Orban Optimod-FM 8300 Operating Manual page 229

OPTIMOD-FM DIGITAL
described.
3. Digital Sample Rate Converter (SRC) and Output Transmitter
Located on Input/Output board
An integrated output sample rate converter (SRC) and AES3 line driver chip,
IC502, converts the 32 kHz 8300 system sample rate to any of the standard 32
kHz, 44.1 kHz, 48 kHz, 88.2 kHz, and 96 kHz rates, and also contains a digital au-
dio interface transmitter to encode digital audio signals using the AES3 interface
format (AES3-1992). This chip is surface-mounted and is not field-replaceable.
4. Composite Output Circuit
Located on the Input/Output board
A composite D/A converter and reconstruction filter drive two digitally controlled
attenuators that permit the levels of the two composite outputs to be set inde-
pendently. The SCA inputs are summed with the composite output before the
digitally controlled attenuators, so the attenuator adjusts the level of the entire
composite signal, attenuating the SCA and stereo signals to the same extent.
The second SCA input can be jumpered to serve as a pilot reference source for
RDS generators, which is how it is shipped from the factory.
Component-Level Description:
We will describe composite output #1. IC300 is a high-speed D/A converter
chip that receives the digital composite signal at a 512 kHz sample rate. It
drives buffer amplifier IC308A. IC308A drives a fifth-order passive LC recon-
struction filter C336-C339, L300-L301, R301-303. (This filter is equalized and
phase-corrected in DSP to obtain excellent flatness and phase-linearity. This
achieves high stereo separation.)
IC302A buffers the output of the anti-imaging filter. IC302B is a servo ampli-
fier to remove DC offset at the output of IC302A. IC401B accepts the SCA in-
puts, summing them with the composite stereo output of IC302A. Any contri-
bution from the SCA inputs is therefore are not indicated on the
L
meter displayed by the 8300, because this meter indicates only the
EVEL
composite signal generated by the DSP.
Digitally controlled attenuator IC402B receives the output of IC401B and sets
the composite output level. IC3B and IC1, a composite high-current buffer
amplifier, receives the output of IC402B and drives the composite output con-
nector J4B through an RFI attenuator network and optional 75
sistor R411.
The pilot reference D/A converter IC400 receives serial data from the DSP cir-
cuitry. After being buffered and low-pass filtered by IC401A, the resulting 19
kHz sine wave signal can be connected to J5A through jumper J400.
The composite line driver amplifiers are socketed for easy field replacement;
all other components are surface-mounted and are not field-replaceable.
TECHNICAL DATA
C
OMPOSITE

build-out re-
6-11