one channel increases, the rate of change in the other decreases. If the circle is large, the rate of change in the analog defection voltage output is large. If the circle is small, the analog voltage will be small. The rate of change is inversely proportional to the contents of the parameter register, which contains the inverse of the circle radius. This causes the generator to produce a circle with the desired radius. The bright-up pulse control is from a quadrant counter in the input/output buffer and control. This turns on the bright-up pulse after the electron beam travels halfway around the circle. This delay compensates for the slow response time of the deflection circuits in the HUD. The bright-up pulse stays on until the quadrant counter counts four times. A short time after the fourth count, the bright-up pulse turns off, and the symbol generator busy signal terminates. This completes the circle mode. Analog-to-Digital Conversion (ADC) Mode.— The third mode of symbol generator operation is the analog-to-digital conversion mode. In this mode, the operation of both the X and the Y channels are the same. Therefore, only the X channel operation is described. At the start of the ADC mode, the most significant bit in the parameter register becomes preset to the one level. The rate and deflection registers are preset to zero and coupled directly to each other. The direct coupling of the two registers provides for duplication of the contents of either register. The third step gates a specific analog voltage from the X channel input selector to one side of the X channel comparator. The fourth step gates the digital-to-analog converter (DAC) voltage from the X channel DAC to the other side of the X channel comparator. The DAC voltage represents the contents of the X channel deflection register. If a difference in voltage level exists between the two applied voltages, the comparator has an output applied to the deflection register. The output of the comparator represents the sign of the difference. This output causes the contents of the parameter register to add to or subtract from the contents of the rate register. On the next operation, the one in the parameter register shifts to the next lower bit location, and the voltage comparison phase repeats. This process continues until the parameter register reaches zero. When at zero, the contents of the rate register (in digital form) are equal to the input selector analog voltage at the comparator. The next operation transfers the contents of the rate register to memory by way of the data feedback (DFB) lines. In later operations, the remaining analog voltages go to the comparator, and the entire process repeats. The ADC mode continues until all analog voltages are converted to digital data and stored in memory. Heads-Up Display Unit Aircraft ac power applied to the low-voltage power supply is rectified, and then divided into seven different dc levels for distribution throughout the display. A transistorized oscillator in the high-voltage power supply receives 24 volts of dc excitation from the low-voltage power supply. The oscillator output is applied to a voltage multiplier and rectifier circuit, which increases the input voltage to 15,000 volts. The 15,000 volts are applied to the CRT anode and a voltage divider within the high-voltage power supply. Outputs from the voltage divider are used to control the oscillator frequency and a comparator output. The comparator senses any differences that may exist between the voltage divider output and a fixed reference voltage. If the high voltage drops below a prescribed level, a fail signal is generated by the comparator and applied to the BITE circuit. OPTICAL MODULE.— The optical module contains the standby reticle, in-range indicator, control panel, autobrilliance sensor, and lens unit. The standby reticle is used only when the HUD is inoperative, and is controlled electrically and mechanically from the control panel. Electrical power used for the operation of the standby reticle is obtained from outside the HUD. Light emitted from the standby reticle is transmitted through the lens unit to the combiner. The in-range indicator indicates when a range discrete signal is received from the forward-looking radar set. Symbols are represented by light emitted from a CRT located in the video module. As symbols are drawn on the CRT face, the emitted light is received by the lens unit and applied to the combiner. The desired level of brightness is selected from the control panel by adjustment of the voltage level applied to the cathode bias circuit. The autobrilliance sensor is used to detect ambient light changes. Any change in ambient light proportionally changes the output voltage level of the autobrilliance sensor. VIDEO MODULE.— The video module contains the bright-up and autobrilliance amplifiers, cathode 5-9