The central unit shall feature selection of number of simultaneously usable station units. The central unit shall be equipped with an All-Mic-On function that simultaneously turns on all microphones for sound collecting and recording of all units. The central unit shall be equipped with a test function which enables easy visual checking of the connection status of all connected units. The central unit shall feature operation of up to six simultaneous speakers. The central unit shall be capable of operation from a 120 V AC 50/60 Hz line. The power consumption of the central unit shall be a maximum of 60 W. The central unit shall provide one microphone input (–60 dB, 600 ohm, unbalanced, phone jack) and one auxiliary input (–20 dB, 600 ohm, unbalanced, phone jack). The central unit shall provide one recording output (–20 dB, 10 k ohm, unbalanced, phone jack and RCA jack × 2). The central unit shall provide one equalizer input terminal (–20 dB, 10 k ohm, RCA jack) and one equalizer output terminal (–20 dB, 10 k ohm, RCA jack). The central unit shall provide one expansion input terminal (–20 dB, 10 k ohm, unbalanced, RCA jack × 2), one expansion output terminal (–20 dB, 10 k ohm, unbalanced, RCA jack × 2), and one control terminal (10 k ohm, voltage output, RCA jack × 2). Central unit controls shall include MIC limiter switch which provides restriction of simultaneously-usable chairman/delegate stations selectable between 0, 1, 3, or 6. The central unit shall provide a selectable time-out function switch adjustable to off, 20 seconds, or 40 seconds. The operating temperature of the central unit shall be 0°C to 40°C (32°F to 104°F) The operating humidity of the central unit shall be under 90 % RH (no condensation) The central unit shall be constructed of pre-coated steel plate with a black finish (30% glossy).