Baseline wander (BLW) is a very concerning and underrated problem, covered in the last log 4B4T: An extended ternary Manchester code and its implications.

MLT-3 has two compelling aspects:

• the spectrum is shifted toward lower frequencies (EMI compliance) and
• the coder/decoder looks quite simple. Roughly.

The MLT-3 encoder is very simple but the decoder is very sophisticated due, indeed, to BLW. So the spectrum spread and the AFE complexity are linked: higher code efficiency and reduced EMI require more high-speed DSP effort.

This project's focus is on low cost, ease of implementation with very affordable and simple parts, not absolute speed, and the receiver's AFE must be kept as simple as possible, to keep the BOM very low. It's not possible to do much analog magic, like active filters. One of the first logs (AGC) shows that the AGC could be done almost with passive parts (and a few diodes) but I'm not sure this feat can be repeated for other functions.

If needed, the clock speed can be reduced to conform to EMI rules. But it is important that overall noise and BLW remain low, low enough that analog filtering is almost unnecessary. I'd say that the level of BLW must be below 1/2 the difference between 0 and +.

This is why I should evaluate the performance of the 4B3T FoMoT code and see if it fits my requirements. The 4-level Running Digital Sum (RDS) (actually -3/+3) looks promising.

I also consider a protocol (passive/implicit or active/explicit) to "drift" the clock and adapt to the line's capacitance/inductance. Transmission would start at a standard, low frequency (10MHz ?) and slowly increase as long as both receivers see some "margin".