Understanding HD

Understanding HD

There are several aspects to high definition video (HD). There are multiple possible frame rates, frame sizes, in both interlace and progressive formats. In addition, its become industry "practice" to use general terminology that's less than precise. This and the next couple articles are devoted to demystifying basic aspects of HD including:

• The basic standards and variations.
• What frame rates we should shoot for various applications.
• How terminology is misused and what people "really" mean.

What Is High Definition Video?

The term high definition today refers to formats that have more resolution than standard definition (SD) video. In this context, when we discuss resolution its meant to describe how many scan lines (horizontal rows of picture information) make up the video image. As we looked at in an earlier section, the two main SD variations, NTSC and PAL, use 480 lines and 576 lines (respectively).

HD video today:

• There are two resolutions: 1080 or 720 scan lines.
• All HD video is widescreen using an aspect ratio of 16x9, also expressed as 1.78:1.
• Video is scanned either progressively or interlaced.
• There are multiple possible frame rates: 23.98, 24, 25, 29.97, 30, 59.94 and 60.

 

Specific Standards

Next we'll look at an overview of the different HD format variations. In following sections, we'll limit our coverage to HD video and its use in the United States, or other NTSC countries. In order to be backwards compatible with NTSC, there are certain limitations to the types of HD standards that should be used when creating video to be distributed in the U.S. (and other NTSC countries).

ATSC: Advanced Television Systems Committee. Established in 1982 to develop digital television standards for the U.S. These include standards for (digital) standard and high definition video. The FCC adopted many of the aspects of the ATSC standards in 1996, and U.S. digital television transmission will use these standards. Below is a list of ATSC formats for HD only.

Note: These are just those HD standards from the ATSC, and don't include other HD standards used around the world. Notably missing are those formats used in PAL countries. These include 25p (25 fps, progressive) and 50i (25 fps, interlaced) for 1080 and 720 variations. In addition, the ATSC defines formats for digital SDTV as well as HDTV standards. The SD variations all have a 4x3 aspect ratio. We're only covering HD standards in this discussion, which all use a 16x9 aspect ratio. In addition, as we'll cover in a later section, just because the above standards were adopted by the ATSC, we generally will not use several of these formats in the U.S. or other NTSC countries.

ITU-R 709: Properly expressed as ITU-R BT.709. The ITU stands for the International Telecommunications Union and sets international standards (called "recommendations") for radio and other telecommunications. Rec. 709 defines how high definition video is encoded. Although called a recommendation in reality it is treated more like a standard. Although Rec. 601 and 709 use the same digital scales for coding, different coefficients are employed.

Among other characteristics, video is encoded in RGB or YCbCr, allows for 8-bit or 10-bit linear coding, square pixels, interlace or progressive scanning.

In addition, the following frame rates are defined:

• 23.98PsF
• 24PsF
• 23.98P
• 24p
• 25i (referred to as 50i)
• 25PsF
• 25p
• 29.97PsF
• 29.97p
• 29.97i (referred to as 59.94i)
• 30i (referred to as 60i)
• 30PsF
• 30p
• 50p
• 59.94p
• 60p