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<             The United Phreaker's Incorporated Proudly Presents            >
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<                     Music On Hold Adapter (Music Box)                      >
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<                            By: The Lost Avenger                            >
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<                           An Upi Production 1990                           >
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Reprinted In File Form On March 4, 1990

Orignally Publised In Radio Electronics August 1989

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                            Music On Hold Adapter
                             By Steve Sokolowski

        Have you ever been in the middle of a phone conversation, and need to
either walk away for a couple of minutes or switch lines?  The kids might be
crying, the washing machine might need fabric softener, whatever.  Here's an
inexpensive "Electronic hold button" controlled by a Touch Tone phone, with
provision for audio for the other party, using either a music synthesizer IC
module or a radio.
        Unlike normal hold buttons, this one doesn't cut your phone handset off
from the other party; you can still hear one another, ans also the audio, if
you use it.  Rather, what it does is let you provide this audio to the other
parrty, and then lets you hang up the handset without losing your party;  you
can also opt for silence.  When you wish to resume conversation, pressing the
pound (#) key shuts off the audio if used, and allows you to hang up and
disconnect when you need to know how hold buttons and Touch Tone phones work.

Placing A Call On Hold

        The normal on hook voltage of a phone line is about 50 volts, depending
on particular central office equipment.  When you lift a phone's handset, 600
ohms is place acriss the phone line, dropping the voltage to 5 volts.
        The hold adapter prevents a disconnection when you hang up the handset,
by placing R6 And LED1 across the phone line using RY1.  The synthesizer
provides music for the party on hold;  it can be left out if desired.

Touch Tone Operation

        Touch Tone Phones produce Dual Tone Multi Frequency (DTMF) signals,
using a 4 X 4 keypad that produces two tocuh tone sinusoidal outputs.  The
frequencies are determined by the key row and column (see Figure 1);  The row
frequencies are lower than those of the columns, and a Touch Tone signal is the
sum of both.  Normal Touch Tone pads have 12 keys, but the operator console
keys (A-D) which normally aren't used.
        Adding two sinusoids is equivalent to multiplying two sinusoids whose
frequencies are the difference if the orginals.  Early Touch Tone pads used
indicators and capacitors; modern versions are crystal controlled intergrated
circuits that create staircase sinusoids.

Circuit Description

        The G8870 represents each tone pair as a 4 bit code, where the column
of a key is the first two bits, and the row of a key is the second two bits;
both pairs of bits range from 00-11.  This 4 bit code is then decoded by a 4514
4 to 16 decoder (IC3) to give the digit dialed.
        Pressing the (*) key on a Touch tone phone adds a 1209Hz sinusoid (a
Row 4 tone), and a 941 Hz sinusoid (a Column 1 tone), to give a binary code of
1100.  Similary, pressing (#) key adds 941Hz And 1477Hz sinusoids, to give a
binary code of 1100.
        If a user preses a key on a Touch Tone phone used to operate the hold