To support the teleoperation of remote robots, this study propose a methodology for modulating vibrotactile signals to enhance human perceptual characteristics. The methodology is needed because the frequencies of the vibrotactile signals occurring on robots are often too high for humans to perceive; thus, our objective is to transmit important information, e.g., the contact characteristics contained in high-frequency vibro-tactile signals, to operators. We considered that the envelope of the vibrotactile signals contains contact characteristics, e.g., the material properties and the robots' motion. Our proposed method modulates the amplitude of the sinusoidal vibration with the upper and lower envelopes of the measured vibratory signals. We propose two modulation types modulated from the original signals (option (a)), which are (b) the envelope of the original signals, and (c) the amplitude-modulated vibration, whose amplitude is modulated with the envelopes of the original vibratory signals and whose carrier frequency range is sensible by humans. Before evaluating our proposed method, we conducted an experiment to determine the carrier frequency of method (c). Finally, we conducted a discrimination experiment whose results showed that our modulation method (c) best improved the human discriminability.