A server for a merchant computer system, the server comprising: a file store for storing a range of audio/video products in respective product files; a dialogue unit having a network connection and operable to invite and receive a client selection from among the products via the network connection; a product reader for reading the product files to generate a digital audio/video signal; a digital signal processing unit having an input connectable to receive the digital audio/video signal from the product reader, a processing core operable to apply a defined level of content degradation to the digital audio/video signal, and an output connected to output the degraded digital audio/video signal from the processing core to the network connection. It is therefore possible for a content provider to change the characteristics of an audio or video data stream supplied over a network to a potential purchaser in a controlled and variable manner. The amount of degradation is sufficient to enable a potential purchaser to appreciate the characteristics of the audio or video product, whilst reducing the perceived quality.