Video Door Phone For Home Security

8/9/2019 Video Door Phone For Home Security

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INDEX

1. Microphone

2. Loudspeaker

3. Audio Am plifier

4. V ideo C amera

5. CRT Monitor


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Microphone

A microphone ( colloquially called a mic or mike; both

pronounced / ma k /) is an acoustic-to-

electric transducer or sensor that converts sound into an electrical

signal . I n 1876, Emile Berliner invented the first microphone used as a

telephone voice trans mitter . Microphones are used in many

applications such as telephones , tape

recorders , karaoke syste ms , hearing aids , motion picture production ,

live and recorded audio engineering , FRS radios , megaphones ,

in radio and television broadcasting and in com puters for recording

voice, speech recognition , V oIP, and for non-acoustic purposes such

as ultrasonic checking or knock sensors . Most microphones today

use electromagnetic induction ( dynamic microphone ), capacitance

change ( condenser microphone ), piezoelectric generation , or light

modulation to produce an electrical voltage signal from mechanical

vi bration.

V arieties

T he sensitive transducer element of a microphone is called its element or capsule . Acom plete microphone also includes a

housing , some means of bringing the signal from the element to other equipment , and often an electronic circuit to adapt the

output of the apsule to the equipment being driven. Microphones are referred to by their transducer principle, such as

condenser , dynamic , etc ., and by their directional characteristics . S ometi mes other characteristics such as diaphrag m size,

intended use or orientation of the principal sound input to the principal axis ( end- or side-address ) of the microphone are

used to descri be the microphone .


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C ondenser microphone

T he condenser microphone , invented at Bell Labs in 1916 by E. C. W ente is also called a capacitor

microphone or electrostatic microphone . H ere, the diaphrag m acts as one plate of a capacitor , and the vi brations produce

changes in the distance between the plates . T here are two types , depending on the method of extracting the audio signal from

the transducer: DC -biased and radio frequency (RF)or high frequency (HF)condenser microphones . W ith a DC -biased

microphone , the plates are biased with a fixed charge (Q). T he voltage maintained across the capacitor plates changes with

the vi brations in the air , according to the capacitance equation (C = Q / V),where Q =charge in coulombs , C =capacitance

in farads and V = potential difference in volts. T he capacitance of the plates is inversely proportional to the distance between

them for a parallel-plate capacitor . T he asse mbly of fixed and movable plates is called an " element " or " capsule ." Anearly

constant charge is maintained on the capacitor . As the capacitance changes , the charge across the capacitor does change

very slightly , but at audi ble frequencies it is sensi bly constant . T he capacitance of the capsule ( around 5100 pF)and the

value of the bias resistor (100 mega-ohms to tens of giga-ohms ) for m a filter that is high pass for the audio signal , and low

pass for the bias voltage. N ote that the ti me constant of an RC circuit equals the product of the resistance and capacitance .W ithin the ti me-frame of the capacitance change ( as much as 50 ms at 20 H z audio signal ), the charge is practically constant

and the voltage across the capacitor changes instantaneously to reflect the change in capacitance . T he voltage across the

capacitor varies above and below the bias voltage. T he voltage difference between the bias and the capacitor is seen across

the series resistor . T he voltage across the resistor is am plified for perfor mance or recording .

RF condenser microphones use a com paratively low RF voltage, generated by a low-noise oscillator . T he oscillator may either

be am plitude modulated by the capacitance changes produced by the sound waves moving the capsule diaphrag m, or the

capsule may be part of a resonant circuit that modulates the frequency of the oscillator signal . Demodulation yields a low-

noise audio frequency signal with very low source i m pedance . T he absence of a high bias voltage per mits the use of a

diaphrag m with looser tension , which may be used to achieve wider frequency response due to higher com pliance. T he RF

biasing process results in a lower electrical i m pedance capsule , a useful byproduct of which is that RF condenser

microphones can be operated in dam p weather conditions that could create problems in DC -biased microphones with

conta minated insulating surfaces . T he S ennheiser " MK H" series of microphones use the RF biasing technique . C ondenser

microphones span the range from telephone trans mitters through inexpensive karaoke microphones to high-fidelity recording

microphones . T hey generally produce a high-quality audio signal and are now the popular choice in laboratory and studio


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recording applications . T he inherent suitability of this technology is due to the very small mass that must be moved by the

incident sound wave, unlike other microphone types that require the sound wave to do more work . T hey require a power

source , provided either via microphone outputs as phantom power or from a small battery . P ower is necessary for

esta blishing the capacitor plate voltage, and is also needed to power the microphone electronics ( i m pedance conversion in

the case of electret and DC -polarized microphones , demodulation or detection in the case of RF/HF microphones ). C ondenser

microphones are also available with two diaphrag ms that can be electrically connected to provide a range of polar patterns

( see below ), such as cardioid , omnidirectional , and figure-eight . I t is also possi ble to vary the pattern continuously with some

microphones , for exam ple the Rde NT2000 or CADM 179.

E lectret condenser microphone

An electret microphone is a relatively new type of capacitor microphone invented at Bell laboratories in 1962 by G erhard

S essler and J i m W est . T he externally applied charge descri bed above under condenser microphones is replaced by a

per manent charge in an electret material . An electret is a ferroelectric material that has been per manently electrically

charged or polarized . T he name comes from electrostatic and magnet; a static charge is embedded in an electret by alignment

of the static charges in the material , much the way a magnet is made by aligning the magnetic domains in a piece of iron.

