Input Circuits - Orban OPTIMOD-FM 8500S Operating Manual

OPTIMOD-FM DIGITAL
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.
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
low-pass 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 8500S, its gain is preset so that
the A/D will clip at +27 dBu with respect to the 8500S'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
solder pads available in the PC board to accept an optional 600
load [R201] on the input signal if the user wishes to install one.) A conven-
tional three-opamp instrumentation amplifier (IC201 and associated circuitry)
receives the input signal. R225-231 and quad analog switch IC203 make up the
circuit that sets the gain of IC201. The switches in IC203 set the gain of the in-
strumentation amplifier by switching resistors in parallel with R223. (Smaller
total resistances produce larger gains.)
IC201 feeds IC205 and associated components. This stage balances, DC-biases,
TECHNICAL DATA
Ω
termination
6-9