Feed-forward equalizer (FFE)

A feed-forward equalizer is a short tapped-delay filter that pre-distorts the signal so the channel's smearing un-distorts it back to crisp. On a transmitter it's called pre-emphasis or de-emphasis: the driver deliberately kicks each bit's first edge harder and pulls back on long runs of the same bit, so that after the channel rubs the edges away, what arrives looks clean. It is the digital equivalent of shouting the first syllable of every word a bit louder because you know the echoey room will swallow it.

An FFE works with a few weighted 'taps' — typically one pre-cursor tap, a main tap, and one or more post-cursor taps — that add scaled, delayed copies of the bit together. Because it acts before any decision, an FFE is the only equalizer that can cancel pre-cursor ISI (echoes arriving early). Its weakness: any boost it applies to the wanted signal also boosts the channel's noise and crosstalk, so its reach is limited and it's almost always paired with a DFE that handles the post-cursor tail noise-free.