The Unit (1) contains a control circuit (2) a digital memory (5) and a Digital to Analogue Converter (6). Incoming digital data (3) is fed via the controller (5) to the memory (2). The image data can be output directly (4) in digital form or can be converted (6) to an equivalent analogue signal. Typical analogue outputs can be in one of the television standard formats, e.g. PAL or NTSC.