Author Topic: How did the SSTV conversion work?  (Read 540 times)

Offline Willoughby

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How did the SSTV conversion work?
« on: January 12, 2018, 12:03:30 PM »
I guess I'm just confused about this.  I have read where the SSTV signal was displayed on a compatible monitor, and a television camera pointed at that monitor and this is what everyone saw live.  I have also read that they had some kind of "converter" that converted the SSTV feed to a signal compatible with television.  Is it possible that everywhere I see a mention of a "converter" that this converter WAS the television camera pointed at a monitor?   

Also, I am confused as to what was actually recorded over.  When my understanding is that the "conversion process" was pointing a camera at a monitor, I thought that the transmission that was feeding directly into that monitor is what was recorded onto telemetry tapes and this is what was recorded over in the 1980s when those tapes were reused. 

Offline Allan F

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Re: How did the SSTV conversion work?
« Reply #1 on: January 12, 2018, 04:45:54 PM »
The converter was the camera pointed at the monitor.
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Offline Allan F

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Re: How did the SSTV conversion work?
« Reply #2 on: January 12, 2018, 04:46:51 PM »
What was recorded over was about 45 tapes of telemetry, containing the embedded tv-signal from the Apollo 11 moon EVA.
Well, it is like this: The truth doesn't need insults. Insults are the refuge of a darkened mind, a mind that refuses to open and see. Foul language can't outcompete knowledge. And knowledge is the result of education. Education is the result of the wish to know more, not less.

Offline raven

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Re: How did the SSTV conversion work?
« Reply #3 on: January 12, 2018, 11:15:18 PM »
The converter was the camera pointed at the monitor.
A broadly similar system called a kinescope using a film camera was used to record TV shows before magnetic tape video recording was available. It really was an ingenious solution to the problem. "We can't convert this in real time, so . . . just point a broadcast camera at a screen that can show SSTV." It's sad so much was lost; you can see so much more detail in the images we have of the original, but, as an ad hoc bodge of a solution, it did what it needed to do.

Offline dwight

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Re: How did the SSTV conversion work?
« Reply #4 on: January 15, 2018, 08:21:53 AM »
From my book, "Live TV From the Moon" (pp129 - 131):

Despite the efforts of Westinghouse, their idea for a converter was ultimately never used during any Apollo mission. A scan converter built by RCA was the unit chosen to convert the slow scan TV signal from Apollo 7, Apollo 8, Apollo 9 and the Apollo 11 moonwalk. Their unit very similar to that devised by Westinghouse, used a stock standard video camera which
had seen use in film-to-video telecine, and in the days prior to videotape was also used to record video onto film (a process known as kinescope). It was a black-and white Vidicon tube camera pointed at a 10” high resolution cathode ray monitor. The monitor had a persistent phosphor which caused the image to remain on the screen for longer than normal. The TK-22 was gated to record 1 frame as it was written onto the high resolution screen.

The output from the camera was a standard interlaced NTSC video signal. 1 full frame of video information was composed from two fields of 262.5 lines which the camera could not properly record from the 10 frame-per-second rate. The first field was recorded correctly, but the second field would be recording off the monitor when the next frame of video information was already being written, resulting in a messy signal which generated a lot of problems in the conversion process.

This snag was overcome by recording the first field onto a video disc recorder which would then repeat the redundant field with a delay built into every second field to allow it to mimic the missing field that the camera was unable to capture. Essentially, the TK-22 recorded the first field, with the disc recorder repeating the fields while adjusting them so that they correctly formed a full NTSC image. This process was repeated to form the “missing” 3 frames of NTSC video and the resulting output was a fully compatible NTSC video signal. There was one major drawback, which unfortunately the technology of the time could not solve. The picture was unavoidably degraded as it was optically converted and this on top of the already reduced resolution of the incoming slow scan TV signal.

The system controls were rather straightforward. A test pattern generator was incorporated into the converter to assist in calibration prior to receiving an incoming signal. The type of slow scan signal could also be selected between the 10 and .625 frames-per-second rates, though in the case of Apollo 11, the high resolution mode, while always available,
was never actually used. A variety of controls relating to video synchronization and video level settings were accessible to adjust the signal, in most cases the controls were separately configured for either the standard or high resolution modes. The final output path allowed image enhancement and adjustment of the video disk recorder. The resultant signal could be made as visually appealing as possible, although despite all the controls, the fact was that the slow scan image was optically converted-and that resulted in a loss of resolution of the picture.
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Offline Willoughby

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Re: How did the SSTV conversion work?
« Reply #5 on: January 15, 2018, 11:41:19 AM »
From my book, "Live TV From the Moon" (pp129 - 131):

Despite the efforts of Westinghouse, their idea for a converter was ultimately never used during any Apollo mission. A scan converter built by RCA was the unit chosen to convert the slow scan TV signal from Apollo 7, Apollo 8, Apollo 9 and the Apollo 11 moonwalk. Their unit very similar to that devised by Westinghouse, used a stock standard video camera which
had seen use in film-to-video telecine, and in the days prior to videotape was also used to record video onto film (a process known as kinescope). It was a black-and white Vidicon tube camera pointed at a 10” high resolution cathode ray monitor. The monitor had a persistent phosphor which caused the image to remain on the screen for longer than normal. The TK-22 was gated to record 1 frame as it was written onto the high resolution screen.

The output from the camera was a standard interlaced NTSC video signal. 1 full frame of video information was composed from two fields of 262.5 lines which the camera could not properly record from the 10 frame-per-second rate. The first field was recorded correctly, but the second field would be recording off the monitor when the next frame of video information was already being written, resulting in a messy signal which generated a lot of problems in the conversion process.

This snag was overcome by recording the first field onto a video disc recorder which would then repeat the redundant field with a delay built into every second field to allow it to mimic the missing field that the camera was unable to capture. Essentially, the TK-22 recorded the first field, with the disc recorder repeating the fields while adjusting them so that they correctly formed a full NTSC image. This process was repeated to form the “missing” 3 frames of NTSC video and the resulting output was a fully compatible NTSC video signal. There was one major drawback, which unfortunately the technology of the time could not solve. The picture was unavoidably degraded as it was optically converted and this on top of the already reduced resolution of the incoming slow scan TV signal.

The system controls were rather straightforward. A test pattern generator was incorporated into the converter to assist in calibration prior to receiving an incoming signal. The type of slow scan signal could also be selected between the 10 and .625 frames-per-second rates, though in the case of Apollo 11, the high resolution mode, while always available,
was never actually used. A variety of controls relating to video synchronization and video level settings were accessible to adjust the signal, in most cases the controls were separately configured for either the standard or high resolution modes. The final output path allowed image enhancement and adjustment of the video disk recorder. The resultant signal could be made as visually appealing as possible, although despite all the controls, the fact was that the slow scan image was optically converted-and that resulted in a loss of resolution of the picture.

Thanks!