gbell@pnet12.cts.com (Greg Bell) (01/28/89)
>Now that we can decode DTMF, how hard would it be to add some functionality >to produce a dial tone? Well, according to my notes, a "precision dial tone" (whatever "precision" means) is a combination of 350 Hz and 440 Hz (ie. those two frequencies at the same time). Greg Bell_________________________________________________________ Hardware hacker | Electronics hobbyist | UUCP: uunet!serene!pnet12!gbell EE major at UC San Diego |
ISW@cup.portal.com (Isaac S Wingfield) (01/30/89)
"Precision dial tone", I think, refers to the fact that, pre-DTMF, the dial tone generator was just that: a rotating machine! With the advent of DTMF, it became necessary to control the harmonic content of the dial tone so it didn't gimp up the DTMF decoder; hence "precision". If you think about the DTMF frequencies, and the precision dial tone, none of the possible harmonics from any of those could be mistaken for an instance of any other tone. BTW, on the precision dial tone: the frequencies are 350 and 440 Hz, with amplitudes chosen so that the lower produces 100% double sideband modulation of the upper. The signals are actually just summed, but if you look with a 'scope, that's what you'll see. Isaac isw@cup.portal.com
rmrin@inuxh.UUCP (D Rickert) (01/31/89)
> > > Well, according to my notes, a "precision dial tone" (whatever > "precision" means) is a combination of 350 Hz and 440 Hz (ie. those two > frequencies at the same time). > > > Greg Bell_________________________________________________________ > Hardware hacker | > Electronics hobbyist | UUCP: uunet!serene!pnet12!gbell > EE major at UC San Diego | In the bad old days, dial tone was produced by a motor generator set and the frequency (and spectral content) of the tone was quite imprecise. -- Dick Rickert AT&T Consumer Products Laboratory
jeffw@midas.STS.TEK.COM (Jeff Winslow) (02/02/89)
In article <14094@cup.portal.com> ISW@cup.portal.com (Isaac S Wingfield) writes: >BTW, on the precision dial tone: the frequencies are 350 and 440 Hz, >with amplitudes chosen so that the lower produces 100% double >sideband modulation of the upper. The signals are actually just >summed, but if you look with a 'scope, that's what you'll see. Hmm. 350Hz 100% DSB modulation of 440Hz gets components at 90Hz and 790Hz; summing gets components at 350Hz and 440Hz. Could you explain that one some more? Jeff Winslow