one section of U402 is used to buffer a 2.3V reference derived from the regulated 5.0V supply.
This external analog reference voltage is marked as "DC_BIAS" on the schematic.
The codec has a mute control output, /MUTEC, which it asserts (active low) immediately whenever
it detects problems with incoming clocks or data and the outputs may become unpredictable (i.e.,
pops, clicks, noise, etc.). This signal is logic-AND'd with the other mute signals from the Sharc that
control the power amplifiers. See the section on MUTE control for more details.
The codec's analog sections operate from +5V supply rail with a ferrite bead and bulk capacitance
decoupling. The digital section is fed from the +5V supply, also. The logic interface section is fed
from the +3.3V supply to interface with the 3.3V-logic Sharc.
2.14 Codec Output Buffers and Low Pass Filters
Sheet 6 of SD254175
The six audio outputs from the codec's DACs are fed into differential amplifiers with a gain of 3.65
(11.25dB) which subtracts out the DC bias of the codec and references the audio signal to the
ground of the power amplifiers. The power amp reference is accomplished by one section of U402
OP Amp which buffers the ground signal "AMP_GND" which is sourced from the Power Amp PCB
via J604, pin 13. This buffered amp ground reference is marked as "GND_REF" on the schematic.
After the audio signals are referenced to the power amplifier's ground, they are passed through a
second order Low Pass filter with a corner frequency of approx. 27 kHz. These lowpass filters are
used to reduce the amount of out-of-band DAC artifacts that get sent to the power amplifiers and
out onto the speaker lines. The outputs of the six LPFs are then passed through a 300 Ohm,
100pF RC lowpass filter for EMI filtering to the Power Amp PCB via J604.
The OP Amps used for the buffers and LPFs are operated off of the +/-VCC supplies (+/-10V).
Maximum (full-scale) audio output from DSP board is approximately 6.8Vpeak (4.8Vrms).
2.15 MUTE Controls
Sheet 8 of SD254175
The Sharc uses its FLAG5 and FLAG6 outputs to separately control the muting of the Right/Left/
Center/Bass amplifiers and Surround amplifiers, respectively. These mute signals are logic-AND'd
with the /MUTEC control output of the codec to provide comprehensive control of the power
amplifiers mute. The Sharc will assert the mute lines (active low) under the following conditions:
•
When 5-Speaker mode is NOT selected, the /LSRS_MUTE is asserted. This keeps the rear
channels from making any sound or background "hiss".
•
When no S/PDIF signal is present. i.e., no audio.
•
When the bass module is in the SmartSpeaker "OFF" state.
•
When there has been a S/PDIF error, interruption, or sample rate change. Through the use of the
S/PDIF error detection methods discussed in the S/PDIF receiver section, the Sharc can "panic
mute" the power amplifiers when conditions occur that might cause undesirable artifacts to come
out of the speakers. During this process, the PSC_PWM signal is also immediately sent to its
minimum value.
Theory of Operation
18
PS18/28/35 Troubleshooting Guide
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