NTSC color encoding is used with the
System M television signal, which consists of 30⁄1.001 (approximately 29.97)
interlaced frames of
video per
second. Each frame is composed of two fields, each consisting of 262.5 scan lines, for a total of 525 scan lines. 483 scan lines make up the visible
raster. The remainder (the
vertical blanking interval) allow for vertical
synchronization and retrace. This blanking interval was originally designed to simply blank 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 (disregarding half lines due to
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-free image at the field refresh
frequency of 60⁄1.001 Hz (approximately 59.94 Hz). For comparison,
576i systems such as
PAL-B/G and
SECAM use 625 lines (576 visible), 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 Hz
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. When color was added to the system, the refresh frequency was shifted slightly downward by 0.1% to approximately 59.94 Hz to eliminate stationary dot patterns in the difference frequency between the sound and color carriers, as explained below in "
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 lines was chosen as a consequence of the limitations of the vacuum-tube-based technologies of the day. In early TV systems, a master voltage-controlled oscillator was run at twice the horizontal line frequency, and this frequency was divided down by the number of lines used (in this case 525) to give the field frequency (60 Hz in this case). This frequency was then compared with the 60 Hz
power-line frequency and any discrepancy 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, 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 multivibrators, 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
thermal drift with vacuum tube devices. The closest practical sequence to 500 that meets these criteria was 3×5×5×7=525. (For the same reason, 625-line PAL-B/G and SECAM uses 5×5×5×5, the old British 405-line system used 3×3×3×3×5, the French 819-line system used 3×3×7×13 etc.)