rfc2431.txt

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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.






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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.






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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.




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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.



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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]