Digital Audio Selector

by R Shankar Switching audio signals digi- tally could be done with the aid of CMOS analogue switches or multiplexers. Simple as this may seem, there is, however, an inevitable loss in the quality of thesoundduetothenoisy nature of CMOS switches. Furthermore, the high on- resistance of these devices together with the large parasitic capacitances generally present in CMOS circuits causes a high susceptibility to crosstalk. The circuit given here is a novel way of selecting one out of ten audio signals digitally without any of the foregoing drawbacks. Asshowninthecircuit diagram, the ten input signals numbered 1-10 are applied to the bases of transistors Tl-Tio via capacitors Cl-Clo respect- ively. The bias voltages for the SELECT( transistors are obtained with the aid of Ri-Rio. Depending on the binary state applied to IC1, one of its outputs Qe-Q9 goes low. For example, if the input code is 0010, Q2 goes low, pulling the baseofT3 to 0V,whilethe bases of all other transistors are r...