1. Field of the Invention
The invention relates to a circuit arrangement for frequency conversion of a color signal to a second carrier frequency, this color signal being present in picture lines and comprising a color burst and a chrominance information signal modulated on a chrominance sub-carrier of the first carrier frequency, the circuit arrangement being switchable in a mode for conjugate-complex formation in which it generates an output signal in which, additionally to the frequency conversion, the color signal is present in a conjugate-complex form.
2. Description of the Related Art
The integrated circuit TDA 9726 marketed by Philips comprises such a circuit arrangement. This circuit arrangement has two mixers. The input signal, having a relatively low frequency of the chrominance subcarrier, is converted, by means of a mixer, to that chrominance subcarrier frequency which is provided for the relevant transmission standard. The first mixer precedes a comb filter. In its turn, this comb filter precedes a second mixer which is used for generating the conjugate-complex signal. This signal is required in a conjugate-complex formation mode for transcoding, for example, an NTSC signal into a PAL signal. This conjugate-complex signal is also required for a PAL signal whose PAL sequence is disturbed. In the known circuit arrangement, the change-over between the signal in its original phase position and the conjugate-complex signal is performed by a switch by means of which either the input signal or the output signal of the mixer is used alternately from picture line to picture line. This state-of-the-art circuit arrangement has the drawback that the carrier frequency, to which the color signal is converted by means of the first mixer, should always be the frequency which is supplied from the output of the circuit arrangement. This is necessary because the output switch alternately switches to the output signals of the first mixer and the second mixer. If the carrier frequency at the output of the second mixer is not the same as the frequency to be supplied from the output of the circuit arrangement, a third mixer would have to be arranged between the second mixer and the subsequent switch, which third mixer would then take care of the conversion to this frequency.