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 Post subject: TONE DECODER
PostPosted: Sun Jul 16, 2017 8:04 pm 
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Joined: Thu Oct 26, 2006 7:11 pm
Posts: 1269
Location: Maryland
Working on a project that would benefit from coded tones for control. I am well aware of, and familiar with encoders and decoders that use a 3.58Mhz crystal for DTMF signaling.

What I am considering is to use a crystal other than 3.58 Mhz to create other groups of tones that are separate from the standard DTMF tones. Since the standard DTMF tone group is actually a 4x4 matrix to create 16 tone pairs, If this would work more tone groups could be used for signaling over the same transmission means.

I have been unable to find any data that refers to crystals other than 3.58, but would expect that other crystals would create different tone groups?

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Len

“To invent, you need a good imagination and a big pile of junk.” (T. Edison)
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 Post subject: Re: TONE DECODER
PostPosted: Tue Jul 18, 2017 6:00 am 
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Joined: Fri Feb 01, 2002 1:01 am
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Location: Mideast USA
Hi Len.
I believe a different crystal would yield different set of tones. Your decoder would need the same shifted crystal. For not coincidental frequencies, some arithmetic is in order to ensure none of the 16 codes would be repeated and they become uniformingly spread on the audio spectrum without stepping into each other detection passband.

If 3.579545MHz yields 770 and 1336 Hz (for #5); the ratios are near 4648 and 2679

Go for it and come back with results. Am curious. I remember learning somewhere there is other than dialing 'hidden' DTMF tones used by the telco for recognizing the coins deposited in a payphone and other billing tasks. Perhaps those are ABCD ?



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 Post subject: Re: TONE DECODER
PostPosted: Tue Jul 18, 2017 6:50 am 

Joined: Fri Aug 22, 2003 12:01 am
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Location: (somewhere), Afghanistan; from Rochester, NY
Quote:
Perhaps those are ABCD ?
No. Coin tone is mix of 1700Hz and 2200Hz in the U.S.. Highest tone in high group of DTMF is 1633Hz. "ABCD", "#" and "*" were/are used for specialized functions in different systems. (i.e. the U.S. military system used A thru D for precedence dialing.) Still other tones were for passing number dialed over analog trunks or older digital trunks. Now done by digital signaling over the separate data channel of multi-channel trunk groups. (no more blue box cheating of the Telco)

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 Post subject: Re: TONE DECODER
PostPosted: Tue Jul 18, 2017 9:16 am 
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Joined: Thu Oct 26, 2006 7:11 pm
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Location: Maryland
I'll do a spreadsheet to figure the 3.58 division factors and the guard bands for the standard DTMF tones then plug in other standard crystal frequencies to see any possible overlap. I am really surprised the chip data doesn't mention other crystal frequencies!
Thanks...



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