Specifications for valid DTMF reception:
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Tone-address signals must adhere to the following guidlines:


 1) Signal Levels:

     Nominal level per frequency: -6 to -4 dBm. Minimum level per frequency:
     Low Group, -10 dBm; High Group, -8 dBm. Maximum level per frequency pair:
     +2 dBm. Maximum difference in levels between frequencies: 4 dB.


 2) Frequency Deviation: +/- 1.5 percent of the values given above.


 3) Extraneous frequency components: The total power of all extraneous
    frequencies accompanying the signal should be a t least 20 dB below the
    signal power, in the voice band above 500 Hz.


 4) Voice Suppression: Voice energy from any source should be suppressed at
    least 45 dB during tone signal transmission. In the case of automatic
    dialing, the suppression should be maintained continuously until pulsing
    is completed.


 5) Rise Time:

     Each of the two frequencies of the signal should attain at least 90
     percent of full amplitude within 5 ms, and preferably within 3 ms for
     automatic dialers, from the time that the first frequency begins.


 6) Pulsing Rate:

     Minimum duration of two-frequency tone signal: 50 ms normally; 90 ms if
     transmitted by radio. Minimum inter-digital time: 45 ms.


 7) Tone leak during signal off time should be less than -55 dBm.


 8) Transient Voltages:

     Peak transient voltages generated during tone signaling should be no
     greater than 12 dB above the zero-to-peak voltage of the composite two-
     frequency tone signal.