NTSC Decoding Basics (Part 1)

Finding a method to combine these lower bandwidth difference components along with the Y component is the essence of the NTSC system. All of this information must fit into the same radio frequency space as the original monochrome television signal. Here is where it gets really interesting. Not only must it fit the space; it cannot cause any undue harm to the image produced by the basic monochrome TV. Here, we must go back to the construction of the basic monochrome television signal to see what we have to deal with. Recall that image information is sent one field every 1/60 th second. Two fields make up a frame, or full picture, which takes 1/30 th second. Our system sends one half the picture in succession with the remaining half so that speed of the system may be kept relatively low and the video luma information fits into the 6 MHz spectrum space. This is the interlaced system. For each horizontal line of video information sent, there is energy associated with that transmission. Therefore, the video energy is associated with, and centered on, the regular intervals of the horizontal scanning time of 15.75 KHz. The radio spectrum of this signal construction looks something like Figure 2. Notice that definite spaces exist with no energy present. This is "wasted" spectrum space.

Figure 2

Since the Y channel represents the original monochrome information, it's transmission method is a given. The agreed upon method for combining the color information is the addition of a radio subcarrier such that its frequency in the available channel spectrum would be high enough so as to be difficult to see on a monochrome receiver; but, capable of providing the required information in its bandwidth-limited form. This subcarrier is the 3.58 MHz sine wave signal.

In Figure 1, the camera sync generator synchronizes the 3.58 MHz oscillator, which supplies the subcarrier. The subcarrier frequency was carefully chosen to satisfy many factors. It is calculated based on odd multiples of one-half the horizontal line rate. The precise arrangement of the subcarrier frequency causes the it's energy to be interleaved, or placed in between the energy points of the Y signal within the channel spectrum, as in Figure 3. Therefore, monochrome receivers will not easily "see" the energy used to carry the color information and will still show a good picture. But, the shared spectrum space is not perfectly balanced, and some small amount of color subcarrier will fall into the luma channel space…more about that later. Basing our color system on this interleaved subcarrier is the reason that the true horizontal line rate is 15.734 KHz (versus the original 15.750 KHz) and the true vertical is 59.94 Hz instead of 60 Hz. This small amount of error is insignificant and still allows monochrome receivers to operate.

Figure 3