It is proposed to copy protect data files for transmission by incorporating DSV data patterns, that is, data patterns which are difficult to encode without causing DSV problems, in the data files. The data in five audio streams and two video streams is placed, by way of encoders (10) and a multiplexer (12) into a stream of blocks of encoded data in the MPEG file format (14). In this format, blocks of video data V are arranged alternately with blocks of audio data A and padding stream blocks P. DSV data patterns (16) are incorporated into the MPEG file (14) by way of an encoder (18) to produce the copy protected data file (15). The DSV data patterns (16) are incorporated into locations within the MPEG file which have no data content such as padding stream blocks P and/or into unused audio data blocks A. The DSV data patterns incorporated within the data blocks fed either to a monitor or to a reproduction system will not interfere with the reproduction such that unfettered access is provided to the transmitted information. However, a CD or DVD writer will generally be caused to write a disc which, when read, will result in a reader returning corrupted data or information signalling a read error. Thus, the data in the MPEG file 14 has been copy protected.