Due to their good perfor mance and ease of manufacture , hence low cost , the vast majority of microphones made today are

electret microphones; a semiconductor manufacturer esti mates annual production at over one billion units . N early all cell-

phone , com puter , PDAand headset microphones are electret types . T hey are used in many applications , from high-quality

recording and lavalier use to built-inmicrophones in small sound recording devices and telephones . T hough electret

microphones were once considered low quality , the best ones can now rival traditional condenser microphones in every

respect and can even offer the long-ter m stability and ultra-flat response needed for a measure ment microphone . U nlike other

capacitor microphones , they require no polarizing voltage, but often contain an integrated pream plifier that does require

power ( often incorrectly called polarizing power or bias ). T his pream plifier is frequently phanto m powered in sound

reinforcement and studio applications . Microphones designed for personal com puter (PC)use , someti mes called multi media

microphones , use a stereo 3.5 mm plug ( though a mono source ) with the ring receiving power via a resistor from ( nor mally ) a

5 V supply in the com puter; unfortunately , a number of incom pati ble dynamic microphones are fitted with 3.5 mm plugs too.

W hile few electret microphones rival the best DC -polarized units in ter ms of noise level , this is not due to any inherent

li mitation of the electret . R ather , mass production techniques needed to produce microphones cheaply don't lend themselves

to the precision needed to produce the highest quality microphones , due to the tight tolerances required in internal

di mensions . T hese tolerances are the same for all condenser microphones , whether the DC, RF or electret technology is used .

Dynamic microphone

Dynamic microphones work via electromagnetic induction . T hey are robust , relatively inexpensive and resistant to moisture .

T his coupled with their potentially high gain before feed back makes them ideal for on-stage use .


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Moving-coil microphones use the same dynamic principle as in a loudspeaker , only reversed . Asmall movable induction coil ,

positioned in the magnetic field of a per manent magnet , is attached to the diaphrag m. W hen sound enters through the

windscreen of the microphone , the sound wave moves the diaphrag m. W hen the diaphrag m vi brates , the coil moves in the

magnetic field , producing a varying current in the coil through electromagnetic induction. Asingle dynamic membrane does

not respond linearly to all audio frequencies . S ome microphones for this reason utilize multiple membranes for the different

parts of the audio spectru m and then combine the resulting signals . C ombining the multiple signals correctly is difficult and

designs that do this are rare and tend to be expensive . T here are on the other hand several designs that are more specifically

ai med towards isolated parts of the audio spectru m. T he AK G D 112,for exam ple, is designed for bass response rather than

treble.[4] I n audio engineering several kinds of microphones are often used at the same ti me to get the best result .

R i bbon Microphone

R i bbon microphones use a thin, usually corrugated metal ri bbon suspended in a magnetic field . T he ri bbon is electrically

connected to the microphone's output , and its vi bration within the magnetic field generates the electrical signal . R i bbon

microphones are si milar to moving coil microphones in the sense that both produce sound by means of magnetic induction.

Basic ri bbon microphones detect sound in a bi-directional ( also called figure-eight ) pattern because the ri bbon, which is open

to sound both front and back , responds to the pressure gradient rather than the sound pressure . T hough the sy mmetrical

front and rear pickup can be a nuisance in nor mal stereo recording , the high side rejection can be used to advantage by

positioning a ri bbon microphone horizontally , for exam ple above cy mbals, so that the rear lobe picks up only sound from the

cy mbals . C rossed figure 8, or Blumlein pair , stereo recording is gaining in popularity , and the figure 8 response of a ri bbon

microphone is ideal for that application .

Other directional patterns are produced by enclosing one side of the ri bbon in an acoustic trap or baffle, allowing sound to

reach only one side . T he classic RCA T ype 77 -DX microphone has several externally adjusta ble positions of the internal

baffle, allowing the selection of several response patterns ranging from "F igure-8" to "U nidirectional ". S uch older ri bbon

microphones , some of which still provide high quality sound reproduction , were once valued for this reason , but a good low-

frequency response could only be obtained when the ri bbon was suspended very loosely , which made them relatively fragile.

Modern ri bbon materials , including new nano-materials have now been introduced that eli minate those concerns , and even

i m prove the effective dynamic range of ri bbon microphones at low frequencies . P rotective wind screens can reduce the

danger of damaging a vintage ri bbon, and also reduce plosive artifacts in the recording . P roperly designed wind screens

produce negligi ble treble attenuation . I n common with other classes of dynamic microphone , ri bbon microphones don't

require phantom power; in fact , this voltage can damage some older ri bbon microphones . S ome new modern ri bbon

microphone designs incorporate a pream plifier and , therefore , do require phanto m power , and circuits of modern passive

ri bbon microphones , i .e., those without the aforementioned pream plifier , are specifically designed to resist damage to the

ri bbon and transfor mer by phanto m power . Also there are new ri bbon materials available that are i mmune to wind blasts and

phantom power .


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C ar bon microphone

A car bon microphone , also known as a car bon button microphone ( or someti mes just a button microphone ), use a capsule or

button containing car bon granules pressed between two metal plates like the Berliner and E dison microphones . Avoltage is

applied across the metal plates , causing a small current to flow through the car bon. One of the plates , the diaphrag m, vi brates

in sy m pathy with incident sound waves , applying a varying pressure to the car bon. T he changing pressure defor ms the

granules , causing the contact area between each pair of adjacent granules to change , and this causes the electrical

resistance of the mass of granules to change . T he changes in resistance cause a corresponding change in the current flowing

through the microphone , producing the electrical signal . C ar bon microphones were once commonly used in telephones; they

have extremely low-quality sound reproduction and a very li mited frequency response range , but are very robust devices . T he

Boudet microphone , which used relatively large car bon balls, was si milar to the granule car bon button microphones .

U nlike other microphone types , the car bon microphone can also be used as a type of am plifier , using a small amount of

sound energy to control a larger amount of electrical energy . C ar bon microphones found use as early telephone repeaters ,

making long distance phone calls possi ble in the era before vacuum tubes . T hese repeaters worked by mechanically coupling

a magnetic telephone receiver to a car bon microphone: the faint signal from the receiver was transferred to the microphone ,

with a resulting stronger electrical signal to send down the line. One illustration of this am plifier effect was the oscillation

caused by feed back , resulting in an audi ble squeal from the old " candlestick " telephone if its earphone was placed near the

car bon microphone .

