precise tone plan造句

• Prior to the Precise Tone Plan, parts of the Bell System and various switching systems used slightly varying signal frequencies and levels.
• In North America, the Precise Tone Plan used today employs two tones of 480 and 620 s, off for 0.5 s ).
• The tone frequencies, as defined by the Precise Tone Plan, are selected such that harmonics and intermodulation products will not cause an unreliable signal.
• The Precise Tone Plan for the North American Numbering Plan of the tones ( 350 Hz and 440 Hz ) which when mixed appear to have a modulation at 90 Hz.