OPTIMOD-FM TECHNICAL DATA
6-11
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 re-
move 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 75µs and 50µs de-emphasis, respectively.
This analog de-emphasis rolls off any digital noise produced by earlier circuitry
and also helps implement independent de-emphasis settings between the analog
and digital outputs.
The buffered and possibly de-emphasized output of IC204 is applied to IC207, a
balanced output line driver. This driver emulates a floating transformer; its differ-
ential 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 de-
scribed.
3. Digital Sample Rate Converter (SRC) and Output Transmitter
Located on input/output board
An integrated output sample rate converter (SRC) and AES/EBU line driver chip, IC502,
converts the 32 kHz 8400 system sample rate to any of the standard 32 kHz, 44.1 kHz or
48 kHz rates, and also contains a digital audio interface transmitter to encode digital au-
dio signals using the AES/EBU interface format (AES3-1992). This chip is surface-
mounted and is not field-replaceable.
4. Composite Output Circuit
Located on the input/output board
Both composite output circuits are entirely independent, including D/A converters. This
permits the composite level to be set independently for each output in DSP.
Component-Level Description:
We will describe composite output #2. 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 reconstruction filter
C336-C339, L300-L301, R301-303, R328. (This filter is equalized and phase-
corrected in DSP to obtain excellent flatness and phase-linearity. This achieves
very high stereo separation.)
IC302 buffers the output of the anti-imaging filter and sends its output to the
small I/O Composite Board on which the two composite output BNC connectors
are mounted, along with line driver amplifiers. These are composite amplifiers
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