On or before April 1 of each year the cable operator shall pay to the City, at the office of the City, in lawful money of the United States, a franchise fee based upon a percentage of the gross subscriber receipts received by the cable operator from the operation of its cable television system within the City during the preceding calendar year. Such fee shall be based on such percentage of said gross receipts as may be approved by the FCC, but in no event shall the same be less than five percent of said gross receipts. Any franchise fee not paid when due shall bear interest at the rate of nine percent per annum from the date due. The franchise fee shall not be considered to be in the nature of a tax.