P iezoelectric microphone

A crystal microphone or piezo microphone uses the pheno menon of piezoelectricity the ability of some materials to

produce a voltage when sub jected to pressure to convert vi brations into an electrical signal . An exam ple of this

is potassiu m sodiu m tartrate , which is a piezoelectric crystal that works as a transducer , both as a microphone and as a sli m

line loudspeaker com ponent . C rystal microphones were once commonly supplied with vacuum tube ( valve ) equipment , such

as domestic tape recorders . T heir high output i m pedance matched the high input i m pedance ( typically about 10 mega ohms )

of the vacuum tube input stage well . T hey were difficult to match to early transistor equipment , and were quickly supplanted

by dynamic microphones for a ti me, and later small electret condenser devices . T he high i m pedance of the crystal

microphone made it very suscepti ble to handling noise , both from the microphone itself and from the connecting cable.

P iezoelectric transducers are often used as contact microphones to am plify sound from acoustic musical instruments , to

sense drum hits, for triggering electronic sam ples, and to record sound in challenging environments , such as underwater

under high pressure . S addle-mounted pickups on acoustic guitars are generally piezoelectric devices that contact the strings

passing over the saddle . T his type of microphone is different from magnetic coil pickups commonly visi ble on typical electric

guitars , which use magnetic induction, rather than mechanical coupling , to pick up vi bration.


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F i ber optic microphone

A fi ber optic microphone converts acoustic waves into electrical signals by sensing changes in light intensity , instead of

sensing changes in capacitance or magnetic fields as with conventional microphones .

During operation , light from a laser source travels through an optical fi ber to illuminate the surface of a tiny , sound-sensitive reflective diaphragm. S ound causes the diaphrag m to vi brate , there by minutely changing the intensity of the light it reflects .

T he modulated light is then trans mitted over a second optical fi ber to a photo detector , which transfor ms the intensity-

modulated light into analog or digital audio for trans mission or recording . F i ber optic microphones possess high dynamic and

frequency range , si milar to the best high fidelity conventional microphones .

F i ber optic microphones do not react to or influence any electrical , magnetic , electrostatic or radioactive fields ( this is

called E M I/RFI i mmunity ). T he fi ber optic microphone design is therefore ideal for use in areas where conventional

microphones are ineffective or dangerous , such as inside industrial tur bines or in magnetic resonance i maging ( M RI)

equipment environments .

F i ber optic microphones are robust , resistant to environmental changes in heat and moisture , and can be produced for any

directionality or i m pedance matching . T he distance between the microphone's light source and its photo detector may be up

to several kilometers without need for any pream plifier and / or other electrical device, making fi ber optic microphones suitable

for industrial and surveillance acoustic monitoring.

F i ber optic microphones are used in very specific application areas such as for infrasound monitoring and noise-canceling .

T hey have proven especially useful in medical applications , such as allowing radiologists , staff and patients within the

powerful and noisy magnetic field to converse nor mally , inside the M RI suites as well as in remote control rooms .) Other uses

include industrial equipment monitoring and sensing , audio cali bration and measure ment , high-fidelity recording and law enforcement .

Laser microphone

Laser microphones are often portrayed in movies as spy gadgets . Alaser beam is ai med at the surface of a window or other

plane surface that is affected by sound . T he slight vi brations of this surface displace the returned beam, causing it to trace

the sound wave. T he vi brating laser spot is then converted back to sound . I n a more robust and expensive i m plementation ,

the returned light is split and fed to an interferometer , which detects movement of the surface . T he for mer i m plementation is a

tabletop experi ment; the latter requires an extremely sta ble laser and precise optics .

A new type of laser microphone is a device that uses a laser beam and smoke or vapor to detect sound vi brations in free air .

On 25 August 2009, U.S. patent 7,580,533issued for a P articulate F low Detection Microphone based on a laser-photocell pair

with a moving strea m of smoke or vapor in the laser beam's path. S ound pressure waves cause distur bances in the smoke

that in turn cause variations in the amount of laser light reaching the photo detector . A prototype of the device was

demonstrated at the 127 th Audio E ngineering S ociety convention in N ew Y ork C ity from 9 through 12 October 2009.


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Liquid microphone

E arly microphones did not produce intelligi ble speech , until Alexander G raham Bell made i m provements including a variable

resistance microphone / trans mitter . Bell's liquid trans mitter consisted of a metal cup filled with water with a small amount of sulfuric acid added . Asound wave caused the diaphrag m to move, forcing a needle to move up and down in the water . T he

electrical resistance between the wire and the cup was then inversely proportional to the size of the water meniscus around

the submerged needle . E lisha G ray filed a caveat for a version using a brass rod instead of the needle . Other minor variations

and i m provements were made to the liquid microphone by Majoranna, C hambers , V anni , S ykes, and E lisha G ray , and one

version was patented by R eginald F essenden in 1903. T hese were the first working microphones , but they were not practical

for commercial application. T he famous first phone conversation between Bell and W atson took place using a liquid

microphone .

M E M S microphone

T he M E M S ( MicroE lectrical-Mechanical S ystem) microphone is also called a microphone chip or silicon microphone . T he

pressure-sensitive diaphrag m is etched directly into a silicon chip by M E M S techniques , and is usually accom panied with

integrated pream plifier . Most M E M S microphones are variants of the condenser microphone design . Often M E M S

microphones have built in analog-to-digital converter (ADC)circuits on the same C M OS chip making the chip a digital

microphone and so more readily integrated with modern digital products . Major manufacturers producing M E M S silicon

microphones are W olfson Microelectronics (W M 7 xxx ), Analog Devices , Akustica (AK U200 x ), I nfineon (S MM 310 product ),

Knowles E lectronics , Memstech ( M S Mx ), NXP S emiconductors , S onion M E M S, AAC Acoustic T echnologies , and Omron.

