Using an eye diagram for dynamic equalization?

Question

I was just watching this video Tutorial on Passive Filters, Data Transmission and Equalization. In the video he shows how one can use the size of the opening in the eye pattern as a way to measure the effectiveness of per-emphasis for overcoming bandwidth limitations of a bus or transmission line.

I assume it is a standard practice to optimize pre-emphasis for known transmission lines once and then just leave the values alone.

What I would like to know is do communications circuits exist where the pre-emphasis is adjusted dynamically as the characteristics of the channel change? I am thinking here of circuits like HF radio links over hundreds or thousands of miles where things like phase distortion vary over time. I assume this would also apply to circuits between satellites and earth stations where the satellites are not in geosynchronous orbit.

Answer 1

Ever since Diskdrives start they used to have pattern sensitive bit shift issues, so prec-comp was done for inner cylinders on MFM using delay lines using +/- 10% bit shift to compensate for 00100 isolated pulses or the classic data pattern 6DB in HEX.

For RF it is the same thing: if there is group delay distortion in the pass band then group delay compensation can be done in either the analog filter domain or digital domain in DSP.

But generally the ideal channel is called the Matched Channel, where the channel bandwidth matches the information channel using Raised Cosine filter than rings at the bit rate such that ringing occurs at the data crossing with little group delay distortion hence no jitter and perfect X X on each data transition.

Of course non-ideal components cause assymetry or bit jitter for which guys like HP have great equipment to analyze the BER and co channel eye pattern and jitter.

Usually if the group delay can be compensated it is. But not every modulation pattern lends itself to this method. Especially wideband channels.. So thats why RF tends to be many parallel narrow bands like 56 KBaud modems and 54 Mbps WiFi. So that group delay is minimized in each of the adjacent channels that aggregate up to a larger bit rate.