Abstract: A silica-based glass rod is inserted into a space portion of a mold. Then, a slurry, which is obtained by dispersing a silica glass powder material in pure water, is poured into a molding space between the glass rod and the inner surface of the mold, whereby a porous glass shaped body is formed around the glass rod. Drying this shaped body results in a silica glass base material which includes the glass rod and the porous glass shaped body surrounding the same.

Abstract: An extruding apparatus having at least two independent inlet ports, one outlet port communicating with the inlet ports, and at least two passages connecting the inlet ports with the outlet port, in which a section of a joint portion of the passages has a central portion and an outer peripheral portion which are concentrically arranged, is prepared. A core material is supplied to one of the passages communicating with the central portion of the joint portion from a corresponding inlet port, and a cladding material is supplied to the other passage communicating with the outer peripheral portion from a corresponding inlet port, thereby causing the two materials to join each other at the joint portion. An integral shaped body having a portion corresponding to a core at its central portion and a portion corresponding to a cladding around the central portion is extruded from the outlet port. Water and a binder and other impurities are removed from the shaped body, and the resultant body is consolidated.

Abstract: A subsignal transmitting system had working lines for respectively transmitting multiplexed signals each including a main signal, and had at least one protection line for transmitting one of the multiplexed signals when it is detected that the transmitting conditions of one of the working lines along which the signals are being transmitted are deteriorated.

Abstract: A multi-level quadrature amplitude modulation and demodulation system includes a first exchange circuit for cyclicly exchanging input digital signals and for outputting exchanged signals to D/A converters. The D/A converters provide analog signals to a QAM modulator which produces a modulated signal. In the receiver side of the system, a QAM demodulator demodulates the modulated signal to produce analog signals which are converted to digital exchanged signals by A/D converters. The digital exchanged signals are exchanged in a manner which is the reverse of that of the first exchange circuit to provide output digital signals on output terminals which respectively correspond to the input terminals. By cyclicly exchanging the input signals in this manner, the transmission quality for each of the input signal channels is maintained equal.