(a) General. Every person is competent to be a witness except as otherwise provided in these rules or by statute. (b) Minimal qualifications. A person is disqualified to testify as a witness if the trial court determines that he: (1) Lacked the capacity to perceive accurately the matters about which he proposes to testify; (2) Lacks the capacity to recollect facts; (3) Lacks the capacity to express himself so as to be understood, either directly or through an interpreter; or (4) Lacks the capacity