Sound recording involves the conversion of sound waves into a form that can be stored and reproduced. In 1877 Thomas Edison invented a phonograph that recorded sound vibrations as indentations made by a stylus on a revolving cylinder wrapped in tinfoil. Emile Berliner's gramophone improved the process by using a zinc disk instead of a cylinder. The volume was amplified by the addition of acoustical horns, which were replaced before World War I by valve amplifiers. In 1901 molded thermoplastic records were introduced and subsequently improved plastics allowed finer grooving which reduced surface noise. In 1927 and 1928 patents were issued in the US and Germany for magnetic recording processes. Later innovations included high-fidelity (hi-fi), stereophonic, and quadrophonic reproduction. Modern recordings on compact disk (CD) employ digital signals (rather than the analog signals used on earlier records).