Complex sentence: Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means; in the United States, the standard "city" dial tone was a 600 Hz tone that was amplitude-modulated at 120 Hz.
Split into simpler sentences: Before modern electronic telephone switching systems came into use, dial tones were usually generated by electromechanical means. In the United States, the standard "city" dial tone was a 600 Hz tone that was amplitude-modulated at 120 Hz.