Editing NTSC (section)

=== Resolution and refresh rate ===
NTSC color encoding is used with the [[CCIR System M|System M]] television signal, which consists of {{frac|30|1.001}}&nbsp;(approximately 29.97)&nbsp;[[interlaced video|interlace]]d frames of [[video]] per [[second]]. Each frame is composed of two fields, each consisting of 262.5&nbsp;scan lines, for a total of 525&nbsp;scan lines. The visible [[raster scan|raster]] is made up of 486&nbsp;scan lines. The later digital standard, [[Rec. 601]], only uses 480 of these lines for visible raster. The remainder (the [[vertical blanking interval]]) allow for vertical [[synchronization]] and retrace. This blanking interval was originally designed to simply blank the electron beam of the receiver's CRT to allow for the simple analog circuits and slow vertical retrace of early TV receivers. However, some of these lines may now contain other data such as [[closed captioning]] and vertical interval [[timecode]] (VITC). In the complete [[raster scan|raster]] (disregarding half lines due to [[Interlaced video|interlacing]]) the even-numbered scan lines (every other line that would be even if counted in the video signal, e.g. {2, 4, 6, ..., 524}) are drawn in the first field, and the odd-numbered (every other line that would be odd if counted in the video signal, e.g. {1, 3, 5, ..., 525}) are drawn in the second field, to yield a [[Flicker fusion threshold|flicker-free]] image at the field refresh [[frequency]] of {{frac|60|1.001}}&nbsp;Hz (approximately 59.94&nbsp;Hz). For comparison, 625&nbsp;lines (576&nbsp;visible) systems, usually used with [[PAL#PAL-B/G/D/K/I|PAL-B/G]] and [[SECAM]] color, and so have a higher vertical resolution, but a lower temporal resolution of 25 frames or 50 fields per second.

The NTSC field refresh frequency in the black-and-white system originally exactly matched the nominal 60&nbsp;Hz [[utility frequency|frequency]] of [[alternating current]] power used in the United States. Matching the field [[refresh rate]] to the power source avoided [[intermodulation]] (also called ''beating''), which produces rolling bars on the screen. Synchronization of the refresh rate to the power incidentally helped [[kinescope]] cameras record early live television broadcasts, as it was very simple to synchronize a [[film]] camera to capture one frame of video on each film frame by using the alternating current frequency to set the speed of the synchronous AC motor-drive camera. This, as mentioned, is how the NTSC field refresh frequency worked in the original black-and-white system; when ''color'' was added to the system, however, the refresh frequency was shifted slightly downward by 0.1%, to approximately 59.94&nbsp;Hz, to eliminate stationary dot patterns in the difference frequency between the sound and color carriers (as explained below in [[NTSC#Color encoding|§{{spaces}}Color encoding]]). By the time the frame rate changed to accommodate color, it was nearly as easy to trigger the camera shutter from the video signal itself.

The actual figure of 525&nbsp;lines was chosen as a consequence of the limitations of the vacuum-tube-based technologies of the day. In early TV systems, a master [[Phase-locked loop|voltage-controlled oscillator]] was run at twice the horizontal line frequency, and this [[frequency divider|frequency was divided]] down by the number of lines used (in this case 525) to give the field frequency (60&nbsp;Hz in this case). This frequency was then compared with the 60&nbsp;Hz [[power-line frequency]] and any discrepancy [[Frequency-locked loop|corrected by adjusting the frequency]] of the master oscillator. For interlaced scanning, an odd number of lines per frame was required in order to make the vertical retrace distance identical for the odd and even fields,{{Clarify|date=May 2022}} which meant the master oscillator frequency had to be divided down by an odd number. At the time, the only practical method of frequency division was the use of a chain of [[vacuum tube]] [[multivibrator]]s, the overall division ratio being the mathematical product of the division ratios of the chain. Since all the factors of an odd number also have to be odd numbers, it follows that all the dividers in the chain also had to divide by odd numbers, and these had to be relatively small due to the problems of [[Frequency drift|thermal drift]] with vacuum tube devices. The closest practical sequence to 500 that meets these criteria was {{math|3&times;5&times;5&times;7{{=}}525}}. (For the same reason, 625-line PAL-B/G and SECAM uses {{math|5&times;5&times;5&times;5}}, the old [[405-line television system|British 405-line system]] used {{math|3&times;3&times;3&times;3&times;5}}, the French [[819 line|819-line]] system used {{math|3&times;3&times;7&times;13}} etc.)