Hawes Mechanical Television Archive by James T. Hawes, AA9DT
How Col-R-Tel Works

Col-R-Tel Drawbacks

High-frequency losses. No technology arrives without posing its costs. Col-R-Tel is a case in point. The simplified electronics lack active equalization that a chroma bandpass amplifier might have made possible. By eliminating peaking networks, Col-R-Tel engineers simplified assembly and setup while reducing costs. Yet wideband amplifiers might have improved color reproduction, particularly at the high end. Without peaking networks, the demodulator and color output probably amplify signals out to maybe 100 kHz. Yet each chroma sideband includes signals up to at least 500 kHz. A narrowband amplifier must lose some of the high-frequency content. Fortunately, most TV content resides in the lowest frequencies. These frequencies contain large details of the picture.

Also, 1950's TV pictures don't contain as many high frequencies as today's pictures do. The sets that I recall probably didn't display much luminance beyond 2 MHz. Hey, I didn't care! Sgt. Bilko and Ralph Kramden are hilarious anyway! I wonder: What is the percentage of 1950's sets that could successfully drive a Col-R-Tel adapter? I've heard that sets that predate compatible color do a better job. These sets are more likely to have the required, broadband IFs. Of course! Before 1954, who worried about filtering out chroma sidebands?

How bad is the problem? How much of a drawback would Col-R-Tel's narrowband color reproduction be? Narrowband TV experimenters have proven that viewers can recognize pictures without high frequencies.(The experimenters seem to be mostly concerned with the luminance channel. Still, the same principle applies to chrominance. The low frequencies are the bulk of the signal.) Typical, 500 kHz chroma sidebands are already bandwidth limited. Let's consider what a typical R-Y and B-Y demodulator can do. The topmost chroma frequencies reproduce details as wide as 1/40 of the screen. (Some reference books say 1/25.) Lower chroma frequencies create broader areas of color. For example, a DC signal would fill the screen with a single color. A 100 kHz signal would reproduce a detail one-eighth as wide as the screen. On a 14-inch, Col-R-Tel screen, such a detail would be 1.75 inches wide.

Low-frequency losses. So much for the high frequencies. Low-frequency losses are another problem for Col-R-Tel. Chroma sidebands from the transmitter include important content down to DC. To derive maximum gain from the chroma demodulator, Col-R-Tel engineers bypassed its cathode. The added gain also favors high frequencies. The demodulator includes no low-frequency compensating network. Such a low-pass network would improve low frequency gain.

Without the low-pass network, we can adjust Saturation Control R22 for maximum gain. This control is in series with the 6BE6 demodulator tube's interelectrode capacitance. At maximum resistance, the control and capacitance form a low-pass filter. Of course, now the high frequencies suffer proportionately. Otherwise, we can rely on the monochrome picture to fill in. Even with the best sets, fine details and outlines only appear in monochrome. Maybe the Col-R-Tel engineers took that principle to heart.

No DC restorer. Col-R-Tel's color-difference amplifier is C5A, one triode section of a 12BH7 tube. The plate of this tube couples to the CRT grid through 0.5 μF capacitor C25. (On some TVs, the CRT cathode couples to C25.) Since Col-R-Tel has no DC restoration circuit, the CRT receives a floating signal. Between color pulses, a DC restorer would maintain a constant, average CRT grid (or cathode) bias. The effect would be consistent rendition of large, static, solid objects. The contrary effect occurs in RC coupling. RC coupling arbitrarily averages the color saturation of large objects. That is, a bright or dark object floats to some "medium" saturation level. Remember that NTSC TV only depicts large objects in color. For this reason, DC restoration is very important. Also, without DC restoration, supply drift could cause large objects to vary in color. DC restoration would also blank the colors during retrace, eliminating chances of colored retrace lines.

Increasing C25 to 5 μF would probably help. Doing that would increase the time constant to 50 milliseconds, about three line times. (50 mS is at the low end of what expert Bernard Grob recommends. Grob, Basic Television Principles and Servicing, 4th ed., p. 305.) Adding a DC restorer would be better yet.

The Col-R-Tel crystal ringer substitutes for a crystal oscillator, phase detector and reactance tube. The circuit doesn't have or need a color killer or automatic color control (ACC). The ringer is the picture of efficient design. TV's rapscallion entrepreneur Madman Muntz would have loved this circuit. After all, he was a champion of no-nonsense circuitry. Still, Col-R-Tel's chroma detection probably suffers from the bare-bones approach. In weak-signal areas, the ringer circuit doesn't perform reliably. There, a set with a phase-locked, 3.58-MHz oscillator would be the superior color receiver.

Col-R-Tel lacks color decompression. According to Alvin Liff, the transmitter avoids overmodulation by compressing R-Y and B-Y signal amplitude. Since the R-Y and B-Y compression factors differ, the amplitude changes also affect color phase. A conventional receiver's matrix or color difference amplifiers restore the correct amplitudes. As long as these amplifiers don't time-delay the signals, phase is no longer an issue. Unfortunately, Col-R-Tel doesn't provide such decompression. Col-R-Tel demodulates the signals separately. Sequential demodulation with one demodulator prevents matrixing. Col-R-Tel could compensate by sequentially varying the gain of its single color-difference amplifier. Yet Col-R-Tel doesn't do that, either.

Alvin Liff, Color and Black & White Television Theory and Servicing, pp. 78 and 80: "In actual practice, the R-Y and B-Y signals are compensated to prevent overmodulation of the transmitter. This is done by reducing their amplitudes to 0.877 (R-Y) and 0.493 (B-Y). This in turn causes changes in the amplitude [p. 80] of the chrominance signals and their phase angles.
"At the receiver the chrominance signal must be adjusted to take this compensation into account."
Liff, page 625: "...the matrix amplifier also compensates for the chroma signal compression that was introduced at the transmitter... Thus, the R-Y amplifier provides a signal boost of 1.414 times, and that of the B-Y amplifier 2.03 times. The G-Y amplifier reduces the level by 0.70 times."

Field-sequential color at one frame every twentieth of a second adds flicker to the image. The color filters steal 60 percent or more of the image brightness. Disc size restricts the image size to only 14 inches across. But Col-R-Tel delivers color at an affordable price. For $150, plus two hours of assembly, you have color! This gadget is worth every 1955 penny.

Go to Page:   1   2   3   4   5   6   7   8   9   Next


Google Search
Web www.hawestv.com

Watch the latest videos on YouTube.com

Copyright © 2005 by James T. Hawes. All rights reserved.

•URL: http://www.hawestv.com/mtv_color/colrtel_drawback.htmWebmaster: James T. Hawes
•Revision—January, 2009  •Page design tools: HTML, Notepad & Explorer