THE FRONT LINE: MARCH 9, 2007

What If…

PETER PUTMAN, CTS

I relocated my offices and studio last fall, and am still missing things I packed up several months ago (so much for my fool-proof archiving system!). But every few weeks, I stumble across another box and mutter to myself, “so that’s where it wound up!”

This month’s column is based on one of those treasure hunts, which resulted in the retrieval of an old copy of Radio News from July of 1945. This magazine was published just after the battle of Okinawa had ended and while the war in the Pacific was winding down to its explosive climax.

The cover features an earnest-looking young man staring into the viewfinder of a prototype CBS TV camera. In itself, that wasn’t such a big deal, but the inset photo under the camera was: It showed a sequential-color scanning system, one that worked at both ends of the transmission system – in the camera, and in the television receiver in your home.

Déjà vu, perhaps? Sequential color is the basic architecture for single-chip, large screen imaging systems, primarily those using Texas Instruments’ Digital Light Processing technology. It’s a pretty efficient way to present color picture information with limited bandwidth, and depends on our persistence of vision and inability to perceive high rates of image flicker.

CBS was so enamored of the sequential color scanning system that it continued development for almost a decade afterwards, proposing that the FCC standardize it for color TV broadcasting.

Unfortunately for CBS, RCA’s competing three-gun CRT system, using discrete red, green, and blue color phosphors, carried the day (not to mention David Sarnoff’s strong political ties to Washington’s bureaucrats), and the sequential color idea eventually faded to black.

But what if? What if CBS’ system had been adopted, after all? It’s fair to say that the entire system of color television would look vastly different today, and might even have better performance than the phosphor-based system that took several decades to finally achieve the level of performance that RCA originally claimed back in the early 1950s.

CBS’ idea used a revolving drum with alternating red, green, and blue filters. There were two segments of each color, resulting in a six-segment color wheel. CBS experimented with several different combinations of field rates (60 to 120 Hz), frame rates (20 to 60 Hz), interlaced ratios, scan lines, and horizontal scanning frequency.

In the end, the combination that produced the best results (minimal color breakup, motion flicker, and frame flicker) used a color field frequency of 120 Hz, a frame rate of 20 Hz, a 2-1 interlace scan ratio, and 375 lines of picture resolution. To pull this off, the horizontal scanning frequency would be increased to 22.5 kHz from the NTSC standard 15.75 kHz.

Across the pond (as the British like to say), John L. Baird was developing a competing system that would transmit two primary colors and 600 lines of image detail with a 50-Hz scanning rate. This system presaged RCA’s development of a three-color CRT, but it never came to market.

According to the Radio News article, “…The majority of engineers here in the United States feel that the simplicity and reliability of the color disc method, as demonstrated by the CBS color system, dictate its probable use in future color systems.” According to a paper presented in 1944 at a technical conference sponsored by the Institute of Radio Engineers (IRE), numerous failures of capacitors and resistors occurred during the CBS scanning color tests, yet the color wheel motor and brake never failed once.

Let’s assume we are in an alternative universe, and that CBS’ idea finally carried the day, dealing a rare setback to Sarnoff and RCA. Starting in the early 1950s, TV sets would have incorporated a six-segment color wheel (just as early DLP rear-projection HDTVs did), synchronized by scanning pulses transmitted as a subcarrier on the 6 MHz broadcast TV channel.

There wouldn’t be any chrominance subcarriers (Pb and Pr), nor would it take 20 years to develop stable, precise YPbPr-to-RGB color space decoders. There would just be a multi-segment color wheel, spinning along at about 120 Hz, filtering the scanned images from a monochrome CRT into full spectral color. (And the infamous hue adjustments that shifted colors from purple to green on early TVs would be banished!)

Figure 1a-b. Here’s the original NTSC color space, as proposed in 1953,
and the actual NTSC color space in use today (SMPTE-C)

Figure 2. Here’s a color space plot for DLP Cinema.
Notice the similarities to the 1953 NTSC gamut!

Now, here’s something to ponder. The original NTSC color gamut from 1953 contained a far greater range of colors than (as it turned out) it was possible to display using the red, green, and blue phosphor formulations available at that time. Would CBS’ multi-segment color wheel be up to the challenge? You bet!

Filtered color has one advantage over a phosphor color system, and that’s brightness. At a certain level of intensity, color phosphors go into saturation, and no amount of electrical energy can stimulate them to glow any brighter.

Color filters, on the other hand, are only limited by the effects of heat and aging. Pump more light through them, and you get brighter colors. As long as most of the heat (infrared energy) generated in the process can be safely dissipated, color images produced by filters are a better choice for color reproduction and wide dynamic range.

Because luminance is a key part of color space values, it’s safe to say that CBS’ system would have been able to create a much larger color gamut, even using a scanning electron beam. This expanded color gamut would have been able to handle more saturated reds, so-called “illegal” colors like orange and turquoise, and greens that would really pop off the screen.

Remember that the NTSC color phosphor system is weighted in accordance with the color response of our eyes. That weighting is approximately 59% green-yellow, 30% red, and 11% blue. Because of the limitations of the 6 MHz TV channel, a weighted color system was a must, given all the other information being transmitted in that packed channel.

But with a scanning color system, only a series of synchronizing pulses needed to be added to the amplitude-modulated luminance and frequency-modulated audio signals broadcast by each TV station. There wouldn’t be any color moiré or loss of high-frequency image detail, because there wouldn‘t be any color burst frequency at 3.58 MHz to filter out with a notch or comb process.

In fact, the terms “composite video” and “component video” might never have entered our lexicon, as there wouldn’t have been any discrete color information transmitted with the black-and-white video pictures – just an additional layer of synchronizing pulses, which could be disregarded for conventional monochrome TV viewing.

Expanded color spaces are all the rage these days, as are comparisons of plasma, LCD, and projection TV color spaces to the NTSC and PAL gamuts, which are both phosphor-based.  Would we still be having these discussions if CBS’ scanning color system had been adopted? Would the xvYCC space have been developed? How about the need for lasers and LED backlights?

Would HDTV have come into existence sooner by eliminating the tricky color subcarrier issues of NTSC? How much resolution would this alternate version of HDTV have? It’s a safe bet that the color gamuts developed for digital HDTV (REC.709) would likely have been much, much wider, approaching if not equaling that of color motion pictures. 

We’ll never know for certain. But it is ironic that CBS’ scanning color system, once rejected by the FCC, is now in wide use for showing HDTV images, and has evolved far beyond what Dr. Peter Goldmark of CBS Laboratories envisioned for it, way back in 1945…