Audio signal An audio signal is a representation of sound waves in a different for m. T ypically this is an electrical voltage, but these signals

can be expressed through alternative mediums such as magnetic particles , when recorded onto analogue tape; or as RF

waves , when broadcast through radio; or even as pulses of light , when trans mitting through fi ber optic cables like TOSLIN K .

An audio signal can be manipulated , stored , trans mitted and reproduced in ways that a sound wave cannot .

Microphones convert sound pressure waves into voltage, an electrical audio signal . T herefore you find the microphone

sensitivity as milli volts per P ascal . Loudspeakers or headphones convert an electrical audio signal into sound . Although

many audio signals have their origin as a sound wave, devices such as synthesizers are designed to create audio signals .

E lectric energy flows through a circuit as voltage. T he opposition to voltage is i m pedance . Im pedance is measured in ohms .

T o measure electric energy in an audio signal , deci bels are used in relation to either power ( d Bm)or voltage ( d Bu or d Bv ,

and d BV).d Bmwas originally used , but is no longer as popular as the other units .

S ignal flow


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S ignal flow is the ter m used to descri be the path an audio signal will take from source (microphone ) to the speaker or

recording device. I t is most frequently in a recording studio setting , where the signal flow is often very long and convoluted as

the electric signal may pass through many sections of a large analog console , external audio equipment , and even different

rooms .

Digital equivalent

As much of the older analog audio equipment has been emulated in digital for m, usually through the development of

audio plug-ins for Digital Audio W orkstation (DAW)software such asP ro T ools or Logic , the ter ms audio signal and signal

flow are also used to descri be the path of digital infor mation through the DAW ( i .e. from an audio track through a plug-in, sent

a bus and an aux , and out a hardware output ).

Digital audio signal being sent through wire can use several for mats

including optical (ADAT, TDIF),coaxial (S/PDIF), XLR (AES/EBU),and ethernet , especially for large digital audio consoles .

T ypes of audio signal P ulse waves:

Digital audio signals

are very quiet to allow them to move quickly between com puter com ponents and create as little heat , through friction

( resistance ) as possi ble. T hey are pulse waves , em ploying rising and falling am plitude to represent 1s and 0 s .

S M PTE signals

are created digitally inside a S M PTE generator device and then am plified (modulated ) to the level of an audio line level signal

in order to be recorded onto audio tape . (Old com puter games em ployed the same syste m to load and save data to cassette

recorders when disc drives were still too expensive for consu mers ).

Audio / line level pulse waves:

can be created by a synthesizers oscillators and used as an effective sound source to create such sounds as pianos and

basses .


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Mic level

Mic level signals are created by microphones . T hey are quiet ( up to 0.0775 volts or -20 d Bu ) and usually need the protection of

abalanced cable. W hen they reach a mixer or hi-fi am p, they are am plified to line level by a pre-am p. R ecord decks also create

mic level like signals , but these differ in a number of ways and must be connected to a phono pream plifier which converts

them to line level .

I nstru ment level

I nstru ment level signals are pri marily created by electric bass and guitar pickups and have a higher level and i m pendence (Z)

then mic level signals .

Line level

Line level signals are much louder than mic and instru ment signals ( up to 2.45 volts or +30 d Bu ) and therefore do not always

need the protection of a balanced cable. Line level signals are the most common audio signals and are produced by .

Key boards

S am plers

S ynthesizers

Drum machines

T ape recorders

CD players

DVD player

FX and out board processors

Mixers


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S oundcard's / audio interfaces

Mini Disc players

DAT recorders

S peaker level

S peaker level signals are very loud in order to power the magnets which move the cones .

Loudspeaker A loudspeaker ( or " speaker ") is an electro acoustic transducer that converts an electrical signal into sound . T he speaker

moves in accordance with the variations of an electrical signal and causes sound waves to propagate through a medium such

as air or water .

After the acoustics of the listening space , loudspeakers ( and other electro acoustic transducers ) are the most variable elements in a modern audio syste m and are usually responsi ble for most distortion and audi ble differences when com paring

sound syste ms .

T er minology T he ter m " loudspeaker " may refer to individual transducers ( known as " drivers") or to com plete speaker syste ms consisting

of an enclosure including one or more drivers. T o adequately reproduce a wide range of frequencies , most loudspeaker

syste ms em ploy more than one driver , particularly for higher sound pressure level or maxi mum accuracy . I ndividual drivers

are used to reproduce different frequency ranges . T he drivers are named subwoofers ( for very low frequencies ); woofers ( low

frequencies ); mid-range speakers (middle frequencies ); tweeters ( high frequencies ); and someti mes supertweeters , opti mized

for the highest audi ble frequencies . T he ter ms for different speaker drivers differ , depending on the application . I n two-way

syste ms there is no mid-range driver , so the task of reproducing the mid-range sounds falls upon the woofer and tweeter .

H ome stereos use the designation " tweeter " for the high frequency driver , while professional concert syste ms may designate


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them as "HF" or " highs ". W hen multiple drivers are used in a syste m, a " filter network ", called a crossover , separates the

incoming signal into different frequency ranges and routes them to the appropriate driver . Aloudspeaker syste m

with n separate frequency bands is descri bed as " n-way speakers " : a two-way syste m will have a woofer and a tweeter; a

three-way syste m em ploys a woofer , a mid-range, and a tweeter .[ citation needed ]

Driver design

T he most common type of driver uses a lightweight diaphrag m, or cone , connected to a rigid basket , or frame, via a flexi ble

suspension that constrains a coil of fine wire to move axially through a cylindrical magnetic gap. W hen an electrical signal is

applied to the voice coil , a magnetic field is created by the electric current in the voice coil , making it a variable

electromagnet . T he coil and the driver's magnetic syste m interact , generating a mechanical force that causes the coil ( and

thus , the attached cone ) to move back and forth, thereby reproducing sound under the control of the applied electrical signal

coming from the am plifier . T he following is a description of the individual com ponents of this type of loudspeaker .

