A. Rutt-Etra Design

Much of my knowledge of scan-processors has come from word-of-mouth: Youtube, the video synth section on Muffwiggler, and the Video Circuits and LZX Video Synth Community Facebook pages. Therefore, I have a patchwork understanding of the detailed workings of the Rutt-Etra scan processor, and will do my best to try to convey which ideas are most important.

One document on Bill Etra & Steve Rutt that is particularly interesting comes from Eigenwelt Der Apparate-Welt: Pioneers of Electronic Art, Ars Electronica, 1992, which can be found here. I believe that this is companion literature to an exhibition of "pioneers of electronic arts through presenting some of the tools and instruments of a particular period of activity"

The linked document contains background and technical information about the Rutt-Etra scan processor. A handy block diagram of the basic scan processor is reproduced below, which is the foundational design for this project:

Anyone familiar with the LZX line of DIY Cadet modules may find this diagram helpful for imagining which modules one might use to recreate the system. Before diving in to the implementation of the above design, I'd like to try to break it down into manageable pieces. Also, I think it's important to note that this is sort of the basic foundation of a vector scan-processor system: by combining other blocks such as oscillators, waveshapers, keyers, etc, throughout the chain, complex results can be achieved according to one's imagination.

0. Multiply/Summing blocks

The first thing that stuck out to me was the amount of multiply and summing circuits in the block diagram: there are 5 multiply blocks and 3 summing blocks. (Something I've learned is that understanding how basic components works really accelerates one's video synthesis journey... I've heard this repeated often by Lars as well as from old videos made by Dan Sandin)

Multiply blocks

There are two kinds of electronic multiplication to consider: 2-quadrant and 4-quadrant multiplication. In 2-quadrant multiplication, 2 unipolar signals, or "factors", are electronically multiplied to create a unipolar "product". For example, if our video luminance is a 0-1V signal, we can multiply this with another control voltage of any kind from 0-1V, and our output will be bounded by 0-1V. Use of bipolar control voltages in a 2-quadrant multiplier will still yield a 0-1V bounded output; -1V * -1V will yield 0V.

On the other hand, we can also perform a 4-quadrant multiplication. Our two signals to be multiplied can be any combination of unipolar and bipolar (-1V to +1V) signals. If we multiply two unipolar signals, we essentially have a 2-quadrant product, as the result will be from 0-1V. If we multiply one unipolar and one bipolar signal, or two bipolar signals, our output bounds are -1 and 1V.

We want 4-quadrant multiplication in this system, because we want the ability to invert signals. The effects of having a -1 to 1 voltage range include negative video image, where black and white levels are switched, as well as mirroring or flipping about the X and Y axes of the raster.

Summing blocks

To me, summing blocks are more mathematically simple to understand. A summer basically adds the voltage of two control signals together. If we have a black video image (0V) and add 0.5V to it, we get a grey field (0 + 0.5 = 0.5). If we have a white field (1V) and add -1V, we will get a black field.

Summing blocks can create interesting effects in a scan processor system; a "classic" effect is adding the video brightness to the vertical ramp signal: brighter portions of the image "pull" the raster upwards, creating a cool 3D displacement type effect. They also serve the purpose of providing control of the horizontal and vertical displacement of the raster.

1. Video Input, Sync, and Ramp Generation

An external video signal is brought into the system; from it, sync information is extracted, which is used to generate two 0-1V ramp signals with frequencies equal to the horizontal scanline frequency, and vertical sync rate. These are the basic components that allows one to "draw" a raster on an XY display.

The top branch indicates the block diagram for additional processing to the video signal. A multiply block multiplies the luminance/brightness information of the monochrome video signal with an Intensity parameter, which could be an external control voltage, a DC voltage from a potentiometer, or a combination of the two. Additionally, brightness is voltage-controlled by summing another signal with the Intensity-multiplied luminance signal. 

Finally, the diagram indicates the sum output should go to the CRT beam, Z-axis, or blanking input of an XY display.

2. Ramp Processing (Size, Depth, Position)

As mentioned before, the multiply and sum blocks provide very useful control within the scan processor system. Namely, we have voltage control over size, depth, and position.

Size

Each ramp is multiplied by a size control voltage; think of this as a scale factor. If we have a unipolar control voltage, at a maximum value of 1V, our ramp is at its original amplitude, and no change in size is observed. However, for values less than 1, we have a linear range of size reductions from 1x all the way down to 0x. The perceived effect of this should be "shrinking" along either the horizontal or vertical axes.

For a bipolar control voltage, we have the same effect of shrinking the horizontal/vertical ramps, but this also inverts them for negative control voltage values. 

• I would be interested in trying is applying a bipolar control voltage to this parameter at a frequency faster than either the horizontal or vertical ramp frequency... would portions of the raster be mirrored/flipped while others remain the same?

Depth
This stage is almost identical to the size stage, with the exception that the Depth control voltage is coupled between both the horizontal and vertical ramp stages. Both ramps should then be scaled in equal proportion, creating a "zoom" effect, I believe.

• CVs to try: external video, hor/vert bars, things that fade from white to black and back

Position
By adding a control voltage to the size and depth-modulated ramp signals, we can finally control the position of the raster on the screen. If using just a DC voltage, we can move the raster to a new fixed location. We could also apply a time-varying waveform to the position control voltage input to modulate the raster position with time.

• This would be a fun parameter to modulate with an oscillator, especially in the horizontal/vertical bar range of frequencies.

This concludes the overview of the Rutt-Etra scan processor block diagram. Next, we will look at implementing this design with the LZX Cadet series of DIY modules.

Further learning: 2- and 4-quadrant multiplication.