| 摘要 |
PROBLEM TO BE SOLVED: To output the audio signal of a fixed level from a speaker on the side of reception at all the time in spite of a change in line resistance caused by the level of audio signal input to a microphone on the side of transmission or the length of a transmission line. SOLUTION: Concerning this interphone system, a public entrance machine 1a, a janitor room base unit 4a and an indoor base unit 3a on the side of transmission are provided with transmission level detecting circuits 10 and 20 for comparing audio signals inputted to microphones 1 and 11 with a reference transmission value and detecting the level, transmission level control circuits 9 and 19 for controlling the transmission level corresponding to the level detected by the transmission level detecting circuits 10 and 20, reception level detecting circuits 8 and 18 for comparing audio signals inputted from buses B1, B2 and B3 with a reference reception value and detecting the level and reception level control circuits 7 and 17 for controlling the reception level corresponding to the level detected by the reception level detecting circuits 8 and 18.
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