T he diaphrag m is usually manufactured with a cone- or dome-shaped profile. Avariety of different materials may be used , but

the most common are paper , plastic , and metal . T he ideal material would be stiff , to prevent uncontrolled cone motions; light ,

to mini mize starting force requirements and energy storage issues; and well dam ped , to reduce vi brations continuing after the

signal has stopped . I n practice, all three of these criteria cannot be met si multaneously using existing materials; thus , driver

design involves trade-offs . F or exam ple, paper is light and typically well dam ped , but is not stiff; metal may be stiff and light ,

but it usually has poor dam ping; plastic can be light , but typically , the stiffer it is made , the poorer the dam ping. As a result ,

many cones are made of some sort of com posite material . F or exam ple, a cone might be made of cellulose paper , into which

some car bon fi ber , Kevlar , fi berglass , or hem p or bamboo fi bers have been added; or it might use a honeycomb sandwich construction; or a coating might be applied to it so as to provide additional stiffening or dam ping.

T he chassis , frame, or basket , is designed to be rigid , avoiding defor mation which would change critical alignments with the

magnet gap , perhaps causing the voice coil to rub against the sides of the gap . C hassis are typically cast from aluminum

alloy , or sta m ped from thin steel sheet , although molded plastic baskets are becoming common, especially for inexpensive ,

low-mass drivers. Metallic chassis can play an i m portant role in conducting heat away from the voice coil; heating during

operation changes resistance , causing physical di mensional changes , and if extreme, may even demagnetize per manent

magnets .

T he suspension syste m keeps

the coil centered in the gap

and provides a restoring

( centering ) force that returns

the cone to a neutral position

after moving. Atypical


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suspension syste m consists of two parts: the " spider ", which connects the diaphragm or voice coil to the frame and provides

the majority of the restoring force, and the " surround ", which helps center the coil / cone asse mbly and allows free pistonic

motion aligned with the magnetic gap . T he spider is usually made of a corrugated fabric disk , i m pregnated with a stiffening

resin . T he name comes from the shape of early suspensions , which were two concentric rings of Bakelite material , joined by

six or eight curved " legs". V ariations of this topology included the addition of a felt disc to provide a barrier to particles that

might otherwise cause the voice coil to rub. T he G er man fir m R ulik still offers drivers with unco mmon spiders made of wood .

T he cone surround can be rubber or polyester foam, or a ring of corrugated , resin coated fabric; it is attached to both the

outer diaphrag m circumference and to the frame. T hese different surround materials , their shape and treat ment can

dramatically affect the acoustic output of a driver; each class and i m plementation having advantages and disadvantages .

P olyester foam, for exam ple, is lightweight and econo mical , but is degraded by exposure to ozone , UV light , humidity and

elevated tem peratures , li miting its useful life to about 15 years .

T he wire in a voice coil is usually made of copper , though aluminum and , rarely , silver m ay be used . V oice-coil wire cross

sections can be circular , rectangular , or hexagonal , giving varying amounts of wire volume coverage in the magnetic gap

space . T he coil is oriented co-axially inside the gap; it moves back and forth within a small circular volume ( a hole, slot , or

groove ) in the magnetic structure . T he gap esta blishes a concentrated magnetic field between the two poles of a per manent

magnet; the outside of the gap being one pole, and the center post ( called the pole piece ) being the other . T he pole piece and

back plate are often a single piece, called the pole plate or yoke.

Modern driver magnets are al most always per manent and made of ceramic , ferrite, Alnico, or , more recently , rare

earth magnets . Atrend in design due to increases in transportation costs and a desire for smaller , lighter devices ( as in

many home theater multi-speaker installations ) is the use of the last instead of heavier ferrite types . V ery few manufacturers

still use electrically powered field coils, as was common in the earliest designs ( one such is F rench ). W hen high field-strength

per manent magnets became available, Alnico, an alloy of aluminum, nickel , and cobalt became popular , since it dispensed

with the power supply issues of field-coil drivers. Alnico was used for al most exclusively until about 1980. Alnico magnets

can be partially degaussed ( i .e., demagnetized ) by accidental 'pops' or 'clicks' caused by loose connections , especially if used

with a high power am plifier . T his damage can be reversed by " recharging " the magnet .

After 1980, most (but not quite all ) driver manufacturers switched from Alnico to ferrite magnets , which are made from a mix

of ceramic clay and fine particles of barium or strontiu m ferrite. Although the energy per kilogram of these ceramic magnets is

lower than Alnico, it is substantially less expensive , allowing designers to use larger yet more econo mical magnets to achieve

a given perfor mance .

T he size and type of magnet and details of the magnetic circuit differ , depending on design goals . F or instance , the shape of

the pole piece affects the magnetic interaction between the voice coil and the magnetic field , and is someti mes used to

modify a driver's behavior . A " shorting ring", or F araday loop, may be included as a thin copper cap fitted over the pole tip or

as a heavy ring situated within the magnet-pole cavity . T he benefits of this com plication is reduced i m pedance at high

frequencies , providing extended treble output , reduced har monic distortion , and a reduction in the inductance modulation


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that typically accom panies large voice coil excursions . On the other hand , the copper cap requires a wider voice-coil gap ,

with increased magnetic reluctance; this reduces

available flux , requiring a larger magnet for

equivalent perfor mance .

