Nippon Telegraph and Telephone Corporation

Nippon Hoso' Kyokai

Ntt Electronics Corporation

Asahi Kasei Microsystems Co., Ltd.

Nhk Engineering Services, Inc.

Rcom Corporation

Japan Kyastem Co., Ltd.

Nippon Telegraph and Telegraph Corporation

Kazuhiro Daikoku

Yasuhiro Ito

Kenji Yamada

Takashi Ando

Yasuaki Nishida

Shigeaki Ogose

Shinichi Hosoya

Tomokazu Takasaki

Toshio Hoshino

To improve the quality of the demodulated signal without altering the form in which an AM radio wave is transmitted, the received amplitude-modulated signal is converted to a single-sideband signal, and the information signal demodulated from the phase term of this converted single-sideband signal.

Amplitude-modulated signal receiving circuit

An automatic dynamic range control circuit is provided with a fixed-gain first circuit means and a variable-gain second circuit means having substantially the same propagation delay as the first circuit means. The automatic dynamic range control circuit holds the peak value of an input signal, selects the first circuit means when this peak value is smaller than a reference signal level, and selects the second circuit means when the peak value exceeds this reference signal level. When the held peak value is smaller than the reference signal level, the second circuit means is operated at the same gain relative to an input signal as the first circuit means by having its gain set by the reference signal. When the held peak value exceeds the reference signal, the gain of the second circuit means is limited by this held peak value and its output level is kept at or below a prescribed level. As a result, when an excessively high input signal level is fed, the first circuit means can be switched to the second circuit means without giving rise to discontinuity in the waveform of the output signal on either the time axis or the amplitude axis.

Automatic dynamic range control circuit

A circuit which limits to a fixed value the amplitude of a pilot signal received along with a modulated signal, mixes the amplitude-limited pilot signal with the modulated signal, and uses the random FM noise contained in the pilot signal to cancel the random FM noise contained in the modulated signal. The desired signal to be received is selected from the input signal at the high frequency band; this output is converted to a low frequency band signal by means of a frequency converter. The extraction of the pilot signal and the cancellation of the random FM noise is performed in the low frequency band, whereby the characteristics required for the high frequency band-pass filter are relaxed.

Circuit for removing random FM noise

A SSB (single sideband) single accompanied by a carrier component is demodulated by the following process: If the carrier level in receiving signals is lower than the SSB signal's, the carrier must be amplified up to the level which is higher than that of the SSB signal. Amplitude variations of SSB signals are completely removed by an amplitude limiter. The output of the amplitude limiter is demodulated by a frequency detector and an equalizer which makes the frequency response flat. A linearizer is employed for demodulated signals to eliminate higher order distortions of the signal. The linearizer may be omitted when a pre-distortion circuit is introduced in a transmitter of the system.

SSB communication system

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Tokyo, Japan

Kazuhiro Daikoku