Technical Reference
What are IRE Units ?
IRE units are the scale defined by the Institute of Radio Engineers to measure the amplitude of a video signal.
Great what does that mean ? Well the Institute of Radio Engineers set the range of a video signal as 1.0 volts peak to peak, or p/p. They divided this range into two areas. The picture portion of the signal and the synchronization ,or synch, portion of the signal. The picture portion is defined as 0 IRE to 100 IRE. The synch portion is defined as -40 IRE to 0 IRE. This means that the total range of IRE values is 140 units. Thus each IRE unit is equivalent to 1/140 volts.
Picture sources are typically set with "black" somewhere between 7.5 to 10 IRE. The maximum signal brightness is usually set to just below 100 IRE. (Except for very brief bursts.)
A third signal is present from +20IRE to -20IRE. This is called a burst signal. It is a short sample of the color subcarrier frequency. It is used to control how colors are displayed on screen. Before color TV the horizontal and vertical frequencies were 60Hz and 15.750Hz respectively. Once color arrived they had to be retimed to accomodate this signal. The current NTSC values are 59.94Hz for vertical scan and 15.734Hz for horizontal scan. (The color subcarrier frequency is 3.58Mhz.)
That is why you have to set your long form projects to 29.97frames per second. Two fields per frame gets you...59.94Hz