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01-16-2019, 03:12 PM
Iain_F Iain_F is offline
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Dear All,

I have a non-VHS video capturing project that is *almost* working for me...

The Source
Is an electron microscope, nearly 30 years old.
It fires a beam of electrons at the sample, captures/measures the electrons coming back from the sample and that signal strength determines what level of grey(gray) is assigned to that pixel. The beam then moves on to the next pixel, and the next, scanning in a raster until it has completed the image. Repeat ad infinitum. Though it's analogue, so the beam moves continuously and there isn't really a "pixel".

In really old machines, the signal was directly displayed on a CRT screen, scanned in synch with the sample scanning, and was never any kind of TV-related video standard. But my machine was at the leading edge for it's time (and is still quite a capable machine), so this is different.

My signal is digitised and put into a frame store (which is a huge circuit board absolutely covered in little memory chips, half a megabyte took up a LOT of space in 1989). This is then read out through a DAC to produce a video signal based on the UK's 50Hz 625 line PAL standard. Except that it's a monochrome signal, so the "PAL" is redundant.

This is presented via a BNC coax connector, and was intended to be connected to a video monitor, probably of the kind that would have been used in a video edit suite of the time (late 80's early 90's). Sadly, when I acquired it, it no longer had its original video monitor and was working through a VGA computer screen driven by a little box that converts a composite video signal to VGA. That worked, but was ugly and gave a poor quality image on screen.

I acquired a 14" SONY CRT professional video monitor of late 90's vintage, and that was much better, but also has problems (suffers from magnetic interference from the microscope and is big enough to make maintenance access tricky). I added a cheap consumer-grade, USB VHS-to-DVD converter so that I could digitally grab stills and could record the video. This worked, and gave superb picture quality on the computer screen, but...
1) There is a significant time lag, making it difficult to use the PC screen for monitoring the image while tweeking focus, position etc. etc.
2) It doesn't capture the full video frame. I can see a lot more picture at the bottom on the analogue monitor than I can on the digitally captured video.

What I need to do is the following:-
Absolutely essential
* Capture and save to hard disc a good quality video of the work that I'm doing, ideally capturing the full frame. (This is an unusual requirement for a microscope of this type, they are usually just used for taking stills).
If at all possible
* Display the captured digital video stream in near-real time, so that I can use this good quality display when adjusting focus/brightness/contrast etc. etc.
Highly desirable
* Grab still frames.

I'm hoping that I can improve on the time lag problem by using an internal capture card instead of a USB (AverMedia CE310B or C725B perhaps?), or by using a Canopus ADVC100 and a fire-wire input card that I have just got, or possibly using a USB3 capture device, if such things exist at an affordable price. The PC I've been using so far is a Core2Duo at 3.1 GHz with 8 GB RAM and windows 7 64 bit, so not too shabby, but could be greatly improved on (though fire-wire support might be poor or non-existent on newer hardware/software, which might take away the ADVC100 option). A old Mac (with firewire) is also an option. Your recommendations would be very welcome.

I have input the signal to the various monitors and USB capture devices that I have tried via a co-ax composite input (BNC or RCA depending on what piece of kit it was) or via the "luma" pins in an S-VHS input (with the "chroma" pin connected to ground). Both seemed to work equally well, probably because the video source is very high quality, the signal isn't transmitted very far, and it has no chroma to get messed up.

I have mainly used VLC to display, grab stills and record video so far. I have tweeked the image cropping settings in VLC, but still I do not get the full frame. With zero cropping I get the full frame at the top of the image, I get the full frame plus a 2-pixel black line at the left side, the full frame plus a 14-pixel black bar at the right side. But at the bottom the digitally captured image cuts off at the top of a menu/databar which is visible in full on all the analogue monitors I have tried. The properties of the captured image say it's 720 X 576. I know that in UK "625 line" broadcast TV there were only supposed to be 576 lines of active, displayed video. The remaining lines were there to accommodate imperfections/misalignments etc. (and eventually were used to transmit data for the "teletext" system).

I suspect that my microscope is generating a lot more than the 576 lines of active video that the 625 line broadcast video standard(s) call for. It may even be generating a truly 625-line picture. Any suggestions on how I might capture that?

Digitally capturing the data direct from the frame store, or intercepting it digitally at the input to the DAC would be an elegant solution and is theoretically possible, but I don't have the skills or probably the resources to do it. I have full circuit diagrams, but no information beyond that. The company that built the microscope probably won't give me information on the signals that I would need to capture, because they would probably prefer that I buy a new one (waaaaay beyond my budget!). Even if they were willing to tell me, everyone who used to know has probably retired by now.

I hope you like weird questions...
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01-16-2019, 05:19 PM
hodgey hodgey is offline
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Normally a capture from a capture card would show more than a CRT would, as some of the image will be hidden by overscan. Professional monitor can sometimes be set to underscan to be able to see the whole image, so I guess the first question is whether it's there is any such setting on the one you are using that makes it display more of the image than normal, maybe you could test it with some other source.

I would also suggest trying to capture from the USB device with VirtualDub in case it's VLC that's doing something weird.

If it is actually sending out video outside the normal number of lines, it is possible on some capture cards to move the capture window around. It's usually not an easily available setting though. On my older PCI Hauppauge card with a conexant 878 fusion chip it was possible with VirtualDub and the Bt8x8 tweaker option, on my Diamond VC500 USB capture card I found I could do it by changing some settings using a command line I2C app on linux.
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01-17-2019, 01:43 PM
dpalomaki dpalomaki is offline
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Does any of the documentation you have give details on the actual image / raster format of the machines native output. (it sounds like the port to using a TV monitor was an after thought to enable use with commonly available gear)?

The ADVC output is a DV stream via firewire and will be limited to the DV raster size (576 lines for PAL). You will not see any of the information outside that raster. On the plus side DV's color limitation will not be an issue for you, but the DV stream is still an 8-bit lossy compression format that might not be what you want for scientific data purposes, although it is arguably better than the analog B&W formats of that era.
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