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.

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.

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. 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.

• 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 article.)

• 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

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