Techniques for employing a decentralized sleep management service are described herein. In such a color TV tube, the horizontal deflection coil is adjusted so that the vertical focus points of the electron beams lie 50 mm or less on the inside and 30 mm or less on the outside of each edge of the effective display screen area along the horizontal axis. A cover plate is fixed to the case to close the inner space. At the transmitter, symbols are simultaneously modulated and transmitted using signals that occupy the same frequency portion of the spectrum, but are distinguishable because different is their position in the wavefield space. Thus, the active noise insulation wall can effectively reduce noises including many frequency components.