Orban OPTIMOD-FM 5500 Operation Manual page 229

OPTIMOD-FM DIGITAL
The digitally controlled gain circuitry was included on the circuit board
for possible use in future products. In the 5500, its gain is preset so that
the A/D will clip at +27 dBu with respect to the 5500'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,
and scales the signal to the proper level for the analog-to-digital (A/D) con-
verter IC208. IC207A and associated components comprise a servo amp to cor-
rectly DC-bias the signal feeding the A/D converter. R253, R255-R257, C217,
C219 make an attenuator / RC filter necessary to filter high frequency energy
that would otherwise cause aliasing distortion in the A/D converter.
The corresponding right channel circuitry is functionally identical to that just
described.
IC201 and IC202 are through-hole parts, which permits easy field replacement
using simple tools. All other circuitry is surface-mounted. Replacement re-
quires surface-mount rework tools and skills to prevent circuit board damage.
2. Stereo Analog-to-Digital (A/D) Converter
Located on input/output/DSP board
The A/D converter, IC208, is a stereo 24-bit sigma-delta converter. (This is a sur-
face-mount part and is not field-replaceable.)
The A/D oversamples the audio, applies noise shaping, and emits a bitstream at
64 kHz sample rate.
3. Digital Input Receiver and Sample Rate Converter (SRC)
Located on input/output/DSP board
The receiver IC501A accepts digital audio signals using the AES3 interface format
(AES3-1992). It applies its output to sample rate converter (SRC) IC503B. This ac-
cepts and sample-rate converts any of the standard 32 kHz, 44.1 kHz, 48 kHz,
88.2 kHz, and 96 kHz rates in addition to any digital audio sample rate within
the range of 32 kHz and 96 kHz. The SRC converts the input sample rate to 64
kHz for processing by the DSP.
TECHNICAL DATA
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termination
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