Modulation/demodulation circuit and communication system utilizing the same

A modulation/demodulation circuit comprising:

• a) a frequency inverting means including:
• a mixing circuit for mixing an input signal with a high frequency signal to produce a mixed signal;
• and
• a low pass filter for passing the lower side band of the mixed signal as an input signal, the low pass filter having a frequency spectrum inverted relative to a frequency spectrum of the input signal,
• b) a high pass filter having a low band cut-off reciprocal to a cut-off of said low pass filter, and
• c) means for selecting and outputting an output signal by frequency inverting the input signal in a modulation mode and for selecting and outputting an output signal filtered by said low pass filter and said high pass filter in a through mode.

A voice scrambler comprising:

• a) means for frequency inverting and scrambling a voice signal, comprising:
• a double balanced modulator for converting the voice signal into a signal having upper and lower side bands;
• and
• a low pass filter permitting only said lower side band to pass therethrough,
• b) a high pass filter having a low band cutoff reciprocal to a cut-off of said low pass filter,
• c) means for determining whether the voice signal is frequency inverted and scrambled and for determining whether the voice signal is filtered by said low pass filter and said high pass filter without being frequency inverted and scrambled, and
• d) means for outputting one of the voice signal frequency inverted and scrambled and the voice signal filtered by said low pass filter and the high pass filter to a transmission channel.

A voice descrambler comprising:

• a) means for frequency inverting and descrambling an input signal, comprising:
• a double balanced modulator for converting the voice signal into a signal having upper and lower side bands;
• and
• a low pass filter permitting only said lower side band to pass therethrough,
• b) a high pass filter having a low band cut-off reciprocal to a cut-off of said low pass filter,
• c) means for receiving a frequency inverted and scrambled signal from a transmission channel, for forming an output voice signal by frequency inverting and descrambling said frequency inverted and scrambled signal, for receiving a not frequency inverted and scrambled signal from the transmission channel and for forming the output voice signal by filtering said not frequency inverted and scrambled signal with said low and high pass filters.