Mini version of Scophony - its do-able

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Re: Mini version of Scophony - its do-able

Postby Gregory » Mon Dec 28, 2020 7:14 am

Scan circuits for line and frame

2020_scan_schematic.jpg
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This is what I used originally which worked for the line scan well, but was unfinished for the frame scan. Both Line and Frame use the same circuit, but with different timing values.

Basically the 4046 operates as a VCO using the internal analogue comparator 1, which compares the signal from the mirror sensor with the incoming sync signal. The 2 mono-stables ensure the mark to space ratio is 50:50 (optimized for 60 lines but less for 32 lines) which the analogue comparator works best at.The output voltage of the comparator varies accordingly and controls the VCO thus synchronizing this to the incoming sync signal. The VCO centre frequency is set to the normal clock frequency required to drive the stepper motor. Q4, R13,R22, and C12 are use to provide the slow ramp up which the stepper motor requires. This probably will not be required for the frame scan as the stepping rate is far lower.

For 32 lines format

Line scan motor speed for 15 sided mirror = 60*32*12.5/15 = 1600 RPM
Frame scan motor speed for 8 sided mirror =60*12.5/8 = 83.75 RPM

For 60 line format

Line scan motor speed 1600*60/32 = 3000 RPM
Frame scan motor speed = 83.75 RPM
Last edited by Gregory on Thu Jan 28, 2021 7:59 pm, edited 3 times in total.
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Re: Mini version of Scophony - its do-able

Postby Harry Dalek » Mon Dec 28, 2020 2:03 pm

DO you have the part values for both circuits come in handy...feed back to a stepper motor clock good idea
As long as you have line scan in sync you have some thing to work with frame scan can be worked on later as you have an image with the line side synced ,perhaps move on to the laser modulation side of things and get back to it ..i would go with PWM go AM and it could kill your laser in a blink of your eye fussy little buggers they are .
I have used this circuit many times for a laser just match your laser to the Zener or its at least an easy to adjust to your wants.
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The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Wed Dec 30, 2020 9:39 am

Very nice and simple circuit Harry, only for 60 lines around 40 - 50kz bandwidth will be needed so the PWM will have to be around 200khz min. This should not be a problem, but may need a faster comparator, I don't know. For the moment I will leave the laser driver for last and will just run the laser diode at very low power 1 mw to test the line and frame raster with a series resistor and zener diode across the laser diode.

I am starting to construct the line and frame scan circuits on breadboard for now, and hopefully will have this completed in the near future.
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Re: Mini version of Scophony - its do-able

Postby Steve Anderson » Wed Dec 30, 2020 2:02 pm

I built a laser-link in 2009 which appeared in the newsletter around that time. PDF attached. I had no problems with the LM311s speed, I would think it's OK even though you are planning to use a higher frequency PWM carrier.

Looking at it over ten years later the receiver could be vastly simplified.

Later...looking at the datasheet for the LM311 it should be quite OK with a 200kHz PWM carrier, perhaps even up to 500kHz. Of more importance will be keeping the capacitance down around the photodiode. In the attached PDF you'll notice I used a common-base arrangement for the photodiode load, (TR201), this means there's very little voltage variation in the voltage across the photodiode negating the bulk of the junction capacitance of the photodiode. Also there's 5-6V of bias across the photodiode reducing its capacitance further.

Whatever photodiode you use make sure it has a clear encapsulation, not the usual black stuff. The BPW34 is available in both clear and IR-only versions. Compared to most it's quite a large-area device which means a large junction capacitance, hence the measures taken to negate it.

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Re: Mini version of Scophony - its do-able

Postby Gregory » Thu Dec 31, 2020 7:25 am

Thanks Steve, a very interesting circuit. What do you think would be the right PWM frequency I could use for 30 and 60 lines resolution?

I can see myself blowing up a few laser diodes before I get there.
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Re: Mini version of Scophony - its do-able

Postby Harry Dalek » Thu Dec 31, 2020 1:57 pm

Run the laser under its rated till you know its working then run it up to correct operating voltage and use a current limiting resistor will do the job as it does for LEDs. If the resistor limits the current to a low enough value you can operate the laser continuously simple as it is used the idea for the mechanical sstv uv laser many years back..and still works .
The PWM operating frequency is some thing i didn't really think that much about apart from matching it to what my lap top could output this could be wrong i have read it should be twice the highest frequency of what your try to PWM...using the idea at the moment mine is at around 40 khz ....i can increase this to what ever my 555 can do ,i suppose the higher the better but i can see Steve just kept it equal to audio in put bandwidth ...i am using PWM phonoVision recording so dealing with it my self at the moment ....so yes to low is no good but to high does it matter apart from able to handle higher bandwidth video ?
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Re: Mini version of Scophony - its do-able

Postby Steve Anderson » Thu Dec 31, 2020 2:20 pm

Gregory wrote:What do you think would be the right PWM frequency I could use for 30 and 60 lines resolution?

For 30/32 lines at a frame rate of 12.5Hz the arrangement in the PDF above is fine, that's what it was designed for. One could argue that the bandwidth is a bit excessive at 18kHz and perhaps could be reduced to 12kHz.

