A method for selectively and sequentially converting
digital video data signals in three formats, namely,
standard definition,
progressive scan and
high definition formats to analogue form such that the same analogue
reconstruction filter can be used for filtering the analogue forms of the signals after conversion. The digital signals prior to being converted are over-sampled at respective over-sampling frequencies for displacing image frequencies to frequencies sufficiently spaced apart from the widest
band width signal so that the analogue
reconstruction filter can be provided for filtering the widest
band width signal as well as the other
band width signals and also can be provided with a relatively low roll-
off rate of attenuation. The
standard definition video data signals are over-sampled at eight times the nyquist sampling frequency, and the
progressive scan signals are over-sampled at four times the nyquist sampling frequency, thus displacing image frequencies of the
standard definition and
progressive scan signals to 108 MHz, so that the centre frequency of the image frequency is displaced to 108 MHz. The
high definition video signals are over-sampled at twice the nyquist sampling frequency, thus displacing the image frequency of the
high definition signal to 148.5 MHz, so that its centre frequency is displaced to 148.5 MHz. This, thus, leaves a
frequency band gap of approximately 66 MHz between the high definition band width signal and the closest image frequency, namely, that of the progressive scan signal. The method can also be applied to the conversion of analogue video signals to digital form whereby the same analogue anti-
aliasing filter can be used for the analogue signals in the three formats.