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(At least) one answer to How were video tape recorders adapted to work in orbit in 1962? argues that the recorder should not be called a "video" tape recorder because the record/playback head was static and not scanning the tape, and links to NASA Technical Note TN D-1542 A Precision Endless-Loop Magnetic Tape Recorder for Space Applications which covers the AC system that maintains the motor's constant speed, but not the recording and playback electronics.

update: it now links to Contract No. NAS5-3173; Final Engineering Report, TIROS-X Meteorological Satellite System, Volume I which may in fact contain the answer to this question.

However if this were a conventional tape recorder and you just took the amplified output of the vidicon and drove an audio record head with it, it would not work when playing back. For example a solid white area or solid back area would result in constant magnetization of the tape and therefore zero output independent of brightness level; the signal during playback would be proportional to dB/dt in the head.

Simply put, conventional tape recorders have a low frequency cut-off; they do not record long stretches of DC levels that a video signal would contain if looking at a blank gray level, which a video tape recorder absolutely must do!

So they must have used some modulation or encoding scheme to convert brightness levels to a modulated carrier tone of some kind (e.g. AM, FM, something else) so that a signal would be present during playback even for constant brightness levels.

Question: How did they do this? How did they make an otherwise audio tape recorder function as a video signal recorder and playback device?

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    $\begingroup$ I know that you can’t do standard parallel track recording, like you do with audio, for the reasons you state. In the 50s, helical read/write was the solution, but I’m no expert. This looks like a lot of good data. books.google.com/… $\endgroup$
    – Anthony Stevens
    Commented Jun 24, 2020 at 12:00
  • $\begingroup$ it was standard single track linear recording @AnthonyStevens - it was only taking one picture every 30 seconds, so writing them to 100 inches of tape over two seconds was quite doable. $\endgroup$
    – user20636
    Commented Jun 24, 2020 at 16:06
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    $\begingroup$ Audio tape recorders have similar issues when used for audio. AC bias has been used to compensate since the early part of the 20th century. $\endgroup$
    – Russell Borogove
    Commented Jun 24, 2020 at 23:20

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According to your linked document the video signal from the vidicon (in record mode)

The composite video resulting from vidicon readout is applied through the emitter follower to the modulator and produces frequency variations in the 85-kc subcarrier oscillator. This FM signal is then applied through the head driver to the video record head of the tape recorder.

The specs for the vidicon video bandwidth of 62.5 kHz allowed an FM range of 85 $\pm$ 31.25 kHz, or 53.75 to 116.25 kHz to be recorded.

This means that the video signal frequency modulated a 85 kHz 'carrier'. The tape recorder operated at 50 inches per second for 2 seconds per image (compared to 7.5 ips for consumer audio recording/playback) and this speed allowed the system to record that relatively high frequency signal for later playback

Edit- Additional trivia...

Data recorders were capable of recording fairly high frequency signals. This Ampex brochure lists some (large) models with bandwidths up to 2 MHz.

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  • $\begingroup$ Wow! That's a heck of a frequency swing. Thank you for your concise yet thorough and well-sourced answer! 50 ips is 1270 mm/sec which means 116250 Hz results in a linear period of 10 microns along the tape which means the tape head azimuth alignment i.sstatic.net/DHhfi.jpg (from here) needed to remain intact despite those incredible vibration testing procedures outlined in NASA Technical Note TN D-1542: $\endgroup$
    – uhoh
    Commented Jun 24, 2020 at 23:35
  • $\begingroup$ "...sinusoidal vibration test levels of 10 g from 0 to 2000 cps for 30 minutes total sweep time, and white noise random vibration of 20 g rms in a frequency bandwidth from 5 to 2000 cps for 4 minutes." $\endgroup$
    – uhoh
    Commented Jun 24, 2020 at 23:36
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    $\begingroup$ Yup. And everything else on TIROS had to hang together under the same abuse. Tape drives can be outrageously rugged... $\endgroup$
    – BobT
    Commented Jun 24, 2020 at 23:41
  • $\begingroup$ This is amazing stuff, I had no idea this existed but it certainly makes sense. I see for example AR-16 series 800 Hz to 2 MHz at 120 ips and FR-1600 2 MHz direct (400 kHz FM) at up to 240 ips! $\endgroup$
    – uhoh
    Commented Jun 25, 2020 at 21:32

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