In my last column (Television Standards - Part I: How Our TVs Work), I gave an overview of how the American (NTSC) color television standard came into being, and how it works. People in other parts of the world, being jealous of our fine color television, decided to see if they could improve upon it and came up with a few other standards. In this month's column I will look at some of these other standards, and talk about how they are the same, and how they differ from NTSC. Next month, we will look at the digital standards in place now which are leading us into the next generation of television.

The European monochrome system was substantially similar to the monochrome system that was used in the U.S., with one major difference. Rather than having 525 lines, 60 frames per second, 2:1 interlaced, the Europeans went for 625 lines, 50 frames per second, 2:1 interlaced. This means that the horizontal line rates are almost the same (15.734 kHz vs. 16.625 kHz.) Since the particular subcarrier frequencies chosen for NTSC were set so that their spectrum falls in such a way as to fall between the spectral lines of the luminance channel (see Fig. below), this same subcarrier frequency would not work with the European standard. Hence, a new color TV standard had to be developed for use in Europe.

PAL

The dominant system for television in Europe is known as PAL, for Phase Alternating Line. You will see why shortly. In an NTSC receiver, there is a phase locked loop (PLL) which oscillates at the subcarrier frequency and is locked onto the colorburst at the beginning of each line. In decoding the color information the phase of the subcarrier is compared to the output of this PLL to determine the hue to display on the screen. In the 1950s and '60s, when the PAL system was developed, it was difficult to design a PLL which would hold a stable phase through the entire line. The result was that if the phase drifted through the line, the picture would have a color shift, left to right, across the screen, which led to the dubbing of NTSC as meaning Never Twice the Same Color. In the PAL system, the phase of the color component is reversed on a line by line basis. In this way, the color shift will be one way on the even numbered lines, and the other on the odd numbered lines, and the eye will average them out to make a better looking picture.

As in NTSC, PAL color is represented with the gain and the phase of the subcarrier. U and V signals are generated by taking the B and R signals respectively, and subtracting off Y (with appropriate scaling factors), and together represent the color content of the image. The trick now was to get the U and V information into the existing Y signal.

The Color Subcarrier:

To get the color information onto the Y (B&W) signal, a new signal was defined:

C = U sin(w t) + V cos(w t) where w is the subcarrier frequency.

and this signal is added to the Y signal.

In PAL, the Phase of the V component is reversed every other line. The result of this phase reversal is that any color subcarrier phase errors create complementary color errors on alternating lines of the picture, giving more precise hue.

As in NTSC the luminance spectrum is a line spectrum with lines spaced by the horizontal rate. Since the V component is being switched at half the line rate, its spectrum consists of only odd harmonics; by situating the color subcarrier at a frequency that is offset by 1/4 line from a multiple of the horizontal rate, Y,U and V are separated in the frequency domain as shown in the figure below:

Any student of history knows that the Europeans have worked long and hard to be able to agree on as little as possible. The PAL standard is a testament to this skill, in that it has several variations, with different video bandwidths, subcarrier frequencies, line rates, placement of the audio subcarrier and one country, albeit non European (Brazil) even uses a variant that has 525 lines and a 59.94 Hz field rate. Table 1 delineates the six major transmission versions of PAL, and a few example countries that use each one:

SECAM

The French looked at how NTSC and PAL had taken advantage of the fact that color had less bandwidth than the Luminance signal, but took this one step further. In NTSC and PAL, only the color bandwidth is limited in the horizontal direction, but since full color information is sent with each line, the full vertical bandwidth is used. In Sequential Couleur Avec Memoire (SECAM), (the logical acronym, using the first letter of each of the four words, hit too close to home), the two pieces of color information are sent on alternating lines. This has the advantage of virtually eliminating crosstalk between the color components, but the receiver has to be able to store one line of video in memory in order to be able to reconstruct the image.

The SECAM system is used over a larger geographic area than any other video standard, primarily because of its adoption by the former Soviet Union, as well as most of the former French colonies. Despite this, I am not going to go into great detail on the SECAM standard

As I mentioned in the first part of this article, I am painting these standards with a very broad brush. Details vital to the proper operation of the standards are being left out in the interest of being able to get an overview of a vast amount of information. If you are interested in delving further into these details, there are several good books that have been written on the topic. I find "Video Demystified" by Keith Jack of Harris to be a good compromise between understandability and being buried in details, but there are several others that cover the topic.

In the next column, I will be wrapping up by looking at the digital standards that are in use, both to represent the standard definition images that I have discussed in these first two columns, but also the High Definition standards that are moving from laboratory to living room.