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In Reply to: RE: DSD [A Reprise]... posted by Tony Lauck on September 12, 2014 at 07:09:23:
OK, you make a valid point. We should bring up 'prior art' and give credit to Gottfried Wilhelm Leibniz who invented the modern version of our binary number system. ;-)
I was hacking phones as a kid in the 1970's. I bought a surplus Ma Bell touch tone decoder board about a foot square in size. It had multiple passive LC tuned circuits on it (audio filters) to decode tone pairs and discrete logic using bipolar transistors at the output.
I later used the LM567 tone decoder PLL chip, 8 of them with additional logic gates to decode all the necessary tone pairs for a 16 key pad. These were OK but the passive RC circuits weren't all that stable over temperature and had to be re tuned (10 turn trimpot) frequently. Not practical for the application I had in mind.
And then a couple companies came out with single chip DTMF decoders using a quartz crystal frequency reference and 4-bit binary output that could decode all 16 tone pairs. I added some logic (74154 series decoder and 7474 series Dual-D flip-flops) to decode touch-tone sequences to latch relays ON or OFF to control devices over the phone or via radio.
It's all done in software now using micro controllers - PIC and others. ;-)
I wasn't really 'hacking' phones. We were building 'auto-patch' systems using FM radio repeater stations located high atop tall mountains tied to telephone lines.
Back in the days before cellphones existed, we were making phone calls from our cars or hand-held FM radio transceivers for FREE. A touch tone sequence from a 16-key pad was sent through the radio's mic to 'bring up' the auto-patch on the mountain. Auto-patch was the automated linking of the radio system to the phone line. Once we heard dial-tone on the radio, we could use the same touch tone pad to dial a phone number. At the end of a call another touch tone sequence was sent to hang-up the line. A watchdog hang-up timer was used to hang-up the phone line in case we lost radio contact and couldn't hang-up the line remotely.
These projects were time consuming and costly so we had a number of people involved working on different aspects of the system. They were called HAM Radio clubs. We collected dues to pay for equipment, electricity, and the phone line. Only members had the touch-tone sequence for accessing the auto-patch system. There were other safeguards as well.
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Follow Ups
- RE: DSD [A Reprise]... - AbeCollins 09:44:13 09/12/14 (0)
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