Bose BUILT-INvisible TA-1 Service Manual page 14

Digital Signal Processor (DSP) PCB (continued)
Signal Path
The signal path through the system takes the following route(s):
For analog signals, the L/R stereo pair is introduced from the source via the input cable from the
Input/Output board and the 14-pin connector J9 [schematic sheet 1, A-D8]. Each signal is
received pseudo-differentially by op-amp U105 [B/C6] and associated components and -6 dB of
gain is applied to match with the maximum in/out signal level of the codec (1Vrms).
Digital S/PDIF input signals enter the DSP board at J9 pin 10 [B8] (which come directly from a
S/PDIF signal source selected from one of four possible inputs on the I/O board), the signal is
routed directly to the codec U100 [C4] via a network of components (D1 [B5] and surrounding)
designed to clamp and filter ESD transients.
The codec takes in either the pair of analog inputs, or the single S/PDIF digital input, and outputs
three digital signals (data, bit_clock, and frame_clock, as described earlier). Since the codec
runs on 5V and the DSPs run on 3.3V, any signals passing between them must be level-shifted
via the buffers U101 [D5] and U102 [D3].
Without going into a lot of detail, DSP1 handles (mostly) Dolby
®
Videostage 5 processing and DSP2 handles (mostly) equalization. With the signal processing
complete, DSP2 formats the six output channels into a single serial bitstream and sends this
back to the output side of the codec U100. The analog buffers following the codec (U103, U104
and associated components) do three things:
• Remove the 2.3VDC reference voltage from the codec's outputs. The codec is a
single supply (+5V) device and references any of its internal analog signals to a
reference voltage at approximately 2.3V, about one-half the +5V supply voltage.
• Reference the signal to the amplifier ground, not the local ground. The power amps are single
ended and are referenced to their own local ground on the amp PCB, which is not the same
as the local ground at the codec on the DSP PCB. Amplifying the differences between the
two grounds will produce audible hum in the output speakers.
• Provide enough gain to increase the output signal for output to the amplifier board.
The signals leave the codec itself at about 1Vrms full-scale, this is amplified up to about
6Vrms full-scale by the output buffers.
Power-Up Sequence
At power-up, all of the microcontroller ports are in a high impedance state, which means that:
• The amplifier mute lines are held low, muting the power amps.
• The 3.3V regulated supply is disabled, reducing the demand on the +20V supply until
things have stabilized.
• The DSP reset line is held low, holding the DSPs in reset.
THEORY OF OPERATION
14
® Digital (AC-3) decoding and