However, an audio signal can be either positive or negative. In this case, the protection diode would sometimes be biased in the forward direction during normal operation, making flawless signal transmission on the line impossible. Therefore, bidirectional protection has to be chosen. In addition, for unidirectional and bidirectional protection, the reverse stand-off voltage of the protection diode has to be higher than the maximum allowed voltage of signal; otherwise the diode could break through during normal operation and again make the signal transmission fail.