HORIZONTAL SCANNING FREQUENCY

In the early days of black and white the horizontal scan rate was a multiple of the number of lines and pictures per second. There were 30 complete pictures per second and 525 lines per picture therefore the horizontal frequency was 30 x 525 or 15.750 KHz. The horizontal scan rate in our NTSC color system is derived from the color subcarrier. The formula is (3,579,545 Hz)(2/455) = 15,734.26 Hz. The horizontal frequency is expressed as 15.734 KHz. The factor of 2 used in the derivation puts the harmonics of the color information in-between the harmonics of the horizontal interval. That allows the use of a comb filter decoder in pulling color information out of the composite signal, while preserving some of the luminance information in that same frequency space. The factor of 455 was chosen in the original system design to put the color subcarrier frequency at a point low enough where it would not interfere with the audio carrier of a television transmitter, and yet high enough allow compatibility with existing black & white TV sets. Each complete horizontal line is 63.55 microseconds (.00006355 seconds) long. About 10.9 microseconds of that time is used for horizontal blanking, the time that the horizontal line is retraced from the right side back to the left. The color burst occupies 2.5 microseconds of that horizontal blanking interval.