Output Circuits - Orban Optimod-FM 8300 Operating Manual

OPTIMOD-FM DIGITAL
receives the input 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 in-
strumentation amplifier by switching resistors in parallel with R104. (Smaller
total resistances produce larger gains.)
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) con-
verter IC107. IC105A and associated components comprise a servo amp to cor-
rectly DC-bias the signal feeding the A/D converter. R137-139, C109, C110
make an attenuator / RC filter necessary to filter high frequency energy that
would otherwise cause aliasing distortion in the A/D converter.
The corresponding right channel circuitry is functionally identical to that just
described.
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 sur-
face-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 8300's DSP performs the final decimation to 32 kHz. This ensures the flattest
frequency response 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 signals using the AES3 interface format (AES3-1992). The built-in sample
rate converter (SRC) accepts and sample-rate converts any of the "standard" 32
kHz, 44.1 kHz, 48 kHz, 88.2 kHz, and 96 kHz rates in addition to any digital audio
sample rate within the range of 32 kHz and 96 kHz. The SRC converts the input
sample rate to 64 kHz. The final, high-quality decimation to the 8300 system
sample rate is done in the system DSP, as was done for the analog input.
This chip is surface-mounted and not field-replaceable.

Output Circuits

This circuitry interfaces the DSP to the analog and digital audio outputs. The digital
audio from the DSP is transmitted to the digital-to-analog converter (D/A) and out-
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-
6-9
TECHNICAL DATA