Orban OPTIMOD-FM 8700, Руководство по эксплуатации

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OPTIMOD-FM DIGITAL TECHNICAL DATA
6-11
put sample rate converter (SRC). The digital-to-analog (D / A) converter converts the
digital audio words generated by the DSP to analog audio. High-speed D/A convert-
ers do the same for the composite outputs, each of whose outputs is smoothed by a
passive LC reconstruction filter. The analog output stages scale and buffer the D/A
output signal to drive the analog output XLR connectors with a low impedance bal-
anced output. The digital output transmitter accepts the digital audio words from
the output sample rate converter (SRC) and transmits them in AES3-format digital
audio signals on the digital output connector.
1. Stereo Digital-to-Analog (D / A) Converter
Located on Input/output board
The D / A, IC211, is a stereo, 24-bit delta-sigma converter. It receives the serial left
and right audio data samples from the DSP at 64 kHz sample rate and converts
them into audio signals requiring further, relatively undemanding analog filter-
ing. IC211 is surface-mounted and is not field-replaceable.
2. Analog Output Stages
Located on Input/output board
The left and right analog signals emerging from IC211 are each filtered, ampli-
fied, and applied to a floating-balanced integrated line driver, which has a 50

output impedance. The line driver outputs are applied to the RF-filtered left and
right analog output connectors. These analog signals can represent either the
transmitter or monitor output of audio processing.
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