Apparatus, including a first signal source (S 1 ) for generating a first signal and a second signal source (S 2 ) for generating a second signal. A cross-over switch (70) is connected between the two sources so as to generate in a direct switch configuration a sum of the first and second signals and in a crossed switch configuration a difference between the first and second signals. The apparatus includes a processor which is configured to receive the sum and the difference and to recover the first signal and the second signal therefrom.
