| 摘要 |
In a key telephone system connectable to both ISDN and PSTN, for the purpose of compensation of a difference in speech signal level between ISDN and PSTN, digital pads are incorporated in an ISDN interface unit mounted on a key service unit of this system. In the digital pads, original PCM-code speech signals received/transmitted from/to the ISDN line are converted into PCM-code speech signal with a compensated speech signal level obtained by adding a selected gain to the original speech signal level. Since the difference in the speech signal level between ISDN and PSTN differs according to the conditions, an appropriate gain is selected from a plurality of previously prepared gains. That is, the relationship between telephone numbers and information signals indicative of the selected gain is registered in the key service unit. Therefore, when conversation is made to a subscriber whose telephone number has been registered through the ISDN, a gain registered in correspondence to the subscriber's telephone number can be selected automatically.
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