Hawes Mechanical Television Archive by James T. Hawes, AA9DT
Two-Color Television Systems, Part 3

Two-Color TV History

Gould. In television, two-color reproduction has a long history. Here are some highlights: In 1930, Leslie Gould patented a two-color, three-dimensional television system. This system reproduces a 360-degree view of an enclosed, finite space. The effect is something like viewing a moving Mercator projection, including naturalistic contours. Although the system uses two colors, it isn't anaglyphic and doesn't require viewing glasses. See the patent drawing, right. Items (231) are the orange-red and cyan lamp tubes. They revolve inside a lattice (215) behind the screen (219). The Gould TV screen (219) is cylindrical like some lamp shades, rather than flat. You can view a realistic image from the front, back or sides of this TV!

Alexanderson. Ernst Alexanderson invented a two-color, NTSC-compatible TV system in the 1940s. If it had adopted Alexanderson's color system, CBS could have avoided its monochrome-compatibility problems. Alexanderson's system accurately reproduced flesh tones. The flicker was very minimal.

Camarena. In 1963, Guillermo González Camarena demonstrated his version of two-color, field-sequential NTSC. He proved his system in actual broadcasts from Mexican station XHGC. Camarena's system could use either a color wheel or a color CRT display. In either case, the electronics were vastly simpler than those for NTSC color. The standard vertical sync signal triggered color field changes.

Patent art: 
       3-D, 2-color TV receiver by Leslie Gould (mechanisches Farbfernsehen: 3-D, Empfänger mit zwei Farben)
Patent drawing for one of Gould's 3-D, two-color receivers. Note the 360°, cylindrical screen! (Item #219, here a cutaway view.)

NRI Schools. For decades, NRI Schools® (National Radio Institute) offered superb correspondence courses in electronics. Budding technicians performed lab experiments on the famous Conar® kits and prototyping stations. Most of the test equipment also bore the dependable Conar brand. In the late 1960s, one NRI course included a two-color color wheel TV kit. This kit was an NTSC-compatible color receiver. It must have included a phase selector, a demodulator, and a synchronized color wheel. Keep your eyes open. This very rare kit might just turn up on eBay!

Two-color NTSC

Two-color mode. Compatible-color NTSC itself includes a two-color mode. The two-color mode is part of the original, 1953 NTSC specification. Since then, due to production costs, few TV receivers have used this mode. Anyway, the two-color mode remains part of the transmitted NTSC signal. The two NTSC colors are approximately orange and cyan. Two-color transmission is the reason why NTSC has a broadband chroma signal that we call “I.” European PAL receivers provide no such broadband, two-color mode. In fact, the PAL color gamut is narrower than the NTSC gamut.

I & Q. In NTSC, signal I is one of two chroma signals. It describes an axis that runs between orange and cyan (so called “flesh tones”). The other color signal is Q. Signal I has a bandwidth roughly three times that of signal Q. Signal I is 1.5 MHz wide, vs. only 500 kHz for Q. Together, signals I and Q describe large details in all three primary colors. As details become smaller, the Q signal tops out. Afterward, the I chroma signal carries on alone. Medium-scale details, then, appear as mixtures of the I-colors, orange and cyan. Meanwhile, the greens and purples of the Q signal don't affect medium details. Eventually, even the I channel tops out. For instance, no color appears in small picture details. In fact, the eye can't discern color in small details. NTSC conserves bandwidth by broadcasting these small details in monochrome.

NTSC method of sending full color on 
       two subcarriers I and Q.
NTSC uses two subcarriers (I & Q) to convey full color.

Col-R-Tel® in Two Colors

With only a few modifications, cyan and red-orange Col-R-Tel seems quite possible. A two-color set would have far less flicker than does the three-color model. Yet the two-color version would still render flesh tones perfectly. Since flesh tones are the most important colors, most people would appreciate the modification.

Change summary. First, the color wheel changes: We must replace the six-segment, RBG—RBG color disc. The new disc also has six segments, but the six color wedges alternate between red and cyan (or orange and blue-green). The electronic changes might not be any more difficult than a Col-R-Tel installation. The main change is the addition of a two-pole, double-throw switch. Here's the step-by-step approach...

Assembly Procedure

  1. Replace the three-color, six-segment disc with a two-color, six-segment disc.

  2. Connect a wire to the cyan (-R-Y) tab on phasing coil L2.

  3. Cut the blue and green (B-Y and G-Y) lines between coil L2 and V3. (On the drawing, points with the marking "X.") From now on, we'll call the transformer side of the cut the "input wire." We'll call the V3 side of the cut the "output wire."

  4. In a convenient location near the cut wires, mount DPDT switch S1. In later steps, you'll wire the switch in series with the blue and green color-phase lines from phase coil L2. When wiring the switch, you might need to add wire. To avoid noise, keep the wiring as short and direct as possible.

  1. To the top-left tab, connect the blue (B-Y) signal input wire.

  2. To the top-right tab, connect the green (G-Y) signal input wire.

  3. To the center-left tab, connect the blue (B-Y) signal output wire. This wire will actually carry the cyan (C) or blue (B-Y) signal. Note that the drawing labels this wire "C or B."

  4. To the center-right tab, connect the green  (G-Y) signal output wire.

  5. To the bottom-left tab, connect the -R-Y signal input wire. The -R-Y signal is an inverted red signal that comes from the bottom transformer phase. Another name for inverted red is cyan.

Mechanisches Farbfernsehen: Modifications to Col-R-Tel phase selector.
  1. Leave the the bottom-right tab open.

  1. Rewire the commutator jumpers, following the Commutator Jumper Diagram, right: Connect all contacts that select the same color (red or cyan). Avoid shorts between red wires and cyan wires or contacts.

  2. Between the Col-R-Tel commutator and connector P6, cut the green wire. See the drawing Col-R-Tel 2-Color Commutator Wiring, right. The cut occurs at the "X" on the drawing. (This cut isolates the green-select diode at tube V3. The new, two-color circuit switches between the other two sections of this tube.)

  3. Tape the cut ends of the green wires.

  4. Set the new switch S1 to its bottom (two-color) position.

  5. Run a regular Col-R-Tel setup as in the manual. The next two steps summarize the process.

  6. With a picture on the screen, adjust your TV for the sharpest picture. Use these controls...

  7. • Fine Tuning: For sharpest picture
    • Contrast: Increase • Brightness: Increase

Mechanisches Farbfernsehen: Col-R-Tel commutator jumpers for 3/c and 2/c pictures. Commutator Jumper Diagram

Mechanisches Farbfernsehen: Modifications to Col-R-Tel commutator. Col-R-Tel 2-Color Commutator Wiring (Mouse over for 3-color wiring.)
  1. With a picture on the screen, adjust the Col-R-Tel hue and saturation controls: (The chassis labels identify these controls as Color Lock and Color Gain.) Continue adjusting until you achieve the best possible flesh tones.

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Page Directory

What is Two-Color TV

Advantages & Disadvantages

Flicker Reduction

Col-R-Tel flicker


2-color gamut


2-color NTSC

2-color Col-R-Tel



TV System Flicker Comparison

2.5-Color TV

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