To make the serial port play music, I added a block after the one-bit quantizer to account for start and stop bits. This block is still inside the feedback loop, and reduces the audible effect of the start and stop bits.
How does it work?
First, the audio is resampled to the baudrate of the serial port.
Look over here for a description of the RS-232 waveform. Good hardware can send bytes “back-to-back”. Two of the bits are determined by the protocol: the Start bit and the Stop bit. I can control the other 8 bits coming out of the serial port.
Sigma-Delta modulation is a way to encode an analog signal into a binary bitstream.
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