Color Television Systems: Which Way Will Europe Go?
July 1966 Radio-Electronics

July 1966 Radio-Electronics [Table of Contents] Wax nostalgic about and learn from the history of early electronics. See articles from Radio-Electronics, published 1930-1988. All copyrights hereby acknowledged.

In July 1966, Radio-Electronics magazine covered Europe's efforts to standardize color-TV at the Oslo CCIR conference, where three competing systems were under consideration: NTSC (the U.S. phase-modulated system, cost-effective but prone to hue errors), SECAM (the French sequential system using FM to eliminate phase distortion but requiring a delay line), and PAL (the German system that corrected NTSC's phase errors by alternating signal polarity each line). While SECAM offered simplicity (no user controls) and PAL provided better color stability under interference, NTSC remained the cheapest option. A last-minute Russian proposal, SEQUAM (a hybrid of PAL and ART), briefly emerged but was ultimately sidelined in favor of SECAM III. The conference sought to prevent a repeat of Europe's fragmented black-and-white TV standards, though political and technical disagreements threatened to prolong the deadlock.

Color Television Systems: Which Way Will Europe Go?

July may bring the end of the long deadlock on European color-TV standards. Some proposed systems have vital advantages over ours.

By Eric Leslie

At one time Europe had 405-, 525-, 625-, and 819-line broadcast systems. Some unfortunate countries - like Belgium - had to build sets that could receive more than one of them.

If European color TV standards are not agreed on at the Oslo CCIR conference (June 22-July 22), the various nations will likely go their own confusing ways, as they did with black-and-white.

Color may not be so mixed up. There is some hope that one system will be adopted for all Europe, and it is not likely that there will be more than two. Meanwhile, the systems - reasonably distinct and different at the time of last year's Vienna convention - have been changing and blending to a point where it is becoming hard to distinguish which exactly is which.

Only One Color System

The advocates of all the systems insist theirs is just like our NTSC. The fundamental principles are identical. The significant differences are in the methods used to modulate the color subcarrier to transmit chrominance information. The color camera filters the scene into three color components - red, green and blue - each beamed to its own camera tube. The electrical outputs of these tubes - scanned like ordinary black-and-white image orthicons - are the familiar R, G and B signals. Mixed together again they form the Y (brightness) signal, which is identical to the signal from a black-and-white camera.

Two difference signals, R - Y and B - Y, carry the color information, and modulate a sub carrier. The subcarrier frequency is about 3.58 MHz for 625-line TV. In fundamental NTSC systems, the color signal is phase-modulated onto the sub carrier. Color (really hue) depends on which portion of the sub-carrier cycle is modulated. A signal near the beginning of the cycle is interpreted as a shade of blue; one at a little more than 90° (quarter way along the cycle) as a red. The NTSC receiver contains an oscillator that is kept in step with the transmitter subcarrier by a burst of 3.58- MHz sub carrier at the start of each scanning line. The two signals follow: the R - Y first and the B - Y a quarter-cycle after it. No green difference signal need be transmitted, since it can be developed at the receiver as red and blue are reconstituted.

Sequence and Memory (French)

The French SECAM differs from American NTSC more than do the other European suggestions. It transmits the color on R - Y and B - Y signals as does the fundamental system. But, instead of sending the two signals at the same time in quadrature, it transmits only the R - Y signal during one full scanning line, the B - Y signal on the next line.

The sub carrier is picked out of the video signal by a bandpass amplifier, as in other systems. The amplifier output goes to an electronic switch, which works like this: Let us suppose that in Fig. 1 we are looking at the switch at the instant a line modulated with the R - Y signal is starting. The R - Y signal goes to the corresponding detector and also to the delay line, where it is stored until the beginning of the next scanning line. As that line (B - Y) comes through the bandpass amplifier, the switch moves to the lower position (on the diagram) and the signal goes through to the blue detector. At the same time, the red signal appears at the output of the delay line and is fed to the red detector, and a blue (B - Y) signal enters the delay line.

Thus, after the first line, two color signals are fed to the matrix simultaneously, though these signals come from two successive scanning lines. Experiments confirm the fact that the difference in content between two successive scanning lines in a 625-line frame is so slight as to be practically imperceptible.

