NASA photo

How the Downlink Converts Moon Signals Lunar vs. terrestrial Col-R-Tel. The lunar color system is terrestrial Col-R-Tel in reverse. Recall that terrestrial Col-R-Tel transforms NTSC television signals into field-sequential signals. The moon downlink station converts field-sequential signals to NTSC. Standard TV sets can reproduce the downlink station's output. This conversion makes viewable pictures for home sets across the world.

Lunar signal conversion is far more difficult than conversion going the other way. While terrestrial Col-R-Tel works by losing color data, moon Col-R-Tel works by making color data. That is, terrestrial Col-R-Tel discards two colors per frame. Lunar Col-R-Tel borrows data from adjacent frames, and actually builds new frames. The field-sequential to NTSC color conversion equipment is another form of mechanical television.

Compensating for Doppler shift

Doppler shift. Before the downlink station can begin its conversion, it must correct for Doppler shift in the lunar signal. The shift varies depending on the relative motion of the earth and the spacecraft. Standard TV sync circuits can't reliably lock a Doppler-shifted picture. Without Doppler shift correction, the pictures would tear and flip. The correction process is partly mechanical and partly electronic.

Two machines. To correct Doppler shift, NASA uses two tape recorders: One machine plays back. We'll call this machine the player. The other machine records. We'll call this machine the recorder.

Tape from the recorder runs to the player. The tape is slack. Without breaking tape, one machine can operate faster than the other.

The recorder tapes the TV signal from the spacecraft. This TV signal includes sync pulses that stabilize the picture. On the tape, the time between recorded sync pulses varies. This time depends on the direction of spacecraft motion with respect to earth: When the spacecraft recedes, intervals between sync pulses grow longer. When the spacecraft approaches, the intervals grow shorter.

Servo. A crystal-controlled servo drives the player. This servo locks on the NTSC color subcarrier frequency, 3.58 MHz. The servo’s other input is the taped horizontal sync pulses from the recorder. Occurring at 15,734 Hz, these horizontal sync pulses follow the NTSC standard. The crystal frequency and the sync frequency have a common quotient of 7,867 Hz. The servo divides both frequencies to achieve two 7,867 Hz signals.

Subcarrier quotient. As its standard, the servo uses the crystal subcarrier quotient. Then the servo compares the two quotient frequencies. Any difference between these two frequencies causes a correction voltage. The servo uses this correction voltage to proportionately vary the player speed. The varying player speed corrects for differing intervals between sync pulses. Output sync matches the NTSC standard. The player output runs to the scan converter.

Conversion from field-sequential to NTSC

Downlink station color conversion. Time for a little review. On the moon, cameras scan one color video field at a time. After three color fields, a full color frame results. This is field-sequential scanning. On Earth, color TV sets scan all three colors simultaneously. Every field contains three colors. Two fields make up a frame. The downlink station must convert the moon video standard to Earth video. The conversion process repeats each moon color field as part of three different frames.

Shifted frames. For every output NTSC field, the process also shifts one lunar video field by a half line. Other downlink circuitry adds normal luminance, chrominance and burst to the moon signal. Despite the data duplication, the downlink equipment never invents new picture elements. Instead of dreaming up the missing data, the process involves distributing received data.

The downlink picture conversion process is entirely analog. To store, repeat and combine lunar video fields, the Apollo missions use six-track, analog disc recorders. Electronics outfits developed this type of recorder for use in sporting events. In the 1960s, such recorders made possible the slow motion replays that football fans love.

Disc operations. Across the disc, six operations proceed simultaneously. Each channel performs a different task. On each successive disc rotation, this task switches by one operation. The "read" operations occur simultaneously and build the output field. Each output field includes two matching moon fields and one that doesn't match. For example, one even and two odd fields, or two even and one odd moon fields. Normal field interlacing causes this mismatch. The system corrects the mismatch by shifting the non-matching field. After shifting, the resulting moon field matches the other two moon fields. Without the shift, the non-matching field would start scanning at the wrong screen location. The shift occurs in a 31.5 microsecond, passive analog delay line.

Record-Playback Sequence. Below is NASA's disc operation sequence. After the second field, the sequence repeats. I base my sequence tables on the 1971 NASA manual for the cameras. I've corrected the data for an error in the color sequence. The manual gives the sequence as RGB (red-green-blue). After checking with Stanley Lebar, I know that the sequence is actually RBG. Peter Goldmark's CBS system and Col-R-Tel both follow the same, RBG order.

Process for Making the Odd Terrestrial Field

Process for Making the Even Terrestrial Field

Reception on Terrestrial Col-R-Tel Set

Terrestrial Col-R-Tel TVs could have picked up lunar Col-R-Tel pictures. This reception method would require no NTSC conversion. Consider that downlink stations included Conrac studio monitors that engineers had modified for field-sequential operation. These Col-R-Tel-like monitors could directly receive the lunar, field-sequential telecast.

