Emulate DX7 "floating point" DAC in Mark I engine

[dronus]

The original Yamaha DX7 uses a digital analog converter made of several parts that provides about 12 bit of linear and 4 bits of exponential resolution.
It also outputs each voice individually and let them mix up in the low pass filter.

For details, see here: https://electronics.stackexchange.com/questions/512992/how-does-the-yamaha-dx7-digital-analog-converter-work and here: https://www.righto.com/2022/02/yamaha-dx7-chip-reverse-engineering.html

Thus quantizing the output sample values into 12bit mantissa and 4bit exponent for each voice individually in the Mark I engine could provide a quite faithful reproduction.

As an addition, the Plogue DX7 plugin features a single "error" wheel to introduce distortion caused by the tolerances of the discrete resistor network used to create the exponential stage of the DAC. Theoretically, there could be three dials to choose the deviations of the 1/2, 1/4 and 1/8 level stage independently.
The error wheel is shown in this video: https://youtu.be/uDlqXe0M2as?si=7w9z-OZixa-000sW