| 摘要 |
To differentiate speech on an incoming line from control tones, such as engaged tone signals, busy signals, or other non-speech frequencies, so that associated recording apparatus will not be activated by non-speech frequencies but will be activated by speech frequencies, the incoming signal is fed to a discriminator which discriminates on the basis of time dependent frequency variations (i.e. in the intervals between changes in frequency which in the case of speech are different and much shorter than in the case of control tone signals), rather than on the conventional basis of control tones and speech being of different frequencies. Control tones, since they are of substantially constant frequency, even though intermittent and separated by pauses, will not activate an associated switching device to start the operation of the associated recorder. On the other hand, signals of varying frequency or modulated frequency, as in speech, will activate the switching device. This is accomplished in one form of system by passing the incoming signal successively through an amplitude limiter, a discriminator, a separator unit, an amplifier, and a comparator stage, the output of which activates a switching device. |
