| 摘要 |
A switching device operable to monitor for the presence of an audio signal on a primary input line to a mixer includes a retriggerable timer responsive to activity on the primary line. When activity ceases for a predetermined time interval, the retriggerable timer connects to the mixer a substitute program, for example background music to be played through a different mixer input channel during performers' break. The switching device is operable for professional-type balanced low impedence audio systems wherein the mixer impresses a DC bias on the signal line to power condensor microphones. The microphones remain operational whether or not the substitute program is enabled. Upon resumption of microphone activity on the primary line, the switching device is automatically reset.
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