Driver design including the particular way two or

more drivers are combined in an enclosure to make

a speaker syste m is both an art and

science . Adjusting a design to i m prove

perfor mance is done using some combination of

magnetic , acoustic , mechanical , electrical , and

material science theory; high precision

measure ments , generally with the observations of experienced listeners . Afew of the issues speaker and driver designers

must confront are distortion , lobing, phase effects , off-axis response , and crossover com plications. Designers can use

an anechoic chamber to ensure the speaker can be measured independently of room effects , or any of several electronic

techniques which can , to some extent , replace such chambers . S ome developers eschew anechoic chambers in favor of

specific standardized room setups intended to si mulate real-life listening conditions .

T he fabrication of finished loudspeaker syste ms has become segmented , depending largely on price, shipping costs , and

weight li mitations . H igh-end speaker syste ms , which are typically heavier ( and often larger ) than econo mic shipping allows

outside local regions , are usually made in their target market region and can cost $140,000 or more per pair . T he lowest-

priced speaker syste ms and most drivers are manufactured in C hina or other low-cost manufacturing locations .

Audio Am plifier T his circuit is an audio am plifier consisting of two stages of am plification. T he 741operational am plifiers is used as a pre-am plifier whose gain is controlled by its feed back resistor R2. T he 386 is a general-purpose power am plifier used as the output driver for delivering power to the speaker . T he volume of the speaker may be controlled via resistor R3. A 0.1 F bypass capacitor must be placed across the power supply to reduce the noise . T he value of R2 must be chosen well to prevent oscillation or distortion .

V ideo C amera C harge-coupled device


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A charge-coupled device (CCD)is a device for the movement of electrical charge , usually from within the device to an area where the charge can be manipulated , for exam ple conversion into a digital value. T his is achieved by " shifting" the signals between

stages within the device one at a ti me. CCDs move charge between capacitive bins in the device, with the shift allowing for the transfer of charge between bins .

Often the device is integrated with an i mage sensor , such as a photoelectric device to produce the charge that is being read ,

thus making the CCDa major technology for digital i maging . Although CCDs are not the only technology to allow for light

detection , CCDs are widely used in professional , medical , and scientific applications where high-quality i mage data are

required .

Basics of operation

An i mage is projected through a lens onto the capacitor array ( the photoactive region ), causing each capacitor to accumulate

an electric charge proportional to the light intensity at that location . Aone-

di mensional array , used in line-scan cameras , captures a single slice of the

i mage , while a two-di mensional array , used in video and still cameras , captures

a two-di mensional picture corresponding to the scene projected onto the focal

plane of the sensor . Once the array has been exposed to the i mage, a control

circuit causes each capacitor to transfer its contents to its neighbor ( operating

as a shift register ). T he last capacitor in the array dum ps its charge into

a charge am plifier , which converts the charge into a voltage. By repeating this process , the controlling circuit converts the

entire contents of the array in the semiconductor to a sequence of voltages . I n a digital device, these voltages are then

sam pled , digitized , and usually stored in memory; in an analog device ( such as an analog video camera ), they are processed

into a continuous analog signal ( e.g. by feeding the output of the charge am plifier into a low-pass filter ) which is then

processed and fed out to other circuits for trans mission , recording , or other processing .

Detailed physics of operation

T he photoactive region of the CCDis, generally , an epitaxial layer of silicon. I t

has a doping of p+ (Boron ) and is grown upon a substrate material , often p++.

I n buried channel devices , the type of design utilized in most modern CCDs ,

certain areas of the surface of the silicon areion i m planted with phosphorus ,

giving them an n-doped designation . T his region defines the channel in which

the photo generated charge packets will travel . T he gate oxide, i .e. the capacitor dielectric , is grown on top of the epitaxial


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layer and substrate . Later on in the process polysilicon gates are deposited by chemical vapor deposition , patterned

with photolithography , and etched in such a way that the separately phased gates lie perpendicular to the channels . T he

channels are further defined by utilization of the LOCOS process to

produce the channel stop region . C hannel stops are ther mally

grown oxides that serve to isolate the charge packets in one column from

those in another . T hese channel stops are produced before the

polysilicon gates are, as the LOCOS process utilizes a high tem perature

step that would destroy the gate material . T he channels stops are parallel

to, and exclusive of , the channel , or " charge carrying", regions . C hannel

stops often have a p+ doped region underlying them, providing a further

barrier to the electrons in the charge packets ( this discussion of the physics of CCDdevices assu mes an electron transfer

device, though hole transfer , is possi ble ).

One should note that the clocking of the gates , alternately high and low , will forward and reverse bias to the diode that is provided by the buried channel ( n-doped ) and the epitaxial layer ( p-doped ). T his will cause the CCDto deplete , near the p-n

junction and will collect and move the charge packets beneath the gates and within the channels of the device.

CCD manufacturing and operation can be opti mized for different uses . T he above process descri bes a frame transfer CCD.

W hile CCDs may be manufactured on a heavily doped p++wafer it is also possi ble to manufacture a device inside p-wells that

have been placed on an n-wafer . T his second method , reportedly , reduces smear , dark current , and infrared and red

response . T his method of manufacture is used in the construction of interline transfer devices .

Another version of CCDis called a peristaltic CCD. I n a peristaltic charge-coupled device, the charge packet transfer

operation is analogous to the peristaltic contraction and dilation of the digestive syste m. T he peristaltic CCDhas an

additional i m plant that keeps the charge away from the silicon / silicon dioxide interface and generates a large lateral electric

field from one gate to the next . T his provides an additional driving force to aid in transfer of the charge packets .

Architecture

T he CCDi mage sensors can be i m plemented in several different architectures . T he most common are full-frame, frame-

transfer , and interline. T he distinguishing characteristic of each of these architectures is their approach to the problem of

shuttering . I n a full-frame device, all of the i mage area is active, and there is no electronic shutter . A mechanical shutter must be added to this type of sensor or the i mage smears as the device is clocked or read out .

