Repurposing a 1970's Vector Scope

A cathode ray vector scope is an analog display device that uses a cathode ray tube (CRT) to graphically represent signals in a two-dimensional space. Unlike a standard cathode ray oscilloscope, which typically displays time-varying signals as a function of time, a vector scope plots one signal against another, typically in an X-Y coordinate format. The result is a visual representation of the relationship between the two signals, often referred to as a vector display.

I was fortunate to have picked this instrument up at a flea market for a giveaway price that I couldn’t refuse. After seeking help online I determined that it was a vector scope display module EV8000 made by Electronic Visuals Limited of Staines in Middlesex England around 1972. The manufacturer’s “handbook” describes the instrument as follows:

I first performed some checks and determined that 68uF electrolytic capacitor on the z-amplifier was open-circuit, and a 2k2 wire-wound resistor was also found to be open-circuit. The ‘power on’ neon indicator was replaced as it appeared burned inside. Slowly powering up the unit using current limiting, I was pleased to see that the instrument appeared to work, displaying a bright orange dot in the centre of the graticule.

CRT vector scopes were widely used in their time for various uses in engineering and physics, such as to study the relationship between two oscillating signals, such as sine waves or waveforms in Lissajous figures. Instead, I had the idea of displaying moving 3D objects on this instrument. But first I somehow needed a way of connecting the vector scope to a computer. I decided to use the Digilent Analog Discovery which is a compact, USB-powered test and measurement device designed for use with personal computers. It is a versatile tool popular among students, hobbyists, and professionals for its ability to function as multiple test instruments, such as an oscilloscope, signal generator, and logic analyzer.

The vector scope “handbook” quotes an input impedance of 50Ω and a sensitivity of 0.5mA/cm for both the vertical (Y) and horizontal (X) deflection amplifiers. The Analog Discovery device has two output voltage waveform generators, but as already mentioned the vector scope deflection amplifiers are designed to accept input currents rather than voltages, and so a signal conditioning board was also needed.

A suitable signal conditioning board was devised for plugging into the edge connector at the back of the EV8000, with a 4-way header for connecting to the X and Y outputs from the Analog Discovery. Using two non-inverting operational amplifiers configured with feedback creating a voltage-controlled current source. The op-amps can accept either a uni-polar or bi-polar voltage input range by either applying an offset voltage to the inverting input depending on the position of the jumpers. This adjustment ensures the output current range matches the output sent by the Analog Discovery. Dual op-amps in a single DIP-8 package have been used for the X and Y signals, the trimmer pots are used to adjust the gains and uni-polar offsets.

Creating a C# application to display 3D objects on a vector scope involves several distinct steps, combining file parsing, 3D geometry manipulation, signal generation, and hardware control.

1. File Loading and Parsing
   The application begins by loading 3D models stored in the Wavefront .obj format. This file format describes a 3D object using lists of vertices, texture coordinates, and faces. The program reads the file and extracts the vertices and face definitions. These vertices represent the 3D coordinates of the object's points, which are necessary for projection into a 2D space.
2. 3D Shape to 2D Projection
   After parsing the .obj file, the program projects the 3D coordinates onto a 2D plane to create X and Y data streams for the vector scope in perspective projection to simulate object depth. The result is a series of 2D points that represent the object's outline or key features in the 2D plane. The object can be rotated about the X, Y or Z principal axes simulating roll, pitch and yaw movement.
3. XY Waveform Stream Generation
   The 2D points are converted into continuous X and Y signal streams that the vector scope can use, the frequency and the amplitude of these are adjustable. To draw the shape’s lines between points, the program interpolates between them, generating intermediate values. This creates a seamless signal that visually connects the vertices, avoiding abrupt jumps. These signal streams are scaled and offset appropriately to match the input range of the vector scope.
4. Controlling the Analog Discovery
   The application interfaces with the Digilent Analog Discovery device using the dwf.dll library provided by Digilent. Through this library, the program configures the device's analogue outputs to generate the X and Y signals. This involves initializing the device, setting up the analogue output channels, loading the signal data, and starting the output. The dwf.dll functions are accessed using P/Invoke, which allows the C# program to call the native library.
5. User Interface and Workflow
   The application provides a user-friendly interface for selecting .obj files, configuring projection parameters, and starting the output. A 2D preview window in the application can display the projected points, giving users a visual representation of how the object will appear on the vector scope. This preview allows for fine-tuning of settings before sending the signals to the hardware.

1. Displaying Unconnected Points as Dots (Red points and traces)
   In this mode, the goal is to render only the individual points that represent the 2D vertices of the object, without any connections between them. To achieve this, each point is directly mapped to specific voltage levels on the X and Y channels of the vector scope. After displaying each point, the signal is kept constant, producing a stepped waveform pattern. This process is repeated for all the points, ensuring that the beam only lights up when a point is being displayed. This creates a pattern of isolated dots on the scope.
2. Displaying Joined Points as Edges (Blue edges and traces)
   Here, the objective is to connect the 2D points with lines, forming the edges of the object. To do this, each pair of consecutive points and calculates a series of intermediate points between them. This smooth transition allows the beam to move continuously from one point to the next, forming a line. Objects are normally is closed, so the last point is connected back to the first. This creates a continuous outline of the object. The intermediate points are calculated using linear interpolation, which ensures a smooth and proportional progression between each pair of vertices.

For both modes, the 2D points are converted into two separate streams of data: one for the X-axis (horizontal deflection) and one for the Y-axis (vertical deflection). Finally, these X and Y signal streams are sent to the Analog Discovery device, which generates the corresponding voltage signals. The vector scope uses these signals to control the beam, rendering the pattern as either isolated dots or connected edges, depending on the mode selected.

Although CRT vector scopes were widely used in their time, they have largely been replaced by digital displays and software tools that offer greater precision, flexibility, and advanced analytical capabilities. However, vector scopes remain iconic for their simplicity and elegance in visualizing complex signal relationships in real time.

This discussion covered a range of topics related to the visualization of 3D objects on a vector scope using a C# application interfacing with the Digilent Analog Discovery. Beginning with an overview of CRT vector scopes and their historical role in rendering vector graphics, we explored how the Digilent Analog Discovery, a versatile mixed-signal device, could be employed to generate precise X-Y signals to drive a EV8000 display module. The conversation delved into the creation of a C# program capable of loading 3D objects in the Wavefront .obj format, projecting the objects into 2D, and generating signal streams for the vector scope.

The project exemplifies how modern software and hardware tools can breathe new life into retro technologies like 1970’s vector scopes. By combining geometry processing, signal generation, and precise hardware control, the software application provides an engaging and artistic visualization of 3D objects. It serves as a testament to the possibilities that arise when old and new technologies are combined, opening avenues for both technical exploration and creative expression.