SC> Today, there are 2 kinds of "PL tones" - the old fashioned low
 SC> frequency audio tones from about 55 Hz to about 250 Hz (yes there are
 SC> specified frequencies - you can't pick a frequency "between" two of
 SC> the specified frequencies) - and since the early '70s a "digital" form
Beg pardon?  Sure you can.  non-standard PL's are used in a number of 
applications, such as a local SO we have, that has had a problem with
a person discovering their input and tone, and disrupting their 
communications.
Non-standard tones, and presto, the problem disappeared.
--- PPoint 1.98
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