W ith a frame-transfer CCD,half of the silicon area is covered by an opaque mask ( typically aluminum). T he i mage can be

quickly transferred from the i mage area to the opaque area or storage region with accepta ble smear of a few percent . T hat

i mage can then be read out slowly from the storage region while a new i mage is integrating or exposing in the active area .

F rame-transfer devices typically do not require a mechanical shutter and were a common architecture for early solid-state


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broadcast cameras . T he downside to the frame-transfer architecture is that it requires twice the silicon real estate of an

equivalent full-frame device; hence , it costs roughly twice as much. T he interline architecture extends this concept one step

further and masks every other

column of the i mage sensor for

storage . I n this device, only one

pixel shift has to occur to transfer

from i mage area to storage area;

thus , shutter ti mes can be less

than a microsecond and smear is

essentially eli minated . T he

advantage is not free, however , as

the i maging area is now covered by opaque strips dropping the fill factor to approxi mately 50 percent and the

effective quantu m efficiency by an equivalent amount . Modern designs have addressed this deleterious characteristic by adding micro lenses on the surface of the device to direct light away from the opaque regions and on the active area . Micro

lenses can bring the fill factor back up to 90 percent or more depending on pixel size and the overall syste m's optical

design .T he choice of architecture comes down to one of utility . I f the application cannot tolerate an expensive , failure-prone,

power-intensive mechanical shutter , an interline device is the right choice . C onsu mer snap-shot cameras have used interline

devices . On the other hand , for those applications that require the best possi ble light collection and issues of money , power

and ti me are less i m portant , the full-frame device is the right choice . Astrono mers tend to prefer full-frame devices . T he frame-

transfer falls in between and was a common choice before the fill-factor issue of interline devices was addressed . T oday ,

frame-transfer is usually chosen when an interline architecture is not available, such as in a back-illuminated device.CCDs

containing grids of pixels are used in digital cameras , optical scanners , and video cameras as light-sensing devices . T hey

commonly respond to 70 percent of the incident light (meaning a quantu m efficiency of about 70 percent ) making them far

more efficient than photographic fil m, which captures only about 2 percent of the incident light . Most common types of CCDs

are sensitive to near-infrared light , which allows infrared photography , night-vision devices , and zero lux ( or near zero lux )

video-recording / photography . F or nor mal silicon-based detectors , the sensitivity is li mited to 1.1 m. One other consequence

of their sensitivity to infrared is that infrared from remote controls often appears on CCD-based digital cameras or

camcorders if they do not have infrared blockers .C ooling reduces the array's dark current , i m proving the sensitivity of the

CCDto low light intensities , even for ultraviolet and visi ble wavelengths . P rofessional observatories often cool their detectors

with liquid nitrogen to reduce the dark current , and therefore the ther mal noise , to negligi ble levels.

T ypes of lenses

Lenses are classified by the curvature of the two optical surfaces . Alens is biconvex ( or double convex , or just convex ) if

both surfaces are convex . I f both surfaces have the same radius of curvature , the lens isequiconvex . Alens with


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two concave surfaces is biconcave ( or just concave ). I f one of the surfaces is flat , the lens is plano-convex or plano-

concave depending on the curvature of the other surface . Alens with one convex and one concave side is convex-

concave or meniscus . I t is this type of lens that is most commonly used in corrective lenses .

I f the lens is biconvex or plano-convex , a colli mated or parallel

beam of light travelling parallel to the lens axis and passing

through the lens will be converged ( or focused ) to a spot on

the axis, at a certain distance behind the lens ( known as

the focal length ). I n this case , the lens is called a

positive orconverging lens .

I f the lens is biconcave or plano-concave , a colli mated beam of light passing through the lens is diverged ( spread ); the lens is

thus called a negative or diverging lens . T he beam after passing through the lens appears to be emanating from a particular

point on the axis in front of the lens; the distance from this point to the lens is also known as the focal length , although it is

negative with respect to the focal length of a converging lens .

C onvex-concave (meniscus ) lenses can be either positive or negative, depending on the relative curvatures of the two

surfaces . Anegative meniscus lens has a steeper concave surface and will be thinner at the centre than at the periphery .

C onversely , a positive meniscus lens has a steeper convex surface and will be thicker at the centre than at the periphery . An

idealthin lens with two surfaces of equal curvature would have zero optical power , meaning that it would neither converge nor

diverge light . All real lenses have a nonzero thickness , however , which affects the optical power . T o obtain exactly zero

optical power , a meniscus lens must have slightly unequal curvatures to account for the effect of the lens' thickness .

C athode ray tube


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C utaway rendering of a color CRT : 1. T hree E lectron guns ( for red , green , and blue phosphor dots )

2. E lectron beams

3. F ocusing coils

4. Deflection coils

5. Anode connection

6. Mask for separating beams for red , green , and blue part of

displayed i mage

7. P hosphor layer with red , green , and blue zones

8. C lose-up of the phosphor-coated inner side of the screen

T he C athode R ay T ube (CRT)is a vacuum tube containing

an electron gun ( a source of electrons ) and a fluorescent screen ,

with internal or external means to accelerate and deflect the electron beam, used to create i mages in the for m of light emitted from the

fluorescent screen . T he i mage may represent

electrical wavefor ms ( oscilloscope ), pictures ( television, com puter monitor ), radar targets and others .

T he CRT uses an evacuated glass envelope which is large, deep , heavy , and relatively fragile.

Overview

A cathode ray tube is a vacuum tube which consists of one or more electron guns , possi bly internal electrostatic deflection plates , and a phosphor target . I n television sets and com puter monitors , the entire front area of the tube is scanned

repetitively and syste matically in a fixed pattern called a raster . An i mage is produced by controlling the intensity of each of

the three electron beams , one for each additive pri mary color ( red , green , and blue )with a video signal as a reference . I n all

modern CRT monitors and televisions , the beams are bent by magnetic

deflection, a varying magnetic field generated by coils and driven by

electronic circuits around the neck of the tube, although electrostatic

deflection is commonly used in oscilloscopes , a type of diagnostic

instrument .

