Videotape recorders need precise alignment to ensure consistent results. Test signals of known level should therefore be recorded at the beginning of every tape. On replay these signals can be used to check the replay systems to ensure that the videotape process is a ‘unity’ system, i.e. the original recorded signal and replay signal are identical. Colour bars are used for this purpose, derived from a test signal generator in the studio complex or when using lightweight cameras, from the camera’s own ‘bars’ generator. Line-up tone is used in a similar way to ensure consistency of audio levels. It is generated within a sound-mixing console or from a lightweight camcorder’s own line-up tone generator.