HomeCorrecting Images of Magnetically Distorted Oscilloscope Display

This is a Sidebar of Main Article, A Closer Look at Fender Optical "Vibrato" (Really Tremolo)

Photographs of the analog oscilloscope screen in the main article about Fender optical tremolo and in the sidebar demonstrating the neon lamp dark effect were skewed by four degrees to compensate magnetic distortion of my 'scope's cathode-ray tube (CRT). Unfortunately, I realized the problem only after the experiments were completed and the photographs were taken. The source of the magnetic field was a Yamaha NS-10M monitor (loudspeaker) resting on a shelf above and to the left of my Hitachi V-212 oscilloscope, only about 14 inches away from the instrument's CRT.

To demonstrate the distortion, I photographed the 'scope while it was displaying a 1250-Hz square waveform on both channels. The "undoctored" images are on the left side of the composite shown below. On the close-up of the CRT (top left photo), take a ruler and hold it up to your screen so that one of the square waveform's state changes lines up between the upper and lower trace. Notice how your ruler is not exactly vertical (it leans toward the left) and that the problem is within each trace as well as between them. If you look carefully you can see the grid lines on the 'scope's screen; note how the waveform's state transition edges are not parallel to the vertical grid lines.

Compensating Magnetic 'Scope Skew in Graphics Software
On the right-hand side of the above composite, a copy of each "undoctored" image was transformed using a four-degree "horizontal skew" in Microsoft's "Paint" graphics software. This is the same method I used to correct (compensate) the oscilloscope photos in Figure 5 and Figure 11 of the main article, and in the dark effect sidebar (after which those images were cropped to give them a rectangular aspect, unlike the "raw" skewed photos above). In the top-right photo above, again use your ruler to line up the waveform's state transitions; notice how the ruler is more vertical than when you inspected the undoctored photo in this manner, at least for most of the display. Sure it's a compromise, because the magnetic distortion is not uniform across the CRT.

After my tremolo and neon lamp experiments, I removed the NS-10Ms from the vicinity of my bench (now using small, cheap speakers there). After re-adjusting the 'scope's trace rotation trimmer, its display is much less skewed. A residual skew of approximately one degree remains, which could be a "memory" of the NS-10M's presence, in which case I may eventually try my old tape-head de-gaussing wand on the 'scope. But we do live on a planet with a magnetic field, and sometimes our analog oscilloscopes are sensitive to that as well.

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Sidebar Demonstrating Neon Lamp Dark Effect

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