| 摘要 |
<p>The telephone system allows any one of a number of subscribers to have access to a common line. A 'hands free' facility is added by providing each subscriber with an additional microphone. An additional contact in each subscriber's access unit connects the microphone to the input of an amplifier, whose output is connected to the line. The amplifier is switched on when any subscriber lifts his receiver. The access units are interlocked by logic gates to ensure that only one subscriber at a time can be connected to the line. If one is using the line and a second lifts his receiver, both are disconnected. An auxiliary unit has a set of logic gates which recognise that two receivers are off-hook, and hence have been disconnected, and connects a dummy across the line to hold the call. It also switches a resistor across the amplifier input.</p> |
