| 摘要 |
PROBLEM TO BE SOLVED: To provide a visual door-phone system which can surely perform the back-tone notification in a simple and compact circuit constitution including plural camera slave units connected together. SOLUTION: In this system, plural door-phone slave units (with camera) 1 (1A, 1B) are connected to an interphone (TV) master unit 2 having a TV via a two-wire cable and the unit 2 performs the supply of power and the back- tone notification to the units 1. Each of units 1 has a 1st voltage switch 14 which controls the start of a talking circuit 123 based on a 1st power voltage level and a 2nd voltage switch 15 which controls the start of a video circuit 112 based on a 2nd power voltage level. Then one of both units 1 inputs a call signal received from the other unit 1 when the unit 2 is talking with one of both units 1. Under such conditions, the power voltage supplied to the other called unit 1 is varied and at the same time the back-tone notification is carried out. |
