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USB Audio Streamer
A Microchip PIC based USB sound card
The hardware for the sound card is based on the Microchip PIC18F2550 USB processor. The processor is clocked at 48MHz which is the maximum rate for this processor and is also a multiple of the audio sample rate. The microcontroller is connected via the SPI port to the Microchip MCP4822 12bit D/A converter. Volume control is implemented by the Microchip MCP41010 variable resistor acting as a voltage divider to attenuate the signal. These voltage dividers are controlled via SPI by the microcontroller via a dedicated bit-banged SPI port. The purpose of using a separate port is so that volume control can run inside the processor at a different priority level then sample output. The Microchip MCP6022 op-amp is used as a buffer before and after the voltage divider. The final op-amp output is connected to a 330uF capacitor which acts as a high-pass filter to remove the DC component of the signal.
Link: USB audio streamer