Orban OPTIMOD-FM 8600, Operating Manual

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OPTIMOD-FM DIGITAL TECHNICAL DATA
6-11
Component-Level Description:
IC201 and associated components filter the left channel signal emerging from
IC211. The purpose of these stages is to reduce the out-of-band noise energy
resulting from the delta-sigma D / A’s noise-shaping filter and to translate the
differential output of the D/A converter into single-ended form. These com-
ponents apply a 3 rd order low-pass filter to the differential signal from the D /
A. This filter does not induce significant overshoot of the processed audio,
which would otherwise waste modulation.
IC203 is used to set the analog output level. It is a digitally controlled gain
block that sets its gain according to signals on its three digital input lines.
IC204B and associated components form a low-frequency servo amplifier to
remove residual DC from the signal. The 0.15Hz
−
3 dB frequency prevents tilt-
induced overshoot in the processed audio.
IC204A buffers the output of IC203 and implements de-emphasis if desired.
FET switches Q200 and Q201 implement 75
µ
s and 50
µ
s de-emphasis respective-
ly. This analog de-emphasis rolls off any digital noise produced by earlier cir-
cuitry and also helps implement independent de-emphasis settings between
the analog and digital outputs.
The buffered and optionally de-emphasized output of IC204 is applied to
IC207, a balanced output line driver. This driver emulates a floating trans-
former; its differential output level is independent of whether one side of its
output is floating or grounded. IC207 and its right channel counterpart IC208
are socketed for easy field replacement. All other circuitry is surface-mounted.
The corresponding right channel circuitry is functionally identical to that just
described.
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 64 kHz 8600 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.
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.