While the number of vertical scan lines within a video system(NTSC/PAL/SECAM) is fixed, the number of dots displayed within each line can vary according to the input source or the ability of the TV monitor itself. This is often referred to as horizontal resolution and is expressed in terms of the lines that the dots create as they are displayed across the screen from left to right.
Using NTSC as an example, there are 525 scan lines (vertical resolution) total, but only 485 scan lines are used to comprise the basic detail in the image (the remaining lines are encoded with other information, such as closed captioning and other technical information). Most analog TVs with at least composite AV inputs can display up to 450 lines of horizontal resolution, with higher-end monitors capable of much more.
The following is a list of input sources and their approximate horizontal resolution capability. Some variations listed are due to range of quality of different brands and models of products using each format.
Video format with horizontal resolution estimates:
220 - 240 lines
250 - 280 lines
270 - 280 lines
Analog TV Broadcast
Analog Cable TV
Standard Digital Cable
330 - 500 lines (Depends on original source of the signal and compression used in downloading to the cable box)
250 - 400+ lines (Depends on recording mode and compression used)
400 - 425 lines
380 - 440 lines
400 - 500 lines
400 - 520 lines
Up to 540 lines
As you can see, there is quite a difference in the detail that different video formats can input into a TV or video display. Basically, VHS is on the bottom end of things, while miniDV and DVD represent the best that analog video can currently produce in terms of detail.
However, another factor that has to be considered is how resolution is stated for Digital and HDTV. For that, just continue on to Page 3...