Hi. What to expect, if a device running with a say 20MHz crystal/oscillator gets a 0.2MHz instead ?
Would its output be 1/100 the speed ?

As in a plain CCTV camera as
----> https://www.aliexpress.com/item/CVB...rch0104.8.80.30c361cbtHeQIV&priceBeautifyAB=0
(shown there without the lens)

 

What would happen depends on the specifics of the device.
Most often I suspect it wouldn't do anything useful.

 

Assuming that all video timing information is derived from the 20MHz xtal, changing the clock frequency would make the CCTV video output useless.

 

Assuming that all video timing information is derived from the 20MHz xtal, changing the clock frequency would make the CCTV video output useless.

What if the intention is obtaining slow scan video, on narrow band audio ? (SSTV)

 

Also, if the scan rate for the optical sensor array is derived from the crystal oscillator (and it almost certainly is), reducing the crystal frequency by a factor of 100 will totally screw up the sensor's operation.

What to expect, if a device running with a say 20MHz crystal/oscillator gets a 0.2MHz instead ?

Complete non-functionality. Don't even bother.

 

I am going to assume this is related to your other thread:
https://forum.allaboutcircuits.com/threads/old-pots-videophones-slow-scan-tv.147473/

If it is a CCD camera then the image would be very poor since the pixels would not hold their charge long enough. A CMOS camera might be better but I am not sure. Maybe @WBahn can be more definitive on this.

Most inexpensive cameras are CMOS. If the CMOS camera does have a problem with leakage in the pixels then cooling the camera will help. This is commonly done in astronomy where long exposures are needed.

Camera modules have gotten extremely inexpensive and the experiment would be easy to do. Another way of doing it might be to modify the auto-exposure signal in the camera. The way this circuit works is by changing the integration time for the light on the pixel capacitance. The longer the integration time the more sensitive the camera is.


p.s. I just saw your reply confirming my assumption...

 

To get slow scan video you have to use a circuit and sensor specifically designed for that task.

 

Hello,

Perhaps a book on how SSTV works might give you some ideas:
http://www.sstv-handbook.com/

There are also many websites around on the topic:
"slow scan tv" schematic

Bertus

 

To get slow scan video you have to use a circuit and sensor specifically designed for that task.

This may be true but what fun would that be?

 

If it is a much older CCTV camera, with a vidicon tube,then possibly there would still be some functionality. But it would not work for slow-scan amateur TV because there is a lot more to it than just slowing things down. Also, finding a monitor would not be possible because slow scan monitors use an entirely different scheme. So while it is an interesting concept the results would just be an education as to what does not work.

 

Silly me! I was thinking about a B/W camera. A color camera will not give any useful color picture when run at any but the correct crystal frequency.

What you really want to do is a "scan conversion". First capture one frame from a standard color or B/W camera. Then dole it out at a slower rate.

The capture is done by digitizing the video into a RAM. For instance, if the image you want to send is 100 lines by 100 pixels you would have to store 10K bytes of pixels. This could be done with just about any reasonably fast microcontroller with an A/D converter and a D/A converter.

 

Wait... do you like such low frequencies because they approach audible ones, and you want to have a bunch of annoying buzzing stuff around you? Because otherwise you want to have the highest frequencies possible to get better computing. Why do you think that 2.4 GIGA hertz is often used for wifi?