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I like to add:
In AC couple, some unwanted DC voltage blocked.
and pure original AC signal passed.
On the other hand, DC couple used in cases, where
we need to pass signal with DC voltage.
it means original signal include DC voltage.
For example composite video signal.
DC couple and AC couple are applied on the necessity of signal transmission. As Davood mentioned, AC couple is to block the DC voltage and just allow AC signal. Let say one block circuit operate with dc 1.2V and the next stage operate with dc 1.5V, so here you have to block the 1.2V and pass AC only, thus you can you use resistive biasing to have 1.5V to next stage operation. As for DC couple, if between stages, the same dc voltage are applied, then there is no cap to block any dc within them.