Orban OPTIMOD 6200, Operating Manual

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OPTIMOD TECHNICAL DATA
6-15
fied audio data with MSB aligned with the leading edge of LRCK. Data is latched
on the falling edge of SCLK. The processed digital output (ANLG_OUT) is pro-
vided by DSP IC707 on its SAI output port SDO2 (pin 45), and is received by the
D/A on pin 10.
A 6.144MHz bit clock is provided from the system clock circuitry to both the fi-
nal DSP and the D/A chips. The DSP output data format is 32 bits per word, two
words per frame. DSP chip IC707 receives a 48kHz frame clock at its WST input
(pin 50) that sets the word transfer rate to two words per 48kHz period. The D/A
receives a 48kHz clock at its LRCK input (pin 7). LRCK delineates the left and
right samples used by the D/A; therefore the D/A uses the first sample received
for the left output and the second sample for the right output. The DSP output
samples are formatted to ensure that the D/A uses samples that represent the si-
multaneously sampled analog input.
2. Analog Output Stages
The left and right analog signals emerging from the digital-to-analog (D/A) converter are
each filtered, amplified, and applied to a floating-balanced line driver, having a 30
Ω
,
±
5% output impedance. The line driver outputs are applied to the RF-filtered left and
right analog output connectors. These analog signals provide a convenient means of
monitoring the processed audio.
Component-Level Description:
The left channel signal emerging from the digital-to-analog (D/A) converter is fil-
tered by IC404-A, IC404-B, IC406-A, and associated components. The purposes
of these stages are to remove common-mode errors including noise, distortion,
and DC offset, and to reduce the out-of-band noise energy resulting from the
delta-sigma D/A’s noise-shaping filter.
IC404-A, IC404-B, and associated components implement a 3
rd
order Chebychev
low-pass filter characteristic to the differential signal from the D/A. The 20kHz
passband nominally has 0.05dB ripple and about 7 degrees of deviation from lin-
ear-phase. This low-sensitivity filter, utilizing tight tolerance components, does
not induce significant overshoot of the processed audio, which would otherwise
waste modulation.
IC406-A and associated components comprise a low-frequency servo amplifier to
remove residual DC from the signal. The 0.15Hz
−
3dB frequency averts tilt-
induced overshoot of the processed audio.
IC404-B feeds the stage consisting of IC403-A, IC403-B, IC402-A, and associ-
ated components, which is a floating-balanced line driver. The floating character-
istic is achieved by complex cross-coupled positive and negative feedback be-
tween two 5532 opamps, and its operation is not readily explainable except by a
detailed mathematical analysis. Opamps may be replaced; resistors are specially
matched and should not be replaced. IC402-A, R408, R409, R412, and C410
comprise a servo amplifier, which centers around ground the average DC level at
output connector J400.