What is the minimum number of knights that can be placed on a standard 8x8 chess board, so that every square (including those occupied by knights) is threatened by a knight and what is such a configuration?

A square is threatened if a knight can move to that square on its next move. A knight moves two squares in one direction and one square in another direction (perpendicular to first direction) to end up on a square of opposite color. The move can occur even if intervening squares are occupied.

Present your answer as an 8x8 grid with ‘o’ representing an unoccupied square and ‘N’ representing a square occupied by a knight.

Hint: the minimum is less than 16.