The first version of the system was monochrome and after several years was improved to support colour video. The standard has not changed in the following years and is still compatible with its first monochrome version.
A complete NTSC image is made of 525 horizontal lines per frame. 525 line limit is due to the functional limitations at the time. The lines are scanned from left to right and from top to bottom. All the redundant lines are omitted. Two scans are required to produce a complete frame with one scan for even lines and one scan for odd lines. Half-frame scanning takes approximately 1/60th of a second, thus, producing a complete image takes approximately 1/30th of a second, and this type of scanning is referred to as interlaced and denoted with “i” interlaced.
Why “approximately” and why “1/60 sec”? ‘1/60’ results from the fact that compared to the European standards, the mains frequency in the US is not 50 Hz but 60 Hz. ‘Approximately’ results from introducing the colour version of the system that required to slow down the 30 Hz monochrome version by 1.001 to add the colour information - the new values were 29.97Hz and 59.94, respectively.
Although 525 lines are scanned, only 480 lines are displayed compared to PAL with 576 displayed lines.
The differences in NTSC, PAL and common HD systems resolutions are shown below.
As you can probably guess ‘x480’ refers to the number of lines displayed by the receiver.
Comparison of NTSC and PAL systems shows that PAL resolution is ~20% higher than its US counterpart. However, the higher refresh rate makes the NTSC images smoother. One of the disadvantages of the NTSC system is its susceptibility to interference and issues with correct colour rendering.