Method for converting a plurality of signals from one of a digital and analogue form to the other form

Abstract

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.