The idea for this circuit came from the question whether there was a simple method for connecting a digital audio output to the TTL input of, for example, a sound card. A typical S/PDIF signal has a level of 0.5 Vpp at 75 ?. This level is of course much too low to drive an input that works at TTL levels. The simplest way to obtain the correct voltage is to use a small transformer to step up the voltage. The input impedance of the circuit should be 75 ? in order to keep distortion of the signal as low as possible.
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