Oscilloscope photography.

[Steve Anderson]

Dear All,

The following is meant to be constructive, not a criticism.

Taking photos of an oscilloscope dispay is a bit of a 'black art', especially when one is dealing with low-repetition waveforms as encountered in NBTV.

First the hardware. It's neccesary to have a moderately good 'scope that reliably triggers on the appropiate point in the waveform to obtain a stable trace. The trace should be not too bright and sharply focused, the graticule illumination should be visible but not dominant.

The photos attached are of my old Goldstar OS8100 'scope I've had since 1993, it has now been retired.

The camera. Assuming that you're using a digital still camera, don't get fixated on Megapixels. Photo 1 (colour) is a reduction of what came out of the camera, it's 1803 x 1455 pixels, i.e. 2.6 Megapixels. Less by far than most cameras nowadays. But it conveys the data well enough.

Photo 2 is a further reduction and conversion to monochome for publication. It's less than 1 Megapixel and yet still conveys all the information needed. The jpg file size is only 115kB.

These photos show about one and a third frames of NBTV, this results in a 6.25Hz repitition rate on the screen. To get at least a single sweep requires an exposure time of at least one 5th of a second. That's assuming you can release the shutter just at the right moment.

A better ploy is to use a much longer exposure, in this case it was about 4 seconds. You'll need a tripod or some other method of keeping the camera firmly stable. This action intergrates the multiple traces to produce the what appears to be a solid stable trace.

The next consideration is the cameras focusing distance, these pictures were taken with a camera to screen distance of about 200mm, this requires the use of a 'Macro' function, allowing you to get closer than needed for the family snapshot.

Keep the extranious light to a minimum so there is no reflection from the screen which reduces the contast, black-out the room or do it at night with the lights off.

Finally, some software to crop/resize/convert the raw picture. The simplist one I know is Irfanview, but Picture Publisher or Photoshop are equally effective (and more flexible).

I hope this helps, happy snapping!

Steve A.

P.S. For those interested the waveforms are of the action (or more correctly the inaction) of many of the DC-restoring circuits published.

[Andrew Davie]

Of course you're right, and I admit the quality of my oscilloscope photography leaves a lot to be desired. My excuse is that I'm shooting the image with camera in my left hand, while holding the probe tip with my right. The camera is not designed to be used one-handed, so it is a bit tricky.

However, the image below shows that I can do better

This is the waveform I see across the + and - of the LED matrix when it is displaying track 20. This definitely doesn't look right to me, and I suspect there's something wrong with my matrix setup.

[Klaas Robers]

Andrew,
does this say that you connected the oscilloscopes ground to the collector of the transistor and the input wire to your +12V or +17V? This explains a lot.

[Steve Anderson]

Hi Andrew,

Yes, that's a big improvement and pefectly good enough! With slower waveforms at NBTV frame rate you'll not be able to hold the camera steady enough. You will need some way of securely fixing it so it doesn't move during the exposure.

A purpose made camera tripod is best, the one I use cost about US$80 or so and a good investment for general photograhy anyway. A good photo shop should have a selection from about half that price upwards.

As for the displayed waveform itself, I don't have the club CDs so I don't know what it should look like, but it sure does look sick! Hope you get it sorted soon before your frustration gets the better of you!

Cheers,

Steve A.