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9 Digital audio input
Optical and coaxial digital audio input is provided. Both of these inputs can support
reception of audio data in IEC60958 consumer mode (S/PDIF) format. This format allows
transmission of two channels of audio data at up to 192kHz. Note the optical receiver
used on the board can only support up to 96kHz in this mode.
Additionally the optical input can receive data in the ADAT optical format. This format
allows transmission of eight channels of audio data at 48kHz. The S/MUX extension to
ADAT allows four channels at 96kHz or two channels at 192kHz.
The coaxial input receiver circuit uses an unbuffered logic inverter configured as an analog
amplifier. This allows the 0.5V pk-pk signal to be amplified up to logic levels required by
xcore.ai. The 100k feedback resistor connected between input and output biases the
input stage to work in the analog region. This circuit does provide a low cost solution but
can use high supply currents (even when no input is present) up to 20mA or more. The
addition of the 68 ohm resistor in the supply line to the logic gate helps to reduce this
current significantly without affecting circuit operation. It works by reducing the supply
voltage to the device to around 2.5V which significantly reduces crossover current. The
output voltage will now only swing to 2.5V in the high state but this actually matches
quite well with the xcore.ai inputs which have a switching threshold around 1.3V typically.
Many other types of receive circuit are possible.
Figure 12:
Digital audio
input
The input signals are fed into two 1-bit ports on the xcore.ai device.
Figure 13:
Board Net
Digital audio
OPT_RX
input xCORE
GPIO
COAX_RX
11
xcore.ai Multichannel Audio Board 1v1 Hardware Manual
xCORE GPIO
Port
X0D38
P1O0
X0D37
P1N0
Description
IEC60958 (S/PDIF) or ADAT optical input
IEC60958 (S/PDIF) input
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