A portable display apparatus selectively operable in a still picture mode or a movie mode includes an operating section for allowing the user of the device to select either one of the two modes. The drive circuit can be disposed in an optical transceiver having a quantum well modulator configured to retro-modulate an incoming optical signal. Then, determining that the help feature has been presented a predetermined number of times; and as a result disabling the help feature. The methods may include pumping the water and selectively pre-treating the pumped water by chemical injection prior to filtration. The tunnel current passing part of the tunnel insulation layer located between the first and second magnetic layers is arranged, at least partially, so as not to overlap the plug in the direction perpendicular to the surface of the substrate. In the system, the stopping or slide movement, the modification of the steel level, the heat currents through the mold walls, the temperature of the liquid metal and the drawing-off speed are measured over the casting time, supplied to a computer and compared with predetermined limit values for an automatic operating mode.