Bose AV28 Manual page 16

Theory of Operation
Analog audio sources to be played in Zone 1 or Zone 2 are selected by U201, the TEA6422 analog
MUX IC. The chip has separate left/right output pairs for each zone, and is controlled by U1 via the
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serial I C interface. Its outputs are fed into the Zone1/Zone2 audio codecs (U303 and U304).
The codecs first digitize the analog signals using onboard 24-bit A/D (analog to digital) converters.
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The digitized result (in I S format) is then simultaneously fed out to the Zone1/Zone2 S/PDIF
transmitters (U306/U406) and back into the codecs. Once back in the codecs, signals are con-
verted back to analog via 24-bit DAC's (digital to analog converters), passed through onboard
volume control cells, then outputted once again as left/right signal pairs. U305 and U405 form
differential amplifiers that increase the codec audio outputs to a full-scale level of 2Vrms, and filter-
out unwanted high-frequency digital noise. The resulting analog signals feed the headphone out-
puts as well as the Zone 1 and Zone 2 speaker outputs. Volume control levels are set by U1 via
the I 2 C interface. Zone 1 signals pass through a set of mute transistors, Q300 through Q304,
which quiet the Zone 1 speaker outputs when the headphones are plugged-in.
The same I 2 S signals which feed the Zone 1 S/PDIF transmitter (U306) also feed U202, a CS4340
DAC. This DAC creates the fixed-level analog signals sent out the analog left/right Record Outputs.
A set of logic gates, U302, U304, U402 and U404 (74LCX157 Quad 2-Input Digital MUX chips) are
used to route I 2 S signals (consisting of a Data line, Master Clock, L/R Frame Clock and Bit Clock)
between the codecs and the S/PDIF transmitter chips. These digital MUX chips are controlled by
U1 using a set of individual logic lines (where a +3.3V level selects the "B" inputs, and a 0V level
selects the "A" inputs). These logic lines have various schematic names, and are connected to the
SELECT pin of each 74LCX157. These same chips route the digital audio signals from U1 (also in
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I S format), discussed next.
Digital audio input streams are selected by U501, the S/PDIF MUX chip, before being funneled into
U1, the CS98K DVD Decoder IC. In this way, the CS98K's onboard DSP can decode the desired
stream if it happens to be in a compressed audio format (AAC, AC-3, MPEG-2, MP3, DTS or
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MLP). U501 selects the desired stream based on I C commands from U1.
Streams played from discs in the DVD-ROM drive are clocked out of the CS98K at a sample rate
equal to the rate at which they were recorded. CD audio (CD-DA) discs all require a 44.1kHz
sample rate. Audio from DVD video discs typically requires a 48kHz sample rate, but may use
other rates. External streams received through the S/PDIF MUX will need to be clocked out of the
CS98K at a sample rate synchronous to the clock encoded into the stream (may be many different
rates). The CS98K's AC-97 port is hard-wired to an external 44.1kHz clock. Therefore, streams
clocked out of this port only support a 44.1kHz sample rate. The CS98K's PCM output ports share
a single clock rate which can be set by U1 to any desired frequency (AUD-DO_0, AUD-DO_1,
AUD_DO_2 and AUD_DO_3, sheet 8 of the schematics, are the data lines).
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