The deeper you get into the HDTV world, the more complex and complicated the whole technical shebang gets - you go from HDTV-versus-analog into strange discussions of lines of resolution, pixels, scanning systems, fixed-pixel displays, scaling systems, and more.
Lines and pixels
You may find discussion of the resolution (the level of clarity and fine picture detail on your screen) of your HDTV - all the horizontal this and vertical that, and lines and pixels - just too much to keep up with at times. But you need to get the lowdown on resolution so you can make the right HDTV choices.
Going with lines
The oldest, and most common, way of describing a TV's resolution comes from the world of CRTs (cathode-ray tubes) TVs. These TVs create an image by shooting an electron beam at the screen and moving (scanning) from left to right and top to bottom. You call the lines that result scan lines.
A TV's vertical resolution (the metric most commonly referred to) is the number of lines that the TV can display in the vertical direction. Think of a stack of pancakes (with each pancake representing a line moving - horizontally; yes, it's confusing - across the screen). The total number of pancakes in the stack (lines on-screen) that the TV can display is its vertical resolution.
If a TV can't display at least 720 vertical lines of resolution (720 delicious pancakes), then it isn't an HDTV-capable TV.
Now take a single one of those lines/pancakes and look across it horizontally. Each TV can display a limited number of individual picture elements (for example, dots of alternating colors) along that line and still keep the image legible. This limit is the horizontal resolution of the TV.
Picking pixels
Many newer TV technologies are fixed-pixel displays. This description means that these TVs, by nature of their physical design, don't have an electron beam moving across a screen, but instead have thousands (or millions) of individual picture elements (pixels) that light up to create the picture. These displays are "fixed" because (unlike a CRT) they don't let you change the number of pixels in the display by re-aiming or refocusing an electron beam.
In a fixed-pixel display, you determine resolution by simply counting up the number of pixels in a vertical stack (from top to bottom or vice versa) - this is the vertical resolution - and across the screen (left to right or vice versa) for horizontal resolution.
The resolution of a fixed-pixel display is usually written out or referenced as "horizontal resolution x vertical resolution." For example, a TV may have a resolution of 1386 x 788 pixels - 1386 pixels in the horizontal direction and 788 vertically.
As with the lines of resolution, you most commonly refer to the vertical number of pixels and use that number when deciding if you can really call a TV an HDTV. A fixed-pixel display must have at least 720 vertical pixels to be called HDTV-capable.
Scanning and interlacing
Resolution doesn't fully define and explain HDTV. Scan types also define HDTV signals (and HDTV TV hardware requirements). Traditional analog CRT monitors, designed for NTSC programming, use interlaced scanning; many (more modern) HDTV designs - including many CRT-based HDTVs - use a system called progressive scanning.
You need to understand two key concepts about scanning:
- Frames: Frames are complete, full-screen images that make up an instantaneous portion of the HDTV picture. In other words, in a 720p HDTV signal, a frame is all 720 lines of the picture. NTSC images are transmitted at 30 frames per second, while HDTV images are transmitted at either 30 or 60 frames per second.
- Fields: Fields are simply half of a frame - the first field within a frame consists of the odd numbered scan lines (1, 3, 5, and so on), while the second field consists of the even numbered scan lines.
When a TV signal (whether it's NTSC or HDTV) is transmitted one field at a time, that signal is interlaced. Interlaced TV video has 60 fields per second - these 60 fields are equivalent to 30 frames per second (fps).
In a progressive-scan signal, both fields that make up a frame are sent simultaneously, allowing the TV to create the entire frame at once. In an interlaced system, only half of the picture is "drawn" on the screen at once, but the short time period between fields (60 fields per second!) means that your eyes see the two fields as one.
So what's better, interlace or progressive? Progressive-scan video has a smoother, more natural appearance. Interlaced video tends to have a slightly flickery appearance, at least when compared head to head with progressive-scan video.
