Phosphor For Dummies
By Vincent Adoxo
 
An oscilloscope is a type of electronic test instrument that allows observation of constantly varying signal voltages—in this case a stereo audio signal.
 
The one we use is a cathode ray oscilloscope, the earliest and simplest type. Our friend Steve (who is also Phosphor’s ‘in house’ film maker) found it in a skip. The name Phosphor comes from this technology; the inside of a cathode ray’s screen is coated with phosphor, the substance that produces green luminescence.
 
In it’s most common mode, an oscilloscope in action may look something like a heart rate monitor. If you don’t know it by name, you’d recognise one if you saw it. Feed it a sine tone you will see a wavy line on it’s screen.

Our Oscilloscope displaying a sine wave
 
But most modern oscilloscopes have two (sometimes more) inputs, and thus can be used to plot one varying voltage against another, useful for graphing what are known as “lissajous figures”. Lissajous figures are an example of how an oscilloscope can be used to track phase differences between multiple input signals. This is frequently used in broadcast engineering to plot the left and right stereo channels to ensure that the stereo generator is calibrated properly. When two inputs are plotted against one another like this, it is known as X/Y mode.
 
In X/Y mode there is a single dot in the middle of the screen, which is constantly lit. When an audio signal is played into the left channel, the dot will move left and right (across the X axis). Similarly, if an audio signal is played into the right channel, the dot will move up and down (across the Y axis). This is all that happens—a dot moves very quickly. There is no image conversion because there is no image, only a moving dot. There are no frames of animation because there is no animation, just the illusion of moving shapes through persistence of vision*.


Offsetting the right channel by 90 degrees produces a circle.
 
A sine tone sent to the X axis (left audio channel) and a second sine tone of the same frequency sent to the Y axis (right audio channel) but 90 degrees out of phase, will produce a perfect circle on the oscilloscope screen. Change the frequency, wave-shape, phase relation, amplitude and number of these tones and you’ve got yourself a party.


Change the sine waves to square waves for a square(ish) shape.


Experiment with mixing tones and adding audio effects

 
This is the basis of everything we do to create the Phosphor show. Almost everything you see is the product of experimenting with simple tones from test tone generators and synthesisers, layering them up and mixing them together in music performance software.
 
In the case of the Clark logo and other words, we used a tiny piece of software called rabiscoscopio, written by a hobbyist hacker called Alex (http://dalpix.com) to convert very simple vector images into wave-shapes that, once combined in X/Y mode, look a bit like said vector image. This is the extent of our ‘cheating’. Once the Clark logo is produced, we drag it into our music software for mixing up with the rest of our weird sounds, in our usual experimentative way. No tutti fruity.

The Music Video

So far we’ve produced two music videos for Clark. Each time, what you see in the film is one shot of the oscilloscope—there are no video effects and no video editing.

On Superscope we took great care to represent the image you see on the oscilloscope screen as closely as possible in the final video, renting expensive cameras to do avoid distortion and flickering as a result of the rolling shutter effect you see with not-so-expensive ones. (Thank you to Focus24 for hire of equipment and space).

This is us filming the scope at Focus24 in London

With the Riff Through The Fog video (co-directed by Steve Bliss), we went a totally different route by using the rolling shutter effect to our advantage. Here's an example of the rolling shutter effect in action on the blades of a spinning fan:

And below is an example of how different the image on the oscilloscope’s screen can look when we steep through the cameras shutter speeds (which also has the effect of darkening the image).