Rotation Reversal, by Inverting Only One Axis.

I could engage in some more speculative thinking.

I could be trying to design a hypothetical analog scheme for modulating color information, that belongs to the Y’UV system for representing colors. But I’d like my system to have as advantage over NTSC, that if the chroma sub-carrier gets phase-shifted, due to inaccuracies with the analog circuits, the result should be a shift in hue, which reverses itself from one TV scan-line to the next, as well as from one frame to the next. Just as viewers don’t normally see dot-crawl when they watch an NTSC-modulated signal on a monochrome receiver, they should also not be able to see the hue-shift, due to analog-circuit issues, with my hypothetical modulation scheme.

Consequently, the receivers for this type of signal should not have a Hue potentiometer.

But I discover a problem in my scheme. The U and V components are to be modulated onto a chroma sub-carrier, using quadrature-modulation, just like NTSC was. And yet, I’ll discover that I can only get the clockwise versus counter-clockwise reversal to take place, if I invert either the U or the V signal-component, but not if I invert both, nor if I just invert the sub-carrier, thereby inverting both U and V:


The problem follows, because every signal which gets modulated onto a sub-carrier, using quadrature-modulation, throws sidebands. Hence, if I was to place the sub-carrier frequency just-beyond the frequencies already being used to encode luminance, I would also need to invert both U and V, by default, to eliminate all the dot-crawl. What can I do?

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1970s TV was usually not distributed on videotape.

I am detecting that our present, retro-style depiction of how life worked in the 1970s, often assumes details which may not be 100% accurate historically. And one such detail would be, that if television stations in the 1970s were disseminating an analog signal, that signal must have been recorded on videotape.

Videotape existed at a much earlier point in time, but was hamstrung in its conception, to not being able to cover color signal-formats. This was due to an inability of the playback-device, to ensure a stable frequency for the color sub-carrier. It was only a much later development, that color videotape formats became possible, because of the ability to use VCOs, PLLs, and other elements of a feedback loop, to Heterodyne the frequency of the color information on the tape, and then to produce an output which had strict control over its frequencies, based on the accuracy of a single quartz crystal in the playback device. We needed numerous Integrated Circuits to accomplish that, and the earliest videotape machines only had tubes.

Early radio-transmitters also needed to have one quartz crystal, for every frequency it was licensed to transmit on. It required later technology, to be able to transmit on numerous accurate frequencies, yet only to possess one quartz crystal. And quartz crystals tended to be expensive, before they started to be mass-produced to resonate at one standard frequency.

What TV stations in the 1970s had was a device, into which 16mm emulsion film was fed, which was also a standard photographic film-format at the time, and that captured video from this photographic movie-film, translated it into an analog video signal – in color – that signal to be transmitted as it was being output from this machine. So content was actually distributed to the TV stations, on film.

And the notion did not exist yet, that in order to capture the film content would require scanning it with a laser. Instead, the same type of video-capture tubes were used in this machine, that were used in video-cameras for live broadcasting, which were also quite large and bulky. And Yes, this required one video-capture tube for each primary color – in practice though not in theory.

For TV, the image on one frame of the film was brought into focus – using a lens – on 3 capture-tubes, the light-input to which was split by reflectors.

This also affects how we watch the old movies today.

Continue reading 1970s TV was usually not distributed on videotape.