defined in BT.656-3) and a unique scan line number.
 The SAV and EAV timing reference codes are removed. Furthermore, no
 line blanking samples are included, so no ancillary data can be
 included in the line blanking period.  It is the responsibility of
 the receiver to generate the timing reference codes, and to insert
 the correct number of line blanking samples.
 Similarly, there is no requirement that the frame blanking samples be
 provided.  However, it is possible to include frame blanking samples
 if such samples contain relevant information, such as a vertical-
 interlace time code (VITC), or teletext data.  In the absence of
 frame blanking samples, the receiver MUST generate true black levels
 as defined above, to complete the correct number of scan lines per
 field.  If frame blanking samples are provided, they MUST be copied
 without modification into the resultant BT.656-3 stream.
 Scan lines MUST be sent in sequential order.  Error concealment for
 missing scan lines or fragments of scan lines is at the discretion of
 the receiver.
 Both 8-bit and 10-bit quantization types as defined by BT.601-5 are
 supported.  10-bit samples are considered to have two extra bits of
 fixed-point precision such that a binary value of 10111110.11
 represents a sample value of 190.75.  Using 8-bit quantization, this
 would give a sample value of 190.  An application receiving 8-bit
 samples for a 10-bit device MUST consider the sample as reflecting
 the most-significant 8 bits.  The two least-significant bits SHOULD
 be set to zero.  Similarly, an application sending 8-bit samples from
 a 10-bit device MUST drop the two least-significant bits.  For a 10-
 bit quantization payload, each pair of samples MUST be encoded into a
 40-bit word (five octets) prior to transmission, as specified in
 Section 6.
 To allow for scan lines with octet lengths larger than the path
 maximum transmission unit (MTU), a scan offset field is included in
 the packet header.  Applications SHOULD attempt path MTU discovery
 [6] and fragment scan lines into multiple packets no larger than the
 MTU.

Tynan Standards Track [Page 3] RFC 2431 RTP Payload Format for BT.656 October 1998

 Fragmentation MUST occur on a sample-pair boundary, such that the
 chrominance and luminance values are not split across packets.  For
 8-bit quantization this gives a four-octet alignment, and a five-
 octet alignment for 10-bit quantization.  As a result, the scan
 offset refers not to the byte offset within the payload, but the
 sample-pair offset.

4. Usage of RTP

 Due to the unreliable nature of the RTP protocol, and the lack of an
 orderly delivery mechanism, each packet contains enough information
 to form a single scan line without reference to prior scan lines or
 prior frames.  In addition to the RTP header, a fixed length payload
 header is included in each packet.  This header is four octets in
 length.
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                           RTP Header                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Payload Header                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                          Payload Data                         |
    |                                .                              |
    |                                .                              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

4.1. RTP Header usage

 Each RTP packet starts with a fixed RTP header.  The following fields
 of the RTP fixed header are used for BT.656-3 encapsulation:
 Marker bit (M): The Marker bit of the RTP header is set to 1 for the
 last packet of a frame (or the last fragment of the last scan line if
 fragmented), and set to 0 on all other packets.
 Payload Type (PT): The Payload Type indicates the use of the payload
 format defined in this document.  A profile MAY assign a payload type
 value for this format either statically or dynamically as described
 in RFC 1890 [4].
 Timestamp: The RTP Timestamp encodes the sampling instant of the
 video frame currently being rendered.  All scan line packets within
 the same frame will have the same timestamp.  The timestamp SHOULD
 refer to the 'Ov' field synchronization point of the first field.
 For the payload format defined by this document, the RTP timestamp is
 based on a 90kHz clock.

Tynan Standards Track [Page 4] RFC 2431 RTP Payload Format for BT.656 October 1998

5. Payload Header

 The payload header is a fixed four-octet header broken down as
 follows:
     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |F|V| Type  |P| Z |     Scan Line (SL)    |  Scan Offset (SO)   |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 F: 1 bit
 When 0, indicates the first field of a frame (line 4 through 265
 inclusive for Type=0 or 2, and line 1 through 312 inclusive for Type=1
 or 3).  Any other scan line is considered a component of the second
 field, and the F bit will be set to 1.  This bit is copied directly
 from the BT.656-compliant stream by the transmitter, and inserted into
 the stream by the receiver.
 V: 1 bit
 When 1, indicates that the scan line is part of the vertical interval.
 Should always be 0 unless frame blanking data is sent.  In which case,
 the V bit SHOULD be set to 1 for scan lines which do not form an
 integral part of the image. This bit is copied directly from the
 BT.656-compliant stream by the transmitter, and inserted into the
 stream by the receiver.  For receivers which do not receive scan lines
 during the vertical interval, BT.656 vertical interval data MUST be
 generated by repeating the quad-sample sequence 0x80, 0x10, 0x80,
 0x10, representing a true black level.
 Type: 4 bits
 This field indicates the type of frame encoding within the payload.
 It MUST remain unchanged for all scan lines within the same frame.
 Currently only four types of encoding are defined.  These are as
 follows;
    0 - NTSC (13.5MHz sample rate; 720 samples per line; 60 fields
        per second; 525 lines per frame)
    1 - PAL (13.5MHz sample rate; 720 samples per line; 50 fields
        per second; 625 lines per frame)
    2 - High Definition NTSC (18MHz sample rate; 1144 samples per
        line; 60 fields per second; 525 lines per frame)
    3 - High Definition PAL (18MHz sample rate; 1152 samples per
        line; 50 fields per second; 625 lines per frame)

Tynan Standards Track [Page 5] RFC 2431 RTP Payload Format for BT.656 October 1998

