When specially designated to exercise such jurisdiction by the district court or courts he serves, any United States magistrate judge shall have jurisdiction to try persons accused of, and sentence persons convicted of, misdemeanors committed within that judicial district.

Any person charged with a misdemeanor, other than a petty offense may elect, however, to be tried before a district judge for the district in which the offense was committed. The magistrate judge shall carefully explain to the defendant that he has a right to trial, judgment, and sentencing by a district judge and that he may have a right to trial by jury before a district judge or magistrate judge. The magistrate judge may not proceed to try the case unless the defendant, after such explanation, expressly consents to be tried before the magistrate judge and expressly and specifically waives trial, judgment, and sentencing by a district judge. Any such consent and waiver shall be made in writing or orally on the record.