| 摘要 |
<p>A dial tone detector of improved performance is provided by precise definition of a passband. In one example, virtually infinite roll off is achieved in less than 5 hertz. An incoming analog signal (23a) of at least a predetermined amplitude is limited to produce square wave pulses (23b) which are counted in a first counter (24) over a period of time determined by a periodic RESET signal. A READ signal occuring just before the RESET signal causes either a valid count state or an invalid count state to be stored depending upon the value of the count in the first counter. When a valid count state is in storage, a second counter (30) counts timing pulses while the valid count state persists, whereby in the second counter a count of greater than a predetermined number indicates the presence of dial tone.</p> |
