Cyrus AV5, Service Manual, Page: 5 / 34

CYRUS AV5 TECHNICAL DESCRIPTION
July 2000 3
Digital/analogue input selection and digital out
The coaxial digital inputs are buffered by differential receivers IC202/401 which convert the input signal level
from 0.5V peak to peak (std SPDIF level) to 5V peak to peak. The output from the coaxial receivers and
the optical receivers are routed to an 8 way multiplexer IC404 which selects a digital input. The output from
the multiplexer is decoded by IC502 and then routed to digital output socket. The analogue inputs are
buffered by IC405/6/7 and then selected by IC404.
Preparation of input signals prior to DSP
Analogue inputs are converted to digital format by the CODEC’s 20bit ADC converters. The digital output
from the CODEC’s ADC is then sent in I2S format to the DSP. When the CODEC is set to decode an
analogue input the 12.288MHz clock (pin 28 IC502) comes from the crystal module (X801) via IC804
(which divides clock frequency by 4). When the CODEC is receiving a digital input the clock is derived from
the digital input using the CODEC’s PLL. The CODEC modes are set by the microprocessor via the SPI
bus pins 3,4,5 and 6.
DSP (digital signal processor).
All signals pass through the DSP (IC802) including stereo, pro-logic, DTS or Dolby Digital. The signal is
received from the CODEC into its compressed data interface.
The DSP can :
When receiving stereo PCM
? Convert to Pro-Logic
? Apply bass management to speakers
? Convert to 3 channel stereo
? Pass the PCM straight through as stereo
When receiving DTS
?
Decode DTS stream into 5.1 format (No bass management or down mixing)
When receiving Dolby Digital
? Apply bass management to speakers
? Convert to 3 channel stereo
? Down mix channels
? Convert to stereo
?
Decode as 5.1
The microprocessor will set the DSP to the appropriate mode (stereo/multi-channel) selected by the user.
The DSP will also send information to the microprocessor to indicate decoding status e.g. receiving AC3
stream. The DSP is controlled by the microprocessor via the SPI bus pins 7, 19, 6, 18 and 20. The program
code for the DSP is stored externally in a memory IC905, connected via IC903/4 acting as address
expanders for the memory.