5196106136

Continued from page 65

LOW GROUP FREOUENCIES' Hz

HIGH GROUP

FREOUENCIES Hz --*--

>3)6 U 77    *33

H3 00l

”°[H 00i ~0 0 0 i] ^,j0 0 0 0

Fig.3: Many modem electronic phones and attachments use dual-tone multifrequency (DTMF) dlalling, rather than the older and slower pulse dialllng. Here are the frequency pairs used tor each digit.

make and break methód of signalling; instead they generate tone-pairs, each of which correspond to one of 16 codes.

Tonc signalling is morę convenient and morc efficient than the old meth-ods. It offers higher rcliability and has uses outside the basie switching func-tion — in answering machines, radio Communications, data transmission and remote control.

With DTMF the telephone handset generates a composite audio signal, madę up by superimposing two tones selected by a line-and-column address-ing of a keyboard. (See Fig.3). The fourth column of buttons using the 1633Hz tonę is not found on most of today’s touch-phones, but it has been set aside for futurę signalling use.

The DTMF frcquencies were chosen so that neither harmonics or intermodu* lation products would fali in any one of the tonę bands. You get about 10% separation between tones. The tones are sent over the lines to the exchange, w here decoder circuitry converts the DTMF signal to a binary format. For the traditional dial-pulse equipment, a further conversion takes place in a tone-to-pulse converter.

DTMF technology lends itself to a variety of remote-control applications, including those using radio-telephone stages Tonę signalling allows radio links to plug-in transparently to the telephone network.

. Remote data entry is also possible, as are automatic credit-card verification Systems, home banking and shopping. Coupled with voice synthesis tech-niques, DTMF brings many of the ad-vantages of ATMs and point-of-sale equipment directly into the home with-out adding subscriber equipment. One can only wonder why we haven’t all been using them for years..    . ®

Now it’s eaśy to design schematics with your Computer

Simply use OrCAD/SDT, the easy to use, totally flexible and affordable schematic design tool (software) - for use with IBM PC/XT/AT or compatible computers.

Easy schematic design

OrCAD/SDT makes the placcmcnt of graphic symbols, intcrconnections. components, and text both fast and casy.

Parts can bc selected by the keyboard or from a pop-up dircctory (and moved or rotated to where you choose).

Fast graphical editing

With powerful editing commands, single objeets or groups of objeets are casily moved, replicated or dclcted. And if an objcct is accidentally removcd, it can bc immediatcly recovercd with an "undo" command.

OrCAD/SDT Features:

• Uniquc parts library • Kubbcrbands wires/buses • Automatic pan • Levels of hierarchy • Graphics options • Zoom levels • Design check • Back annotation • t)eMorgan equivalents • String searching • Part rotation & mirroring • Keyboard macros • Color graphics • On-line part browsing • Net list translations • List of materials

OrCADS

Systems Corporation r

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Prometheus Software Developments Pty Ltd. 191 Riversdale Road Hawthorn Vic. 3122. Telephone: [phone]    Fax: [phone]

ELECTRONICS Australia. September 1987