The two difference signals are combined with the brightness signal, which comes directly to the matrix from the video stages, to obtain the red and blue signals. The remainder is the green signal as in NTSC. Special synchronizing pulses (analogous to the color burst of NTSC) keep the "switch" at the receiver in step with that at the transmitter.

The frequency-modulated subcarrier (with a swing of 700 kHz) is so insensitive to phase and amplitude distortion that color controls are not needed. The receiver is as easy to operate as a black-and-white set.

PAL resembles the American system most closely, and may be thought of as NTSC with cancellation of phase errors. Hue depends on the relative phases of the elements in the color signal, so anything that changes those phase relationships anywhere between the camera and the TV screen will change the color. For example, suppose that the camera is focused on a red object and that the correct phase for red is the dashed line of Fig. 2. If the color signal is delayed in passing through any of the networks in the transmitter or receiver, it may arrive a few degrees late (Fig. 2-a), or toward the green.

PAL corrects this by reversing one of the modulation axes (the I axis on the original PAL version, the R - Y axis on the latest variation) and then shifting it 180°. This puts the red signal as much ahead of correct phase in the second line as it was behind in the first. Hue would now be shifted toward the magenta, and the average of the two would give the correct color (Fig. 2-b).

A receiver can actually be operated this way, with the eye acting as a sort of matrix and averaging the line to get correct color. The cheaper PAL system (VolksPAL) works just that way. But for large phase errors there is a peculiar Venetian-blind effect and "eye fatigue" sets in. The better circuit uses a delay line, as SECAM does. Fig. 3 is a block diagram of that circuit. Each line is averaged electronically in the matrix with the preceding line, and errors are thus cancelled.

Since phase errors are hue errors, PAL needs no hue control. PAL without the control has truer color than NTSC with it. But, since correcting large phase variations cuts down brightness, PAL does need a saturation control.

An ARTful Device

Phase distortion may also vary with the Y-level (strength) of the signal, Thus the color may be correct at the beginning of a line and wrong in the middle of it, if the brilliance in the scene changes greatly. No manual or line-by-line type of automatic hue control can cope with this kind of color error.

Another German system supplies an additional reference signal to synchronize the subcarrier oscillator in the receiver. This system is referred to as NTSC with Additional Reference Transmission, or ART. The reference oscillator is in phase with one of the color signals (the I or R - Y), and rides up and down with color-signal strength, instead of being fixed at the sync backporch level like the NTSC color burst. The reference-transmission signal is reversed for each alternate line and fed into the demodulators through a delay line, as with PAL, so that distortion is cancelled.

Unfortunately, ART is not as compatible with black-and-white as any of the other systems, and is harder to record on tape than straight NTSC. (It is interesting to note that, should we wish to improve our American color, NTSC transmitters and receivers can readily be adapted to PAL or ART.)

Which is best? As in many other things, there is no best. NTSC costs the least and has a long record of practical use to prove its reliability, A SECAM set is simplest to operate (like a black-and-white receiver) and the SECAM signal is easiest to record - can be recorded on an ordinary black-and-white video recorder. PAL is claimed by its advocates to show slightly better color under adverse conditions. It is a question which of these factors engineers consider is most important.

But, just as the various committees, subcommittees and conferences had finally agreed that all the parameters have been set and that no more tests are needed, and they were now ready to thresh out their differences at Oslo, a voice was heard from the South. A new Russian system was discussed in Rome by the French delegates at a conference of the European Broadcasting Union. There is no word on whether it has been tested, or even exactly what it is. Called NIIR by the Russians and SECAM IV or SEQUAM by the French, the details revealed so far lead one to believe it is a sort of cross between PAL and ART, with the phase correction of the one and the additional reference transmission of the other. The features of the older SECAM seem to have disappeared in the merger.

Apparently the main features of the new system were political, combining as they did the two competing systems. However, France and Russia later officially joined in approving SECAM III. (The III refers to certain specifications concerning the direction of modulation of one of the color signals and of the amount of deviation). Therefore, it is not likely that SEQUAM will be one of the systems introduced at the Oslo conference. However, it remains in the background as an interesting dark horse.