OPTIMOD-FM TECHNICAL DATA
6-9
Note that the small RFI “tee” filter assemblies connected to the input and output connec-
tors are socketed and user-replaceable.
Component-Level Description:
The left channel balanced audio input signal is applied to the filter/load network
made up of L100-103, R100-109, C100-103, CR100, and CR103. (There are sol-
der pads available in the PC board to accept an optional 600Ω termination load
[R106] on the input signal if the user wishes to install one.) A conventional three-
opamp instrumentation amplifier (IC100 and associated circuitry) receives the in-
put signal. R110-114 and quad analog switch IC101 make up the circuit that sets
the gain of IC100. The switches in IC101 set the gain of the instrumentation am-
plifier by switching resistors in parallel with R104. (Smaller total resistances pro-
duce larger gains.) The state of the switches is set by the outputs of digital latch
IC108.
IC100 feeds IC104 and associated components. This stage balances, DC-biases,
and scales the signal to the proper level for the analog-to-digital (A/D) converter.
IC105A and associated components comprise a servo amp to correctly DC-bias
the signal feeding the A/D converter. R137-139, C109, C110 make an attenu-
ator/RC filter necessary to filter high frequency energy that would otherwise
cause aliasing distortion in the A/D converter, IC107.
The corresponding right channel circuitry is functionally identical to that just de-
scribed.
IC100, 101, 102, 103 are socketed for easy field replacement. All other circuitry
is surface-mounted and is not field-replaceable.
2. Stereo Analog-to-Digital (A/D) Converter
Located on input/output board
The A/D converter, IC107, is a stereo 24-bit sigma-delta converter. (This is a surface-
mount part and is not field-replaceable,)
The A/D oversamples the audio, applies noise shaping, and filters and decimates to 64
kHz sample rate. (An Orban-designed synchronous sample rate converter in the 8400’s
DSP performs the final decimation to 32 kHz. This ensures the flattest frequency re-
sponse to 15 kHz without aliasing.)
3. Digital Input Receiver and Sample Rate Converter (SRC)
Located on input/output board
The integrated receiver and input sample rate converter, IC500, accepts digital audio sig-
nals using the AES/EBU interface format (AES3-1992). The built-in sample rate con-
verter (SRC) accepts and sample-rate converts any of the “standard” 32 kHz, 44.1 kHz,
48 kHz rates in addition to any digital audio sample rate within the range of 25 kHz and
55 kHz. The SRC converts the input sample rate to 64 kHz, with the final, high-quality
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