Howie, The original Marine selective call used two tones which toggled controlling a mechanical escapement. At the end of each 0-9 digit the hold tone was kept on & a pawl would hold the escapement wheel if a pin was placed for that number in the wheel. The next digit would then be dialed. If it was the correct number the escapement would move around to a fixed contact that would sound a bell. If it was the wrong digit the wheel would return to the start position. This system was used on Marine and land mobile phone systems until the SS revolution in the 70's

Art, KC6UQH

-----Original Message----- From: amsat-bb-bounces@amsat.org [mailto:amsat-bb-bounces@amsat.org] On Behalf Of Howie DeFelice Sent: Friday, January 10, 2014 12:18 PM To: amsat-bb@amsat.org Subject: [amsat-bb] Re: DTMF on HF?

Bob,

I don't have any experience using DTMF on HF but back in the day before satcom, commercial marine HF SSB coast stations used a dual tone selective calling system made by Lorain Electronics. The encoder used 11 tones, one each for the digits 0-9 and a carrier tone. Each digit was transmitted along with the carrier tone and the difference in frequency was detected to decode the digit. This compensated for small frequency variations. These worked very well. You could probably do a similar thing with a single row or column of DTMF if you didn't need more than 4 digits.

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