Abstract: An air quality measurement unit comprising: a housing having an inlet for receiving an air flow and an outlet for venting said air flow; a plurality of sensors positioned in said housing forming an array of sensors, each sensor having a sensor inlet for receiving a portion of the air flow and a sensor outlet for venting said portion of air flow; each sensor arranged to measure an air quality parameter; wherein said sensors are placed such that the sensor inlet is positioned adjacent to the inlet.

Abstract: Huffman coding (302) is first performed on uncompressed data (402) to obtain Huffman data (406). Additionally, the frequencies of one or more variable-length bit strings are also determined (408). At least one bit string is selected (304) when compression savings provided by the at least one bit string compares favorably with compression savings provided by the Huffman data. The Huffman data (406) is modified to account for the selection of the at least one bit string. Thereafter, compression is performed (306, 308) on the uncompressed data based on token replacements for the at least one bit string and, where necessary, on the modified Huffman data (420). In this manner, performance will be no worse than those techniques that employ Huffman coding. Using the present invention, greater compression efficiencies may be achieved without knowledge of the data being compressed while still preserving random accessibility of the data being compressed.