 Further encodings can only be defined through the Internet Assigned
 Numbers Authority (IANA).  For more information refer to Section 8,
 "IANA Considerations".
 P: 1 bit
 Indicates the required sample quantization size.  When 0, the payload
 is comprised of 8-bit samples.  Otherwise, it carries 10-bit samples.
 This bit MUST remain unchanged for all scan lines within the same
 frame.
 Z: 2 bits
 Reserved for future use.  Must be set to zero by the transmitter and
 ignored by the receiver.
 Scan Line (SL): 12 bits
 Indicates the scan line encapsulated in the payload.  Valid values
 range from 1 through 625 inclusive. If no frame blanking data is
 being transmitted, only scan lines 23 through 310 inclusive, and
 lines 336 through 623 inclusive SHOULD be sent in the case of Type=1
 or 3.  For 525/60 encoding (Type=0 or 2), scan lines 10 through 263
 inclusive and lines 273 through 525 SHOULD be transmitted.
 If a receiver is generating a BT.656-3 data stream directly from this
 packet, the F and V bits MUST be copied from the header rather than
 being generated implicitly from the scan line number.  In the event
 of a conflict, the F and V bits have precedence.
 Scan Offset (SO): 11 bits
 Indicates the offset within the scan line for application-level
 fragmentation.  After doing PMTU discovery, if the path MTU is less
 than the required size for one complete scan line, the data SHOULD be
 fragmented such that a given RTP packet does not exceed the allowable
 MTU.  The offset for the first packet of a scan line MUST be set to
 zero.  The scan offset refers to the sample-pair offset within the
 scan such that for a scan line width of 720, the maximum scan offset
 is 359.

6. Payload Format

 In keeping with the 4:2:2 color subsampling of BT.656 and BT.601,
 each pair of color-difference samples will be intermixed with two
 luminance samples.  As per BT.656, the format for transmission SHALL
 be Cb, Y, Cr, Y.  The following is a representation of a 720 sample
 packet with 8-bit quantization:

Tynan Standards Track [Page 6] RFC 2431 RTP Payload Format for BT.656 October 1998

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Cb0      |      Y0       |      Cr0      |      Y1       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |      Cb1      |      Y2       |      Cr1      |      Y3       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    .
                                    .
                                    .
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Cb359     |     Y718      |     Cr359     |     Y719      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 1144 and 1152 sample packets SHOULD increase the packet size
 accordingly while maintaining the sample order.
 For 10-bit quantization, each group of four samples MUST be encoded
 into a 40-bit word (five octets) prior to transmission.  The sample
 order is identical to that for 8-bit quantization.  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 byte
 order, that is, left-to-right as shown.

Tynan Standards Track [Page 7] RFC 2431 RTP Payload Format for BT.656 October 1998

7. Security Considerations

 RTP packets using the payload format defined in this specification
 are subject to the security considerations discussed in the RTP
 specification [1].  This implies that confidentiality of the media
 streams is achieved by encryption.  Because the payload format is
 arranged end-to-end, encryption MAY be performed after encapsulation
 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.

8. IANA Considerations

 The four encoding types defined by this document relate to specific
 schema defined by ITU-R Recommendation BT.656-3.  Future revisions of
 the recommendation may create further encoding types which need to be
 supported over RTP. The "Type" field is four bits wide allowing for a
 total of up to sixteen possible encodings, with twelve currently
 reserved for future use.  Due to the small number of possible
 encodings and given that it is very unlikely that future revisions of
 BT.656 will introduce any new schema, requests to extend the Type
 field MUST be vetted by the Internet Assigned Numbers Authority.
 Furthermore, implementors SHOULD check the IANA repository for new
 definitions of the Type field in order to comply with this document.
 Applications for a new Type value MUST be submitted to the IANA and
 include the requestors name and contact information, the reason for
 requesting a new Type and references to appropriate standards, such
 as an updated version of ITU-R Recommendation BT.656.  Furthermore,
 in the unlikely event that the new Type will lessen the security of a
 compliant implementation, such security risk MUST be detailed in the
 application.  The application will be reviewed by a Designated Expert
 and if appropriate, a new Type will be assigned.  This type will be
 listed in the IANA repository for future implementations.

Tynan Standards Track [Page 8] RFC 2431 RTP Payload Format for BT.656 October 1998

9. References

 [1]   Schulzrinne, H., Casner, S., Frederick, R., and V. Jacobson,
       "RTP: A Transport Protocol for Real-Time Applications", RFC
       1889, January 1996.
 [2]   Interfaces for Digital Component Video Signals in 525-Line and
       625-Line Television Systems operating at the 4:2:2 Level of
       Recommendation ITU-R BT.601 (Part A), ITU-R Recommendation
       BT.656-3, 1995.
 [3]   Studio Encoding Parameters of Digital Television for Standard
       4:3 and Wide-Screen 16:9 Aspect Ratios, ITU-R Recommendation
       BT.601-5, 1995.
 [4]   Schulzrinne, H., "RTP Profile for Audio and Video Conference
       with Minimal Control", RFC 1890, January 1996.
 [5]   Bradner, S., "Key words for use in RFCs to Indicate Requirement
       Levels", BCP 14, RFC 2119, March 1997.
 [6]   Mogul, J., and S. Deering, "Path MTU Discovery", RFC 1191,
       November 1990.

10. Author's Address

 Dermot Tynan
 Claddagh Films Limited
 3 White Oaks
 Clybaun Road
 Galway
 Ireland
 EMail: dtynan@claddagh.ie
 Phone: +353 91 529944

Tynan Standards Track [Page 9] RFC 2431 RTP Payload Format for BT.656 October 1998

11. Full Copyright Statement

 Copyright (C) The Internet Society (1998).  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 Society 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
 followed, 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 IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS 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.

Tynan Standards Track [Page 10]

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