Orban OPTIMOD-FM 5518, Operating Manual

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6-8
TECHNICAL DATA ORBAN MODEL 5518
3. LED Meter Circuits
Located on display board
The meter LEDs are arranged in an 8x16 matrix, in rows and columns.
Each row of LEDs in the matrix has a 1/8 duty cycle ON time. The rows are multi-
plexed at a fast rate so that the meters appear continuously illuminated. Via the
general-purpose bus, the DSP sends meter data values to the control microproc-
essor, which sends the appropriate LED control words (eight bits at a time) to the
data latches that drive the LEDs directly.
Component-Level Description:
The meter LED matrix consists of ten 10-segment LED bar graph assemblies
(CR1-CR9, CR16) and one discrete LED (CR10). Row selector latches IC4, IC5,
IC6, and IC9 are controlled by the host microprocessor and alternately sink cur-
rent through the LEDs selected by column selector latches IC1 and IC2, which
are also controlled by the SC520. IC1 and IC2 drive the selected row of LEDs
through current limiting resistor packs RP1 and RP2.
Input Circuits
This circuitry interfaces the analog and digital inputs to the DSP. The analog input
stages scale and buffer the input audio level to match it to the analog-to-digital
(A/D) converter. The A/D converts the analog input audio to digital audio. The digi-
tal input receiver accepts AES3-format digital audio signals from the digital input
connector and sample rate-converts them as necessary. The digital audio from the
A/D and SRC is transmitted to the DSP.
1. Analog Input Stages
Located on input/output/DSP board
The RF-filtered left and right analog input signals are each applied to a floating,
balanced amplifier that has an adjustable (digitally controlled) gain. Analog
switches set the gain. The outputs of a latch set the state of the switches. By writ-
ing data to the latch, the control circuits preset the gain. This circuit feeds an RC
lowpass filter that applies the balanced signal to the analog-to-digital (A/D) con-
verter.
The digitally controlled gain circuitry was included on the circuit board
for possible use in future products. In the 5518, its gain is preset so that
the A/D will clip at +27 dBu with respect to the 5518’s analog inputs.
Note that the small RFI “tee” filter assemblies connected to the input and output
connectors are socketed and user-replaceable.
Component-Level Description:
The left channel balanced audio input signal is applied to the filter / load net-
work made up of the various passive components preceding IC201. (There are