RTP Payload Format for BT.656 Video Encoding
RFC 2431

Document Type RFC - Proposed Standard (October 1998; No errata)
Author Dermot Tynan 
Last updated 2013-03-02
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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.


   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",
   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
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