Ancient slow scan television, narrow band...

Can you suggest how can this be done ? Analog SSTV, NO computers, no high speed, nothing digital. Same way it was done in the sixties...

Any sample circuits for the modulator and the demodulator sections that you can link ?

What considerations are missing ? A modern surveillance camera does not scan slowly... How is its video interfaced into the modulator ?

SSTV is still widely used. The classic method is to digitize from a normal video source at normal speed then read samples from the image at slow speed. As far as I know, there are no 'live' converters but as each frame typically takes 15 seconds it isn't possible to send motion images anyway.

sources of information:
http://www.nbtv.wyenet.co.uk/
http://www.batc.org.uk/

Brian.

A lens can project a focussed image onto a grid of photosensors.
The photosensors could be scanned by doing these steps in rapid succession:

* read one sensor at a time,
* transmit its value,
* go on to the next sensor,
* after reading last sensor transmit 'end of frame'
* go to first sensor

The sensor value can be plain Hi or Low (light or dark), or it can be fed to ADC if you can make the system fast enough to process a few bits per sensor.

The sensor grid can be:
* homemade from individual components,
* coarse Charge-Coupled Device (old articles tell how experimenters might fabricate this),
* illumination unit from an early cheap low-resolution VCR projector (but only if it can be made to output voltages as you illuminate its grid).

These ideas are only speculation on my part.

Thank you, Brian.
Am fine with just a few images per minute.
What device/circuitry will take the analog camera video signal and 'slowly' feed the modulator one image/frame; when done repeat with the next current camera frame ? Any links to sample circuits would be helpful.
How was a camera interfaced in the sixties ? Thanks for the links. I asked for assistance to the batc one.


Thanks, Brad.
If a typical monochrome camera like this has, say a 20 MHz crystal oscillator built-in for operation; and produces 30 frames per second. What would happen if the oscillator is replaced with a 0.2MHz oscillator ? Would the scan speed be 1/100 of 30 frames per second?

- - - Updated - - -

Expanding the post...
With a plain CCTV camera and a plain small LCD video monitor; how can I transfer images like these ?

----> https://www.youtube.com/watch?v=u3k6Xt30Z7g

I don't expect that any recent CCD camera can be easily operated at 1/100 scan speed. Classical CRT (Vidicon) cameras could do it, redesigning the deflection circuit is neither simple, but it probably would better fit this retro project.

A todays solution would be to replay the video data out of a digital frame buffer.

I would be very surprised if you could slow down a CCD that way. There are other consequences of dropping the clock speed such as much longer pixel exposure and wrong line to frame speed ratios.

Older systems used slow current ramps to the deflection coils around vidicon/saticon tubes as FvM stated but finding them these days is difficult. Also consider that most SSTV these days is in color using field sequential scanning (line 1 - R,G,B line2 - R,G,B ...) which is not the same format that CCD cameras produce.

If you are prepared to use digital techniques, a frame buffer is the answer but you might want to experiment with software generated SSTV first. There are many programs for PCs that use the OS to read in a camera image (or a still image from disk) then do the sampling and sound generation in software. I use "MultiPSK" which is for Windows but works well in an emulated environment from Linux too.

Brian.

On a lucky strike, found a camera that directly outputs the SSTV in audio form. :-D

If you can suggest the second half, please advise : Decoding the SSTV audio into video for a plain monitor. No computers involved.

The application is very unusual:
Camera audio------amplifier------speaker . . . . . . . . . microphone------preamplifier------decoder-----videomonitor

So far, have found only a SSTV decoder application to display on a smart phone.

It's not possible with a plain analog monitor. Either need a storage CRT or some kind of digital video storage that refreshes the picture periodically.