A digital audio playback system with USB interface
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A high performance sound card is designed and implemented using a USB enabled microcontroller and an external dataconverter. Data is retrieved either via USB or S/PDIF. The sampling clock is generated by a precision clock synthesizer. This is programmable and can be adapted to different sampling rates of USB data. The system supports 24 bit, 192 kHz audio. Signal attenuation is performed through a relay based stepped voltage divider with constant output impedance. 64 dB attenuation in steps of 1 dB is available. An extensive power supply is made to support the range of required voltages. The signal to noise ratio of the power supply was measured to be 93 dB in the audio frequency band. The microcontroller has been programmed to handle the USB communication and provision of control signals for the system. The whole system is assembled on PCBs and tested. The audio performance measurements show a dynamic range of 105 dB, measured at the system output in a noisy environment. The total harmonic distortion plus noise was 0.0011 %.