Apparatus, including a first signal sourcegenerating a first signal and a second signal sourcegenerating a second signal. A cross-over switch isconnected between the two sources so as to generate in adirect switch configuration a sum of the first and secondsignals and in a crossed switch configuration adifference between the first and second signals. Theapparatus includes a processor which is configured toreceive the sum and the difference, and to recover thefirst signal and the second signal therefrom.
