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The Art of Waves: Image creation on a scope

27 Jul 2015  | Arash Ushani

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Oscilloscope images
Lissajous curves are great, but what if you could draw a face, or some other arbitrary image, on an oscilloscope? By providing the appropriate "X" and "Y" signals, you can create any pattern. In fact, this is exactly how old cathode ray tube TVs worked. To produce an image on the oscilloscope screen, for a pseudo triangle signal on the "Y" input, the "X" input can be computed based on the image's digital information. The tricky part is now how to generate the appropriate arbitrary signal to draw the desired image. At any given time, the vertical sweep specifies where in the image we are currently drawing. For example, assume that our vertical sweep ranges from -1 V to +1 V. When it's at -1V, the CH1 or "X" signal should be drawing the bottom of the image. When it is at 0V, the CH1 or "X" signal should be drawing the centre row of the image. Because we know that the vertical sweep is synchronised to the arbitrary function generator, at any given sample in the arbitrary waveform, we can predict exactly where the vertical sweep will be. In this manner, we can create our arbitrary waveform. This is similar to how some video formats work.

At times, the arbitrary signal needs to be both -1V and +1V at the same time. What can you do then? You can tackle this by quickly cycling through the different voltages you require. The signal can stay at -1V for a short period of time, and then quickly jump to +1V and stay there for a short period of time, and repeat. You will, however, need to make sure that your arbitrary function generator has a fast rise time and the oscilloscope has a sufficiently high bandwidth.

You can take any black and white image (ie, a pixel map of where to draw and where not to draw with your signal.) I've written a simple python application that does this and converts a given image to a waveform, saved in a csv file that you can load into a function generator. This arbitrary waveform itself looks like gibberish. But, when viewed in XY mode with the vertical sweep, the original images appear. Figures 4 and 5 show the results.

Figure 4: Dolphins on an oscilloscope? Who knew?

Figure 5: What would Elvis say if he saw himself on an oscilloscope?

Generally, images with sharp edges such as cartoons produce sharper plots. In these plots "snowy" effects are the result of the generator's output filter limiting the bandwidth. In an ideal situation, you need infinite bandwidth both for the generator and the oscilloscope to eliminate the effect. Also, improving the digital-signal-processing algorithm will enhance the quality of the plots. Nevertheless, seeing the images on the oscilloscope screen are indeed enjoyable. I encourage you to try this out yourself. But be warned: it can be a lot of fun and somewhat addictive. There's nothing quite like using state-of-the-art technology to draw the face of your favourite EDN editor, see the video below.

About the author
Arash Ushani contributed this article.

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