NotTaR of Television Sets : Why is the NTSC color subcarrier such a ..  
 Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

     << How did the (vertical) fr.. |  Index  | What is the maximal allow.. >>

Why is the NTSC color subcarrier such a weird frequency?

(The following is from Bob Myers (myers@fc.hp.com).)

Actually, if we wanted to define the rates to the Nth degree, the important starting point is the field rate. The NTSC color frame rate was defined as (60 * 1000/1001) Hz, which is a bit more than 59.94 Hz. From this rate, all the others in the system are defined. The line rate is 262.5 times this, and the color subcarrier is defined as 455/2 times the line rate. This is often given as simply 3.579545 MHz, but the the color subcarrier was actually derived from the line/field rates rather than the other way around.

The whole thing was done so as to avoid (or at least minimize) interaction between the luminance, chrominance, and audio subcomponents in the standard color signal. This could have been achieved by moving either the audio subcarrier or adjusting the line and frame rates as described above. Unfortunately, the latter route was chosen, leaving us with this very strange looking set of rates.

The precise color burst frequency winds up being 3.579545.4545... under this definition, but giving it to the nearest Hz is within the tolerances of the system.