[PaulHoule]

Usually the top performing codec is OFDM or something similar, which is a lot like FSK with a huge number of channels.

You have to equalize that signal to match the capabilities of the channel. I think you can record well past 18k bandwidth with vinyl, this was used to encode quad audio in the classic rock era. That part of the signal rubs off when you play it unfortunately.

[AdmiralGinge]

Cheers for the heads up about OFDM, I hadn't heard of that before! I wonder if that 18 kHz I've heard is per channel ie the sum and difference channels are both 18 kHz?