Converters cancel. The Col-R-Tel converter and moon-downlink converter cancel each other. Thus without NTSC in between the converters, neither converter is necessary. The TV pictures would probably come in just fine.

Experiment. Someone could prove my statement about Col-R-Tel moon pictures. The experiment involves transmitting moon pictures on a standard TV carrier. Then the experimenter would reproduce the unconverted moon signal. The picture would appear on a Col-R-Tel unit without hue and saturation conversion electronics. Both the lunar and terrestrial units would maintain their disc-sychronization systems. Cliff Benham has now performed this experiment, and it works! Cliff's homemade mooncam has reproduced color TV pictures on an original Col-R-Tel set. Since the camera pictures were field sequential, the Col-R-Tel converter worked barefoot! That is, without the adapter electronics. See... Homemade mooncam. Graphic: Col-R-Tel Commutator. Top-Left: Hue points. Bottom-Left: Disc sync points.

How the Standards Differ

Comparison. Below is a comparison of the lunar and terrestrial color standards vs. the CBS color standard. You can see how close terrestrial Col-R-Tel signals are to lunar Col-R-Tel signals. Note that both signals use the same vertical and horizontal frequencies. In both cases, the CBS system differs. This critical comparison underscores my statement that...

• The CBS system didn't go to the moon.

• The CBS system couldn't lock a lunar TV signal.

Incidentally, this statement applies both before and after the downlink conversion. Either way, the CBS system is incompatible with Apollo color moon pictures. On the other hand, Col-R-Tel borrows extensively from both the CBS and NTSC color systems.

Two-Color Mooncam

Simpler Conversion. If Stanley Lebar had chosen a two-color system, he could have avoided using the disc-based converter. Two-color, field-sequential pictures transmit at the same frequency as normal NTSC does. That is, one frame every 30 seconds. In terms of speed, this rate is a huge improvement over the three-color, field-sequential system that Apollo actually used. Recall that the three-color pictures transmitted at only 10 frames per second.

Still not live. Addition of the standard NTSC subcarrier would still occur at the downlink. Also, Doppler correction would still be a requirement. Doppler correction involves a very slight tape delay between two video recorders. This delay would remain. Yet two-color pictures could have been slightly closer to "live."

Why go with two colors, then? Because by Newtonian theory, a three-color system has a broader gamut than does a two-color system. That is, the three-color system can display pictures with more colors and better color fidelity. By the way, Edwin Land would contest this statement. About the same time as the color Apollo flights, the Spectrac^® TV color converter proved Land's point.

Resolution. Another disadvantage of the two-color system is that frames transmit at half resolution. There is no left and right field for each color. Instead, for each color, only one field transmits. As with Spectrac, probably nobody would notice the difference.

Which 2 colors? A typical two-color system displays orange and cyan fields. A standard CRT makes the orange by turning on its red and green guns. To get cyan, the CRT activates its blue and green guns. The result is much like the standard, NTSC I-subcarrier. RCA engineers invented the I-subcarrier because it is in the optimum color range for accurate flesh tones. The eye is more sensitive to flesh tones than to any other color. Very early color TVs included a two-color mode using the I-subcarrier. Today, few if any TV sets can decode this mode.

For more about two-color television systems, click TV in two colors.

• Apollo 10 introduces first color cameras.

• "The Color War Goes to the Moon." Stanley Lebar's story of the Westinghouse cameras. Includes a photo of the color wheel.

• Apollo 15-16-17 cameras, by RCA Project Engineer Sam Russell

• Color wheel similarity: Compares Apollo & IBM color wheels. The article mistakenly names the Apollo color filters as cyan, yellow and magenta (CYM). These are the complements of the true colors. As Stanley Lebar and Cliff Benham explained to me, in some photos, the Apollo filters appear to be CYM. This is because the filters are dichroic filters. Dichroic filters reflect the non-absorbed colors. The strongest rejected colors are the complements CYM.

• NASA Apollo Lunar Surface Journal: Mooncam articles, press releases, manuals, plans, etc.

• Many Apollo mooncam articles (See end of this link.)

• Video short: Stan Lebar with the Apollo TV cameras.

• Apollo & Soviet moon programs: Details of both programs.

• Mission summary from NASA

• Saturn V moon rocket

• Smithsonian Saturn V exhibit

• Color TV history summary. Excellent brief on NTSC color TV system development (Ed Reitan)

• Detailed color TV history. NTSC TV development in more detail

Alleged "Moon-Landing Hoax"

• Bad Astronomy.com. True story behind make-believe controversy.

• Clavius.org examines hoax claims. What hoax believers don't understand

• Astronaut harassment to promote hoax believer's video

• Public nuisance. Belligerence vs. national heroes

Copyright © 2006 by James T. Hawes. All rights reserved.

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