WEBVTT - How does Caller ID work?

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<v Speaker 1>Audible dot com is the leading provider of digital audio

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<v Speaker 1>stuff works dot com where smart Happens. Hi Am Marshall

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<v Speaker 1>Brain with today's question, how does collar i D work?

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<v Speaker 1>If you have a color i D box attached to

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<v Speaker 1>your phone, or if the color i D box is

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<v Speaker 1>built into your phone, or if you have a cell phone,

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<v Speaker 1>then an amazing thing happens. Every time your phone rings,

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<v Speaker 1>the number, and sometimes even the name of the calling

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<v Speaker 1>party appears on the display right after the first ring.

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<v Speaker 1>It turns out that the process of making the color

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<v Speaker 1>i D display possible is remarkably simple at your end

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<v Speaker 1>of the line. If you've read the house stuff Work

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<v Speaker 1>article entitled how modems work, then you know that the

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<v Speaker 1>early modems used a technique called frequency shift keying to

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<v Speaker 1>transmit bits over the phone line. Frequency shift keying is

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<v Speaker 1>extremely simple. One tone, for example, like twelve hundred hurts

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<v Speaker 1>or beep represents a one, and another tone LIKEEP represents

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<v Speaker 1>a binary zero. A modem changes frequencies depending on whether

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<v Speaker 1>it wants to send a one or a zero. How

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<v Speaker 1>quickly it changes frequencies determines the speed or the bod

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<v Speaker 1>rate of the modem. To send caller i D information

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<v Speaker 1>to your home, the phone company uses a frequency shift

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<v Speaker 1>keying technique identical to a two modem from a million

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<v Speaker 1>years ago, and it sends asky character data to the phone.

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<v Speaker 1>The modem message is sent between the first and the

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<v Speaker 1>second ring, so the phone rings once and if you

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<v Speaker 1>could listen to the phone line just after that ring,

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<v Speaker 1>you would hear bleep sound about half a second long,

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<v Speaker 1>and if you decoded the bleep, you would find that

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<v Speaker 1>it contains first a series of alternating ones and zeros

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<v Speaker 1>to help the caller i D box get the timing down,

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<v Speaker 1>then a series of a hundred and eighty ones, and

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<v Speaker 1>then a bite representing the type of message that's about

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<v Speaker 1>to be sent, and then a bite representing the length

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<v Speaker 1>of the message, then a pair of bites representing the month, day, hour,

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<v Speaker 1>and minute that the message was sent, and then the

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<v Speaker 1>tendiget phone number in ten bytes, followed by a checksum

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<v Speaker 1>bite so the phone can make sure the message came

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<v Speaker 1>through okay. There's also a more advanced system that contains

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<v Speaker 1>the caller's name, but its technique is identical. Each character

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<v Speaker 1>is sent with a standard eight bit asky character, preceded

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<v Speaker 1>by a zero start bit and followed by a one

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<v Speaker 1>stop bit. The phone contained the twod modem to decode

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<v Speaker 1>the bits, a little circuit to detect the ringing signal,

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<v Speaker 1>and a simple processor to drive the display. That's really

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<v Speaker 1>all the into the whole thing. For more on this

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<v Speaker 1>and thousands of other topics, visit how staff works dot com.

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<v Speaker 1>Audible dot com is the leading provider of digital audio

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<v Speaker 1>books and spoken word, with over one thousand titles to

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<v Speaker 1>choose from. Audible lets you listen to your favorite books anywhere, anytime.

0:03:20.639 --> 0:03:23.679
<v Speaker 1>Go to audible podcast dot com slash stuff brain to

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<v Speaker 1>get a free audio book download of your choice when

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<v Speaker 1>you sign up today.