The invention relates to apparatus for manufacturing an optical fiber preform from a tube of vitreous material, the apparatus comprising: means for holding said tube during manufacture of said preform; and heater means supplying said tube with the heat energy required for manufacturing said preform. The valve assembly draws fluid from a compartment and discharges the drawn fluid into a designated compartment to displace the designated compartment with respect to the surface. In a presently contemplated implementation, the block of data is a reference macroblock of decoded MPEG video data that is used in motion compensation operations, and each of the fetch commands is an AB fetch command, where A represents the number of columns of data and B represents the number of rows of data to be fetched in response thereto, and wherein further, A>B. Also disclosed are ways to keep temperatures constant and to use temperature sensors to accurately measure the temperatures of the chambers.