Can Morse Code be encoded using "silence" for dots and dashes?
i used to be a ham radio person (like more than 40 years ago) and still know Morse code.  it's like learning to ride a bicycle; you don't forget.  between the elements ("dits" and "dahs") of a single character, the space of silence is supposed to be the same as the ON time of a single "dit".  call that a single unit of time.  so a string of dits would be at exactly twice the frequency as a stream of dahs.  but the space of silence between characters is supposed to be three units, the same length as a dah.

so, if you encode only the onsets of each element, whether it's a did or dah, you can't tell the difference between a lot of dahs in a row in the same character (like zero or "0") or the same number of dits in separate letters (like five adjacent "E" in sequence).  from just the onsets of the beeps, a single "0" (which is "dah-da-dah-dah-dah") sounds the same as "EEEEE" (which is "dit-dit-dit-dit-dit" at the same rate).  nor could you hear the difference from "EENE" ("dit-dit-dah-dit-dit" at the same relative onset times).

if you make the space between letters long enough that there can be no confusion between separated characters and the closer elements within a character, you can figure it out from just knocking.  but, given the element and character spacing set forth by the format of Morse Code, you cannot tell "0" from "EEEEE" from "EENE" with just knocks.