UnDecima Audio Output from Arduino

I already have one project where arduino outputs audio signal to USB speakers via software 10-bits PWM. In first, I was not satisfied with quality of sound generated via PWM. There are just not enough speed in arduino engine to run PWM well. For example for 20. 000 Hz audio, PWM has to be at least 2 3 times higher above normal frequency range,
UnDecima Audio Output from Arduino - schematic

or 40 60 kHz. If we multiply this value with 10-bits resolution, we would get 40 60 MHz, that is too much for small arduino to drive. In second, the idea to create multichannel audio system was boggling my mind for about a year now. This is how UNDECIMA project was born. 1 + 11, or 12 channel ! audio system running on arduino UNO board with full 10-bits resolution maximum available with internal ADC. Project gets its name because there is 1 Master channel, and 11 linearly delayed copy of the same audio stream, or Puppets channels, In its essence, this is acoustic Phase Array. As you can see on posted drawings, the heart  of the project is 10-bit multiplying (parallel) digital analog converterDAC1022. Output of the IC than buffered with OPA and demultiplex`d  via two 74HC4051 8-channel analog switches. Outputs of the switches loaded by sampling and hold capacitors 0. 01 uF, to filter out unwanted sampling frequency noise. DAC isconfiguredfor single supply power line. Usually, they recommend to buffer output with / high input impedance / high slew rate / rail to rail / OPA in such configuration. Which I don`t have, and it costs about half a price of DAC itself, So, this is why I implemented two variable voltage references based on NE5532 and couple of pots. Difference in voltages between tworeferences forms a span . Lower voltage creates an off-set for cheap non rail-to-rail OPA LF351, with adequate slew rate 13 V/usecond....

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