Everyone gets confused by the term “lines of horizontal resolution,” also known as LoHR or TVL. It’s a carryover from analog video, it’s poorly understood, and it’s inconsistently measured and reported by manufacturers, but we’re stuck with it until all video is digital and we can simply report resolution in pixels.
Technically, lines of horizontal resolution refers to visually resolvable vertical lines per picture height. In other words, it’s measured by counting the number of vertical black and white lines that can be distinguished an area that is as wide as the picture is high. The idea is to make the measurement independent of the aspect ratio. Lines of horizontal resolution applies both to television displays and to signal formats such as that produced by a DVD player. Most TVs have ludicrously high numbers listed for their horizontal resolution.
Since DVD has 720 horizontal pixels (on both NTSC and PAL discs), the horizontal resolution can be calculated by dividing 720 by 1.33 (from the 4:3 aspect ratio) to get 540 lines. On a 1.78 (16:9) display, you get 405 lines. In practice, most DVD players provide about 500 lines instead of 540 because of filtering and low-quality digital-to-analog converters. VHS has about 230 (172 widescreen) lines, broadcast TV has about 330 (248 widescreen), and laserdisc has about 425 (318 widescreen).
Don’t confuse lines of horizontal resolution (resolution along the x axis) with scan lines (resolution along the y axis). DVD produces exactly 480 scan lines of active picture for NTSC and 576 for PAL. The NTSC standard has 525 total scan lines, but only 480 to 483 or so are visible. (The extra lines contain sync pulses and other information, such as the Closed Captions that are encoded into line 21). PAL has 625 total scan lines, but only about 576 to 580 are visible. Since all video formats (DVD, VHS, LD, broadcast, and so on) have the same number of scan lines, it’s the horizontal resolution that makes the big difference in picture quality.