all-analog design using two integrators: X and Y. The computer sets the integration rates using a digital-to-analog converter. The computer controls the integration time by momentarily closing electronic analog switches within the operational-amplifier based integrator circuits. Voltage ramps are produced that the monitor uses to steer the electron beam over the face of the phosphor screen of the cathode ray tube. Another signal is generated that controls the brightness of the line. The cathode ray tube is a Samsung model 240RB40 monochrome unit measuring 9 × 11 inches, displaying a picture of 240 mm diagonal; it is an off-the-shelf picture tube manufactured for small black/white television sets. The brightness of the CRT is controlled using a circular knob on the back of the display. A vector CRT display such as the one in the Vectrex does not require a special tube, and differs from standard raster-based television sets only in the control circuits. Rather than use sawtooth waves to direct the internal electron beam in a raster pattern, computer-controlled integrators feed linear amplifiers to drive the deflection yoke. This yoke has similar, if not identical inductances, unlike a TV deflection yoke. The yoke uses a standard TV core. The high-voltage transformer also uses a standard core and bobbin. There is special circuitry to turn off the electron beam if the vector generator stops or fails. This prevents burning of the screen's phosphors. This design is a great deal smaller than the electronics found in the free-standing, full-sized Asteroids arcade machine. During development, for a short period of time, the possibility of using the 6502 processor was considered, but later its performance was considered insufficient. Early units have a very audible "buzzing" from the built-in speaker that reacts to the graphics generated on screen. This is due to improper production grounding of signal lines of the low-level audio circuitry, and was eventually resolved in later production models. A "ground loop" had been created by a grounding strap added in production to meet U.S. Federal Communications Commission signal radiation requirements. This idiosyncrasy has become a familiar characteristic of the machine. Peripherals The 3-D Imager, invented by John Ross, turns the 2-D black-and-white images drawn by the Vectrex into a color 3-D experience. The imager works by spinning a disk in front of the viewer's eyes. The disk is black for 180 degrees and in some cases has 60 degree wedges of transparent