Specifications

LEVEL METERING DESCRIPTION (Cont.)

Meter Ballistics

When S4 is open and S3 closed, the PPM signal drives the display circuitry. This signal is created by IC4B and associated

circuitry. A positive signal from the rectifiers causes the opamp output to go high turning on Q1. This charges C10 through

a reverse protection diode (D2) and a resistor (R18). When the signal level decreases, Q3 turns off and C10 discharges

through R15 and R9. Closing S4 and opening S3 allows the VU signal to drive the bargraph drivers. This signal is generated

by IC3a and associated components configured as an active filter to provide the proper VU characteristic. See Fig-1c (page

21) for a chart showing the ballistics DIP switch settings. A chart can also be found on a label on the top cover of the ATSC-

Bargraph Driver

The signal (PPM or VU) directly drives the LM3916, which drives the upper bargraph display. IC4a provides an additional

16dB of gain to the lower bargraph display driver (LM3915). D1 provides the reference supply to all opamps and both LM391x

display drivers. The LED drive current (brightness) is about 10x the current taken from pin 7. Pins 6 and 4 are the top and

bottom connections to the reference dividers inside the display drivers. The voltage across R1, P1, R2, and DP1 is the reference

against which the rectifier output applied to pin 5 is compared. The meter zero level is calibrated by adjusting P1. Refer to

National Semiconductor Linear data books or website for a detailed discussion of the LM3915 and 3916 bargraph display

drivers.

NOTE: The driver PCB connects the LEDs in each bargraph in two SERIES strings (one for the upper, and another for the

lower). The anode of the LED corresponding to the LOWEST input level in each string (-10 or -40) is connected to V+, with

the other 9 LEDs (8 in the upper one) following. The outputs of the 3915/16 are connected to the node between each LED.

Only ONE output of the 3915/16 is active (LOW) at any one time (“dot” mode), so if the LED string is “broken” (open) at

any point, the bargraph in that half of the display (only) will go OFF whenever the input level is AT OR ABOVE the level

corresponding to the break!

IC5A is configured as a comparator, nominally set at 6dB above “0” to drive the topmost segment of the upper bargraph display

as a peak indicator. The peak segment's threshold is set by P2.