Clock with DC coupled and AC/DC coupled?

I'm looking at a datasheet with clock input level range description.

It is saying
(1) Square wave, DC coupled Vih = .... Vil=....
(2) Sine wave, AC/DC coupled amplitude p-p = .... Vp-p

Could someone please advise what "DC couple" and "AC/DC couple" mean here?

Thanks for your help.

Hello,

DC couple means that your clock input is non-centered around zero. It has a DC component.

AC couple means that your clock input is centered around zero.

I hope this helps.

In case of a clock input, AC coupled usually means centered round a bias voltage set chip internally, or may be by extern bias resistors. But ussually a reference circuit is given in the datasheet that clarifies this point. Some chips have a differential clock input and the bias voltage is identical to reference input voltage.

Thanks.

I still have one question.

In which case I need a DC blocking capacitor?

Thanks for your kindly help.