Video On Compact Cassette

[G6Tanuki]

Originally Posted by 19Seventy7

I've never heard of slow-scan TV before, but I've just googled it. Is it a software and phone application, or are there special TVs for this?

Slow-scan TV - as used by radio-amateurs - was initially displayed on specially built receivers using "long persistence" cathode-ray-tubes, often ex-military ones designed for Radar applications. A SSTV "frame" took around 20 seconds to be sent, and so the phosphor of the tube needed to have a long after-glow.

The on-screen images were very dim, faded out quickly, and usually green or yellow because of the phosphors used. Some people saved the images by using Polariod cameras originally designed for use with oscilloscopes.

Later on, various 'frame store' circuits were developed which digitised the incoming SSTV and stored it line-by-line in solid-state memory [RAM] - this could then be rescanned/re-modulated at a frame-rate suitable for displaying on a normal TV.

These days a SSTV receiver is easily implemented using the audio 'line in' on a PC running suitable software.

See here: https://www.essexham.co.uk/sstv-the-basics

for more info.

[electronicskip]

Here is the PXL-2000 in action.

https://www.youtube.com/watch?v=TCXJ5twf5tM

[19Seventy7]

Hi,

I've since watched that video and found it quite interesting,

I'm glad this topic was brought up again as I had a thought.

Could one of those little 5" b/w Chinese TV's be converted to accept video from a compact cassette?

I think it'd make for an interesting project, if it's somehow possible.

Thanks

[Graham G3ZVT]

Originally Posted by G6Tanuki

Slow-scan TV - as used by radio-amateurs - was initially displayed on specially built receivers using "long persistence" cathode-ray-tubes, often ex-military ones designed for Radar applications. A SSTV "frame" took around 20 seconds to be sent, and so the phosphor of the tube needed to have a long after-glow.

The on-screen images were very dim, faded out quickly, and usually green or yellow because of the phosphors used.

I built such a monitor forty-mumble years ago. It used a 5FP7 radar tube with an amber filter. Scan coils from an old 70 deg TV, EHT from a Thorn Jellypot with a doubler tray driven by, if I remember correctly, ECC82 - 6BW6 the rest was all solid state.
I also built a not very successful Flying Spot Scanner for transparencies, that also used a 5FP7 this time with a blue filter and a 931A photomultiplier. Versatile things those 5FP7s!

There was a little known analogue sampling technique whereby you could convert a standard videcon security camera to SSTV, and I had a lot of success with it.

Let's see if I remember the principle.

You turn the camera on its side so that the 15.625 ls scanning vertically and the 50Hz horizontally. The only mod I had to do to the camera itself was put a simple unijunction transistor relaxation oscillator in the frame drive (which is now the line drive, remember?) and set the frequency so it effectively divided by 3 giving you your 16.666Hz SSTV line frequency. A toggle switch restored normal 625 operation when required.

The sampler was an external board, it had two ramp generators one triggered by the 15.625kHz from the incoming video and the other free running at 1/8Hz (or 8 seconds period).

The two waveforms are mixed and differentiated to produce a narrow
pulse of about 1 microsecond that slides across (actually down) the
fast-scan raster.

This sliding 15.625kHz pulse is used to open a gate, typically an FET,
and let samples of the video through. These are integrated to remove
the quantization, and SSTV syncs added using monostables triggered
from the 1/8Hz osc and the 16.66Hz from the camera.

The baseband signal modulates an audio oscillator and level controls
provided to adjust for correct sync, black and peak white AF tones.

[G6Tanuki]

My SSTV kit was all solid-state except for the CRT [an American 1960s RADAR-tube with a screen around 12 inches across - it was at least square so the image could be made to fill all the screen ] - I used 741 op-amps for the signal-processing and pairs of AD161/162 Geranium transistors to drive the line/frame coils [which were salvaged from an old 405-line TV].

These days it's all so much simpler - an extremely-obsolete laptop running Linux and associated freeware SSTV-decoding software. DSP to 'clean-up' the incoming image, save it as JPEG, downscale the image you want to send from the high-resolution original, overlay captions etc - all so easy! No more flying-spot scanners or transparency hassle.