HDTV Signals

Unlike NTSC and PAL, which are used to define both broadcast television and generic video input standards, HDTV defines the hardware's capabilities (video and audio input) and broadcast formats separately. Technical information about the properties of standard Hi-Def signals is covered in more detail in another Next-Generation Multimedia guide titled What Is Hi-Def? For the purposes of discussing HDTV we'll concentrate on the standard signals used for broadcasting, as well as those used in the displays themselves.

Digital and Analog

Although storage and broadcast of HDTV is in the digital domain, at some point the digital data must be converted into an analog form for the human eye. Although this conversion may be performed by the display itself, it may be done by an intermediate device that provides the signal to the HDTV. Although processing, including IVTC and deinterlacing, is best handled with digital content, analog signals have advantages as well. For example, while digital video frames consist of discrete pixels in fixed positions, the horizontal lines of an analog frame or field is composed of a continuous waveform. In order to resize digitally pixels must be removed or added, which in most cases requires interpolation. Interpolation of saying a mathematical formula is used to make an educated guess as to what a particular pixel should look like based on the values of surrounding pixels. Analog video can be resized by simply stretching lines vertically or drawing them more slowly.

DTV and EDTV

Not all digital television is HDTV. DTV, or Digital Television, refers to any digital television system, whether it supports hi-def resolutions or not. Many digital televisions have been sold that are EDTVs instead. EDTV, or Enhanced Definition Television, is a sort of bridge between legacy NTSC/PAL analog TV systems, also known as Standard Definition TV (SDTV), and HDTV. While they don't deliver hi-def resolution, they are still an upgrade from SDTV for most digital sources. EDTV resolutions are based on square pixel approximations of anamorphic DVD content. Since 16:9 (widescreen) DVDs contain frames that are meant to be stretched horizontally for playback, EDTV resolution is either 852x480 (NTSC) or 1024x576 (PAL). Although EDTV is neither NTSC nor PAL, it's largely designed with content designed for one or the other in mind. It's also designed to be compatibile with the all-digital future of television in much of the world.

Frame Type

Although much hi-def content is progressive, HDTV signals can also be interlaced. While progressive frames are better visually, originally the highest standard HDTV resolution, 1920x1080, has only recently started catching on for progressive signals. The lower resolution of 1280x720 has always been used for progressive content. A less well known resolution of 1440x1080 (interlaced) is used for some camcorder formats.

Framerate

1920x1080 video may have 23.976 (24/1.001), 24, 25, 29.97 (30/1.001), or 30 progressive frames per second. Interlaced video at that resolution may have 59.94 or 60 fields (half frames) per second. In terms of the digital video this is actually 29.97 or 20 interlaced frames since pairs of fields are stored together to form a complete frame. 720p video (1080x720 progressive) may have framerates of 23.976fps, 24fps, 25fps, 29.97fps, 30fps, 50fps, 59.94fps, or 60fps.

Aspect Ratio

The Aspect Ratio, or AR, of HDTV is 16:9 or 1.78:1 widescreen, compared to SDTV's 4:3 or 1.33:1. All 1920x1080 or 1280x720 HDTV video is widescreen, although fullscreen (1.33:1) video may be pillarboxed, adding black borders on each side of the original 4:3 frame.

HDTV Systems

Although there are various standards in place around the world, unless you're broadcasting it's not necessary to know them as your HDTV should automatically support whatever system you have access to. The most common are ATSC (Advanced Television Systems Committee) from the U.S. and DVB (Digital Video Broadcast) from Europe. Japan also has their own system that borrows from both ATSC and DVB.

Signal Quality

Like any other video technology, the quality of an HDTV signal is affected by transmission limitations. The highest quality you can get in a particular situation is determined by available transmission bandwidth, video resolution and framerate, and the type of compression Since using uncompressed RGB video is out of the question. The most common video compression for HDTV is MPEG-2 - similar to what's used for DVD. Unfortunately the limitations of MPEG-2 are much more visible at hi-def resolutions, and standards are slowly shifting to AVC (MPEG-4 part 10) , which is not only more efficient in terms of bitrate, but actually higher quality as well.

Sometimes the biggest enemy of signal quality are the steps in the broadcasting process itself. Signals may be broadcast in different resolutions than they were produced in. Making things worse, some HDTV receivers for cable or satellite may only output a single resolution. Every conversion along the way will reduce picture quality.