Bose BUILT-INvisible TA-1 Service Manual page 9

Input/Output (I/O) PCB (continued)
Power supply failure monitor level translator
There are four power supplies on the power amplifier PCB, derived as two pairs of symmetrical
bipolar supplies. By tying each of these supplies into a node by appropriate resistor values, we
have a node that sits at zero volts when all supplies are normal. If one or more supplies deviates
sufficiently, we want to detect this as an error. The node PROTECT is brought out of the amplifier
PCB at J701 pin 11 [amplifier sheet 2, A7] and onto the I/O PCB at J22 pin 11 [D2] and connects
to Q1 and Q2 [D2]. Q2 turns on when PROTECT exceeds about 0.5V, while Q1 turns on when
PROTECT exceeds about –0.5V. This dual transistor's output (Q1 collector) is normally high,
reflecting the state where all six power supplies on the power amplifier PCB are within normal
limits. When either transistor turns on, the output signal BUF_PROTECT_N out of Q1 goes low,
indicating a failure. The PROTECT signal also becomes active when there is a DC offset failure
in the bass speaker amplifier.
When the PROTECT signal becomes active, the microcontroller (U5) [C4] on the I/O PCB
responds by placing the unit into the Standby state, where the main power transformer is de-
energized. The Fault LED D29 [D4] flashes on and off when this occurs. The next time a user
command is issued, it attempts to turn on, and will stay on if the fault has been cleared; other-
wise, it will again switch to the Standby state.
Microcontroller U5 and related circuits
The I/O PCB requires its own microcontroller to monitor the serial interfaces, the installer DIP
switches, the TV sensor, etc. It is responsible for passing along control commands to the
microcontroller U202 [DSP schematic sheet 2, C5] on the DSP PCB. The I/O PCB's micro-
controller U5 is clocked internally by an oscillator using the crystal Y1 [C3]. The microcontroller
can write and read to the optional (not installed) EEPROM U6 [C3]. A reset generator U4 [C4]
ensures that the microcontroller wakes up and executes its reset routine when power is turned
on or momentarily interrupted. The microcontroller handles the 3 LEDs [D4] Command (D28),
Fault (D29), and Setup (D30). It also reads the configuration DIP switches contained in S7 [D4],
as well as the speaker EQ switches S4 [B3]. Other functions (bass and treble PWM outputs,
etc.) have been mentioned elsewhere in this presentation.
Digital Signal Processor (DSP) PCB
Signal Processing
Note: Unless otherwise noted, refer to the schematic diagrams for the DSP board for the follow-
ing information. The references in brackets [ ] are the schematic sheet and grid coordinates for
the component.
The Digital BUILT-INvisible
PCM (digitized 2-channel audio) or the AC-3 output from up to four separate digital sources.
AC-3, also known as Dolby
allows 5 wideband channels and one low frequency bass channel (5.1) to be compressed into
less bandwidth than would be required for 2-channel PCM.
The Videostage ® 5 processing used in the amplifier expands a L, R input pair into L, C, R, S
and bass, but adds a few new features over previous versions of Videostage, such as stereo
surround channels and a mono to 5 channel mode. The mono to 5 channel mode is a way of
processing a mono soundtrack such that all 5 speakers receive parts of the signal. The amplifier
will also accept two analog inputs directly at the input jacks.
THEORY OF OPERATION
®
amplifier has the ability to receive a digital S/PDIF bitstream, either
® Digital, is a perceptual coding and data compression technique that
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