Flying spot scanner idea...

[dominicbeesley]

I'm not sure if this has been done but I just had a thought when reading Harry Dalek's interesting flying spot thread. If you could modulate the spot at a highish (say 1MHz) frequency then you could also tune the pickup cell (assuming you're using something like a photo transistor or diode) and amplifier to this frequency.

Then the scanner will work in day light?

Just an idea

D

[AncientBrit]

Hi Dom,

Nice idea.
I suspect the reason it's not been attempted before was the lack of an emitter than would handle such frequencies, but with LEDs it may be a goer. Ditto pickup devices.

Are you going to take the idea further?

Edit
Does it need to be as high as 1MHz. How about 40kHz?

Cheers,

Graham

[Panrock]

The only possible problem I can think of is light saturation of the pick up device. And obviously, using a PMT here is a no-no.

Given the above, I can't see why a flying spot wouldn't work anyway in conditions of ambient light - without any carrier frequency. Have I missed something?

Steve O

[AncientBrit]

Hi Steve,

Thinking about it a little more I think you are right.

If the pickup device saturates from ambient light no signal will be recoverable, modulated or not.

A further consideration is the freq response of a large PE array.

I seem to remember Steve A mentioning that the capacitance of large PE arrays is quite substantial and would benefit from an I to V input stage.

Cheers,

Graham

[Panrock]

If you don't mind a 'pinpoint lighting' effect then small collection areas would be OK.

I understand the frequency response of traditionally sluggish Cds photoconductive cells (ORP12 etc.) becomes adequate for NBTV when light bias is present. Spectral response matches the eye quite well too, with no excessive infrared. The only problem using 'em this way might be poor sensitivity though...

Steve O

[Harry Dalek]

Hi Dom
Very interesting idea i have used a defocused laser before in the green ...lasers work very well as you can defocus it to the scanning area size where as a normal light gos every where.

I think the rethinking of the problem your right still need the light to be detected ...i remember Gary saying the flying spot can camera does work in room light i think so too its one of those things if you can see it so should your detector...more extra light i suppose more signal noise ...

Mines about done we only have lap tops and I this I that in this house and my current laptop only has a mic input i have to hook it up to the old one with line input see how it go's again ...i think its working just hearing the result from the subject reflections but need to see to believe again.

[dominicbeesley]

Thanks lads,

Graham,

I'm not sure if I'll get a chance to do anything this side of Christmas! Too much work and too many other projects...

The reasons it needs to be quite a high frequency is that you'd need to be able to set up a filter or resonant circuit that just picks up the modulated light without adversely affecting the changing picture signal. At 40kHz it's a bit close to the nbtv bandwidth, especially if you're going for 60 or more lines. I'd steer clear of 40kHz anyway as that's the frequency of infra red remote controls.

Steve,

A flying spot won't work very well in ambient lighting conditions as you pick up the whole scene with your sensor. So any signal from DC to your bandwidth could either be movement or the spot changing brightness. There's no way of separating them. However there's not many things that would be moving at 1MHz type frequencies.

CdS and Photovoltaic cells are out, they're too slow. The sensor will need to respond to 1MHz and beyond for this idea. I was thinking more along the lines of the detectors that go on the end of fibre-optic links.

Harry,

That's kind of my thinking. I'm not sure of the frequency response/sensitivity of various sensors when they're picking up a lot/reasonable amount of light. They'd have to have a frequency response up to the modulation frequency then there would be a sharp filter about that frequency(+/- our NBTV bandwidth) to remove all the normal light and just leave our modulated light. Then a simple IF amplifier or synchronous detector.

Picking out a slowly varying signal on top of an ambient signal is difficult but picking out a weak carrier from a lot of noise is fairly easy - its what radios do....

Infra red remote controls for TVs are pretty sensitive for these reasons. If they just sent their digital signal not on a carrier it would be quite difficult to pick them out from the background.

I'll keep this on the back burner and see if I can find a suitable sensor...A suitable light source should be easy. On my first search I found a laser that is good for 20Mhz analogue or 100MHz digital modulation!

[AncientBrit]

Hi Dom,

Not sure of the maths on this but here goes...

Your return signal out of the light detector after filtering will be a low level 1MHz carrier, amplitude modulated by the scene content..

Detecting this signal to produce the embedded NBTV might be a problem due to the low levels.

If you heterodyned this signal with either the 1MHz reference or a derivative, say 31/32 * 1MHz, you could down convert the signal to a more manageable carrier frequency before amplifying and detecting.
(A similiar process to that employed in a Direct Conversion Rx.)

I seem to remember you did a considerable amount of such RF work in your NBTV subcarrier experiments.

Edit.

Okay, forget the above, I have just re-read your post. I see you're thinking of a synch demod which is in essence my idea above

Kind regards,

Graham

[dominicbeesley]

An IF strip for a radio running at 1MHz should be easy enough...thing is how much noise it would pick up.

I'm not sure what the noise / frequency profile of various sensors is so not sure if the S/N would improve with frequency...

Steve Anderson probably has more experience here as I know he's done a fair amount of work transmitting/receiving using modulated light.

D