A respiratory gas supply circuit for an aircraft carrying passengers and/or crewmembers in a cabin, comprising: - a source of breathable gas (R1, R2), - at least one supply line connected to said source, - a first actuating device (6, 60, 12) provided on said supply line to open at least partially or close the supply of breathable gas, - a respiratory device adapted to be worn on one passenger or crewmember's face, said respiratory device being connected to said supply line, and further comprising an ambient air inlet, said respiratory device providing to said one passenger or crewmember a respiratory gas to be inhaled corresponding to ambient air and/or breathable gas, - said ambient air inlet further comprises a second actuating device (4, 11) to open at least partially or close said ambient air inlet; and both said first and second actuating devices are operated so that at least one of said first and second actuating devices is closed at each instant of time: - a first pressure sensor is provided in the aircraft cabin to provide a cabin pressure signal representative of the cabin pressure to an electronic circuit, said electronic circuit controlling at least the second actuating device and said electronic circuit, upon reading said cabin pressure signal, determine a first time and a second time intervals (T-T1), - said electronic circuit operating the second actuating devices to open, respectively close, it during a duration corresponding to said first time interval, to close, respectively open, it during a consecutive second duration corresponding to said second time interval, - the respiratory gas supply circuit causes the first actuating device to close during the first time interval, respectively the second time interval (T-T1). - a source of breathable gas (R1, R2), - at least one supply line connected to said source, - a first actuating device (6, 60, 12) provided on said supply line to open at least partially or close the supply of breathable gas, - a respiratory device adapted to be worn on one passenger or crewmember's face, said respiratory device being connected to said supply line, and further comprising an ambient air inlet, said respiratory device providing to said one passenger or crewmember a respiratory gas to be inhaled corresponding to ambient air and/or breathable gas, - said ambient air inlet further comprises a second actuating device (4, 11) to open at least partially or close said ambient air inlet; and both said first and second actuating devices are operated so that at least one of said first and second actuating devices is closed at each instant of time: - a first pressure sensor is provided in the aircraft cabin to provide a cabin pressure signal representative of the cabin pressure to an electronic circuit, said electronic circuit controlling at least the second actuating device and said electronic circuit, upon reading said cabin pressure signal, determine a first time and a second time intervals (T-T1), - said electronic circuit operating the second actuating devices to open, respectively close, it during a duration corresponding to said first time interval, to close, respectively open, it during a consecutive second duration corresponding to said second time interval, - the respiratory gas supply circuit causes the first actuating device to close during the first time interval, respectively the second time interval (T-T1). - a first pressure sensor is provided in the aircraft cabin to provide a cabin pressure signal representative of the cabin pressure to an electronic circuit, said electronic circuit controlling at least the second actuating device and said electronic circuit, upon reading said cabin pressure signal, determine a first time and a second time intervals (T-T1), - said electronic circuit operating the second actuating devices to open, respectively close, it during a duration corresponding to said first time interval, to close, respectively open, it during a consecutive second duration corresponding to said second time interval, - the respiratory gas supply circuit causes the first actuating device to close during the first time interval, respectively the second time interval (T-T1).