Bose FreeSpace Business E4 II Series Service Manual page 96

THEORY OF OPERATION
The analog section of the DSP board contains the two stereo codecs (U503, U507) and their
associated analog input and output circuitry. Line 1 and 2 each have two inputs that are resis-
tively mixed and buffered through difference amplifiers (U529, U532). Each of the difference
amplifiers feed a channel of the volume control (U531) that in turn drives the two dual op-amps
(U501, U504) that convert the single ended signals to differential drive for the codec.
Mic in and Page in are balanced inputs, each driving an instrumentation amplifier (U537, U540).
These two channels each have a voltage regulator (U536, U539) that can be enabled by the
host to provide phantom power to external mics. The gain of the voltage regulator is controlled
by the host switching on analog switch sections (U541, U542, U543) that connect gain control
resistors. The range of gain is 0 to 60 dB in 10 dB increments.
The outputs of the instrumentation amplifiers are each served by ½ of an op amp (U538) to a
fixed DC offset of 2.5V to provide one half of the differential drive to the second codec. Each
channel is inverted by half of a dual op-amp (U505) to provide the second phase of differential
drive to the codec. Each of the four input channels is fed to a multiplexer (U535) that selects
one channel under host control for output as music on hold.
The four differential outputs of the codecs drive four identical circuits built around switchable
amplifiers (U509, U510, U511, U512). One differential input of each is configured as a low-pass
filter and differential to single ended converter and is fed from a codec output. The second input
is connected to a common input signal, the direct input. A PNP transistor circuit (Q502) detects
ground on the PTT input and switches the switchable amplifiers to the direct input while also
signaling the PIC to inform the host that an override is occurring. The four switchable amplifiers
drive the power amplifier inputs from the selected input signal path.
The outputs from the switchable amplifiers also drive four channels of amplitude sense circuitry.
Each sense path consists of an op-amp gain stage followed by a low pass filter, a high pass
filter and a precision rectifier/average circuit. The op-amps are contained in dual op amps
(U700, U706) and quad op-amps(U702, U703, U704, U705).
3. LED Board
The LED board contains the front panel indicator LEDs, the standby switch and the USB con-
nector and interface circuitry. Power is applied to this board even when the E-4 Series II is in
standby and is regulated to 5V by U105 voltage regulator and to 3.3V by U107 voltage regula-
tor. Serial input from the host on the DSP board drives a sequence of three serial in, parallel out
shift registers (U102, U103, U104) to drive the LEDs. To reduce power consumption three
inverters from U101 are configured as an oscillator to multiplex the LED drive. Two more of the
inverters are used to buffer and debounce the standby switch. When the standby button is
pushed the signal is detected by the PIC processor on the DSP board, which then communi-
cates over the I 2 C bus with the host processor. The I 2 C lines also come onto the front panel
board to allow the host processor to communicate with an external PC through the USB inter-
face IC (U106).
4. Auto-volume Microphone Sense Board
The auto-volume microphone sense board contains the input and level control circuitry for four
sense mics and an RS-232 connector to allow the E-4 system to communicate with an external
PC. Power is not applied to this board when the E-4 is in standby. The board regulates analog
power supply voltages with a positive 15V regulator (U216) and a negative 15V regulator
(U215). There is also a positive 8V regulator (U204) to provide power to the external sense
mics.
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