For 60 lines with an equal increase in line resolution at the same frame rate will require four times the bandwidth, 72kHz. But if 12kHz is used as a baseline it becomes 48kHz. So if we also increase the PWM carrier frequency by the same factor the PWM carrier becomes 200kHz as per your idea, perhaps a little less, say 150kHz.

Whether those circuits could be nudged up in frequency to those figures, I'm not sure. I think I did try a higher PWM frequency but the oscillator/ramp arrangement became 'not so good' if I recall correctly.

As I mentioned, certainly the receiver could be massaged to quite an extent...

I should try and find a replacement for the BF450 which is not so common, a 2N3906 looks promising...maybe...though Cricklewood Electronics in the UK still list the BF450. An ideal replacement would be a PNP silicon (not Germanium) transistor used in I.F. stages. Any suggestions? Preferably one still in current production.

There remains the question whether these laser diodes can be modulated to 200-odd kHz. There's only one way to find out!

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Re: Mini version of Scophony - its do-able

Postby Gregory » Fri Jan 01, 2021 7:01 am

Thanks for all the info Steve.
One more question whilst on the subject of bandwidth. For 60 line resolution I have seen that wav files can be converted to 60 res using Video2NBTV V3.0 software.

Now for the 50-70 kz bandwidth required, can this be produced by a PC (perhaps with a special sound card) or what other device currently on the market ( and not too expensive) can one easily use for this ?
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Re: Mini version of Scophony - its do-able

Postby Gregory » Fri Jan 01, 2021 7:17 am

Harry Dalek wrote:Run the laser under its rated till you know its working then run it up to correct operating voltage and use a current limiting resistor will do the job as it does for LEDs. If the resistor limits the current to a low enough value you can operate the laser continuously simple as it is used the idea for the mechanical sstv uv laser many years back..and still works .


Thanks Harry, yes initially I will try running the laser diode this way you suggested, by limiting the current using a resistor to limit the maximum current and some sort of current driver, I have seen the LM317 used in several circuits.

Later when I get courageous I may try the PWM option, as this may give a better and more linear performance.
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Re: Mini version of Scophony - its do-able

Postby Steve Anderson » Fri Jan 01, 2021 4:24 pm

Gregory wrote:Now for the 50-70 kz bandwidth required, can this be produced by a PC?

If your PC can sample audio at 96kHz, in theory the bandwidth should be 48kHz, plus/minus any filtering built in or required. Some (but few) PCs can sample at 192kHz - plenty, yes, but BIG files as a result. Though not that big in todays age. But if you wanted to upload them to this forum you'd hit the 20MB limit per file very quickly. About 52 seconds stereo (V/A) at 96kHz, twice that if mono (video only).

I'm not familiar with the software you mention, perhaps others here are...

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Re: Mini version of Scophony - its do-able

Postby Gregory » Sat Jan 02, 2021 3:48 am

I'm not familiar with the software you mention,

Sorry software should be Video2NBTV Version 3
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Re: Mini version of Scophony - its do-able

Postby Harry Dalek » Sun Jan 03, 2021 8:26 am

Gregory wrote:
I'm not familiar with the software you mention,

Sorry software should be Video2NBTV Version 3


With all this we are stuck with what your PC sound card can output so what ever software we use it can only output that, results 60 64 line are still pretty good with what must be some loss due to this .

FreeNBTV you can adjust line and frame rates so you could do a no standard test card or video that should fit what ever you have 45 line 50 line or 32 with different frame rate what ever you like .

It sort of go's into another project such as a closed circuit system camera or test card generator where its fed direct to the monitor but the results out of a pc at 64 line look pretty good to me as is at least till you have something better.
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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Re: Mini version of Scophony - its do-able

Postby Gregory » Sun Jan 03, 2021 10:29 pm

Thanks Harry - I downloaded freeNBTV but unfortunately it comes up with dwmapi.dll file missing. Maybe its because the OS is Windows XP. If necessary I will install Windows 7 as freeNBTV sounds very useful.

Back to the electronics I managed to dig up the old electronics used 4 years ago but one stepper drive is missing. What a mess so this will need rebuilding, luckily I have spare stepper drives.

old electronics1.jpg
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STEP 1
To build Line scan and frame scan circuits on one of the breadboards, with some sort of chassis and front panel for the controls
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Re: Mini version of Scophony - its do-able

Postby smeezekitty » Mon Jan 04, 2021 8:38 am

Gregory wrote:Thanks Harry - I downloaded freeNBTV but unfortunately it comes up with dwmapi.dll file missing. Maybe its because the OS is Windows XP. If necessary I will install Windows 7 as freeNBTV sounds very useful.

Yeah it requires VIsta or newer. XP is 20(!!) years old this year
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Re: Mini version of Scophony - its do-able

Postby Harry Dalek » Tue Jan 05, 2021 5:58 pm

XP i love but sounds like it doesn't work with FREENBTV :( ,look for Free Tiny Windows 7 iso !
What steppers are you using ? bipolar ,unipolar ..hard to get the the line speed with a stepper be fine for framing mirror drum ,i used a pulley system to get the line speed when i used that idea but gave the idea up using just a DC motor no clogging...never tried the Chinese stepper drivers might be better than my go Keep working on it Watching with interest !
The electromagnetic spectrum has no theoretical limit at either end. If all the mass/energy in the Universe is considered a 'limit', then that would be the only real theoretical limit to the maximum frequency attainable.
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