RFC 2431 (rfc2431) - Page 2 of 10

RTP Payload Format for BT

Alternative Format: Original Text Document
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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.






Tynan                       Standards Track
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