Two Ways to Draw Light
Every CRT (cathode ray tube) display works by firing a beam of electrons at a phosphor-coated glass screen. When electrons hit the phosphor, it glows. The fundamental difference between raster and vector displays is how they steer that beam.
Raster: Row by Row
A raster display scans the electron beam in a fixed pattern — left to right, top to bottom, line by line, typically 262 lines per field at 60 fields per second (for NTSC). The beam visits every point on the screen whether anything needs to be drawn there or not. Brightness is modulated as the beam sweeps to create the image. Think of it like a typewriter — it goes everywhere, in order, every time.
Vector: Point to Point
A vector display — also called an XY display or random-scan display — does something fundamentally different. The beam goes only where it needs to go. To draw a line from point A to point B, the deflection system steers the beam directly between those coordinates. To draw a triangle, it traces three lines. Nothing is drawn in the blank space between objects.
Inside the XY Monitor
The heart of a vector arcade machine is its XY monitor. Unlike a television, which uses magnetic deflection yokes driven by sawtooth waveforms to create a raster scan, an XY monitor's deflection system is driven by arbitrary analog voltages from the game's vector generator.
The Electron Gun
At the back of the CRT, an electron gun heats a cathode to emit electrons. These electrons are focused into a tight beam by a series of anodes and accelerated toward the screen at high velocity. The beam intensity (brightness) is controlled by the Z-axis signal — the game can turn the beam on and off as needed, making it invisible when repositioning between shapes.
Deflection: Steering the Beam
Two pairs of deflection plates (or coils) control the beam's position. One pair handles horizontal (X) deflection, the other handles vertical (Y). By varying the voltage on these deflection elements, the beam can be steered to any point on the screen almost instantaneously. To draw a line, the game smoothly ramps the X and Y voltages from the start coordinate to the end coordinate while the beam is turned on.
The deflection amplifiers in vector monitors had to be exceptionally fast and linear. Atari's Quadrascan XY monitor, used in games like Asteroids and Battlezone, could position the beam to any point on screen in approximately 1 microsecond and draw up to 40,000 points per frame.
The Vector Generator
The game's processor doesn't draw directly to the screen. Instead, it writes a display list — a sequence of instructions like "move to (100, 50), draw line to (200, 150), move to (300, 100)..." — into a vector generator chip. The vector generator converts these digital instructions into the analog voltages that drive the deflection amplifiers. Atari developed several vector generators: the Digital Vector Generator (DVG) used in Asteroids, and the more advanced Analog Vector Generator (AVG) used in Tempest, Star Wars, and later titles.
Phosphor: The Glow
The screen of a CRT is coated with phosphor — a substance that emits visible light when struck by electrons. Different phosphor types produce different colors and have different persistence — how long the glow lingers after the beam moves on.
Most vector arcade monitors used P22 phosphor (for color tubes in later games like Tempest) or single-color phosphors. Early monochrome vectors typically used P31 (green) phosphor, which is why many vector games — including Battlezone — had that distinctive green glow. Some games used blue-white P4 phosphor.
Persistence and the Flicker Problem
Here's the critical constraint: the phosphor glow fades quickly — typically within a few milliseconds. The vector generator must redraw the entire display list fast enough that the eye perceives a stable image rather than flickering lines. At 60 frames per second, the beam has about 16.7 milliseconds to draw everything.
This means the more lines on screen, the dimmer and more flickery the image becomes. Each line takes time to draw. The beam can only move so fast. If a game puts too many vectors on screen at once, either the refresh rate drops (causing visible flicker) or lines must be drawn more quickly (making them dimmer). This was the fundamental technical limitation of vector displays — and the reason vector games tend toward sparse, geometric aesthetics.
Why They Looked So Distinctive
Several factors combined to give vector games their unmistakable visual character:
Infinite resolution. Because lines are drawn mathematically — from coordinate to coordinate — they have no pixels. A vector line is perfectly sharp at any scale. There are no jagged edges, no aliasing, no stairstepping. A diagonal line on a vector display is as clean as a horizontal one.
The bloom. When the electron beam excites the phosphor, the glow isn't perfectly contained. It spreads slightly, creating a soft halo around each line — the famous "phosphor bloom." This gives vector graphics a luminous, almost ethereal quality that flat digital displays can't naturally reproduce.
True black. The space between vector lines isn't a dark pixel — it's unlit phosphor. It's as black as the inside of the tube. This extreme contrast ratio — bright glowing lines against absolute darkness — is part of what makes vector displays so striking.
Analog warmth. Vector lines have subtle variations in brightness along their length. Corners glow slightly brighter where the beam decelerates. Long lines may be slightly dimmer than short ones. These analog imperfections give vector graphics an organic quality that feels alive in a way pixel grids don't.
Vector vs. Raster: At a Glance
| Property | Vector (XY) | Raster |
|---|---|---|
| Drawing method | Point-to-point lines | Row-by-row pixel scan |
| Resolution | Effectively infinite | Fixed pixel grid |
| Filled shapes | Not practical | Yes |
| Line quality | Perfectly smooth | Aliased (jagged) |
| Complexity limit | Number of lines per frame | Screen resolution |
| Phosphor bloom | Natural glow effect | None |
| Cost (1980s) | High | Low to moderate |
| Durability | Fragile, high voltage | Relatively robust |
Color Vector
Early vector games were monochrome — typically green or blue-white. Color was introduced by Atari with their Color Quadrascan monitor, first used in Tempest (1981). This monitor used a shadow-mask CRT similar to a color television, with red, green, and blue phosphor triads. The AVG (Analog Vector Generator) could specify color per vector, allowing games like Tempest, Star Wars, and Major Havoc to paint the screen in vivid multi-colored lines.
Color vector was technically demanding — the shadow mask reduced brightness compared to single-phosphor tubes, and the deflection had to be even more precise to hit the correct color triads. But the visual results were spectacular.
Legacy in Modern Technology
Vector display principles live on in unexpected places. Laser light shows use galvanometer-driven mirrors to steer a laser beam — essentially the same XY deflection principle. Oscilloscope art (drawing images on oscilloscopes driven by audio signals) is a direct descendant of vector display technology. And every modern GPU still draws geometry using vectors (vertices and edges) before rasterizing them into pixels — the fundamental process hasn't changed, just where the rasterization happens.