Oscilloscope CRT s


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I n oscilloscope CRT s , electrostatic deflection is used , rather than the magnetic deflection commonly used with television and

other large CRT s. T he beam is deflected horizontally by applying an electric field between a pair of plates to its left and right ,

and vertically by applying an electric field to plates above and below .

P hosphor persistence

V arious phosphors are available depending upon the needs of the measure ment or display application . T he brightness , color ,

and persistence of the illumination depends upon the type of phosphor used on the CRT screen . P hosphors are available with

persistences ranging from less than one microsecond to several seconds . F or visual observation of brief transient events , a

long persistence phosphor may be desira ble. F or events which are fast and repetitive, or high frequency , a short-persistence

phosphor is generally preferable.

Microchannel plate

W hen displaying fast one-shot events the electron beam must deflect very quickly , with few electrons i m pinging on the

screen; leading to a faint or invisi ble display . Oscilloscope CRT s designed for very fast signals can give a brighter display by

passing the electron beam through a micro-channel plate just before it reaches the screen . T hrough the pheno menon of

secondary emission this plate multiplies the number of electrons reaching the phosphor screen , giving a significant

i m provement in writing rate (brightness ), and i m proved sensitivity and spot size as well .

G raticules

Most oscilloscopes have a graticule as part of the visual display , to facilitate measure ments . T he graticule may be

per manently marked inside the face of the CRT, or it may be a transparent external plate. E xternal graticules are typically

made of glass or acrylic plastic . An internal graticule provides an advantage in that it eli minates parallax error . U nlike an

external graticule , an internal graticule cannot be changed to accommodate different types of measure ments . Oscilloscopes

commonly provide a means for the graticule to be side-illuminated , which i m proves its visi bility when used in a darkened room or when shaded by a camera hood .

C olor CRT s


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C olor tubes use three different phosphors which emit red , green , and blue light respectively . T hey are packed together in

stripes ( as in aperture grille designs ) or clusters called " triads" ( as

in shadow mask CRT s ). C olor CRT s have three electron guns , one for each

pri mary color , arranged either in a straight line or in a triangular

configuration ( the guns are usually constructed as a single unit ). Agrille or

mask absor bs the electrons that would otherwise hit the wrong

phosphor . Ashadow mask tube uses a metal plate with tiny holes , placed so

that the electron beam only illuminates the correct phosphors on the face of

the tube. Another type of color CRT uses an aperture grille to achieve the same result .

C onvergence in color CRT s

T he three beams in color CRT s would not strike the screen at the same point without convergence cali bration . I nstead , the set

would need to be manually adjusted to converge the three color beams together to maintain color accuracy .

Degaussing

Most CRT television sets and com puter monitors have a built-in degaussing ( demagnetizing ) coil , which upon power-up

creates a brief , alternating magnetic field which decays in strength over the course of a few seconds . T his degaussing field is

strong enough to remove most cases of shadow mask magnetization .

V ector monitors

V ector monitors were used in early com puter aided design syste ms and in some late-1970 s to mid-1980 s arcade games such

as Asteroids . T hey draw graphics point-to-point , rather than scanning a raster .

CRT resolution

Dot pitch defines the maxi mum resolution of the display , assu ming delta-gun CRT s . I n these , as the scanned resolution

approaches the dot pitch resolution , moir appears , as the detail being displayed is finer than what the shadow mask can

render . A perture grille monitors do not suffer from vertical moir, however , because their phosphor stripes have no vertical

detail . I n smaller CRT s, these strips maintain position by themselves , but larger aperture grille CRT s require one or two

crosswise ( horizontal ) support strips .


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G amma

CRT s have a pronounced triode characteristic , which results in significant gamma ( a nonlinear relationship in an electron gun

between applied video voltage and light intensity ).

T ypes of CRT s

C at's eye

I n better quality tube radio sets a tuning guide consisting of a phosphor tube was used to aid the tuning adjust ment . T his was

also known as a " Magic E ye" or "T uning E ye". T uning would be adjusted until the width of a radial shadow was mini mized .

T his was used instead of a more expensive electromechanical meter , which later came to be used on higher-end tuners when

transistor sets lacked the high voltage required to drive the device .

C haractrons

S ome displays for early com puters ( those that needed to display more text than was practical using vectors , or that required high

speed for photographic output ) used C haractron CRT s . T hese incorporate a perforated metal character mask ( stencil ),which

shapes a wide electron beam to for m a character on the screen . T he syste m selects a character on the mask using one set of

deflection circuits , but that causes the extruded beam to be ai med off-axis, so a second set of deflection plates has to re-ai mthe

beam so it is headed toward the center of the screen . Athird set of plates places the character wherever required . T he beamis

unblanked ( turned on ) briefly to draw the character at that position . G raphics could be drawn by selecting the position on the mask

corresponding to the code for a space ( in practice , they were si m ply not drawn ),which had a small round hole in the center; this

effectively disabled the character mask , and the syste m reverted to regular vector behavior . C haractrons had exceptionally-long

necks , because of the need for three deflection syste ms .

N i mo

N i mo was the trademark of a family of small specialised CRT s manufactured by I ndustrial E lectronics E ngineers . T hese had

10 electron guns which produced electron beams in the for m of digits in a manner si milar to that of the charactron . T he tubes

were either si m ple single-digit displays or more com plex 4- or 6 - digit displays produced by means of a suitable magnetic

deflection syste m. H aving little of the com plexities of a standard CRT, the tube required a relatively si m ple driving circuit , and

as the i mage was projected on the glass face, it provided a much wider viewing angle than com petitive types ( e.g. nixie

tubes ).


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Z eus T hin CRT Displays

I n the late 1990 s and early 2000 s P hilips R esearch Laboratories experi mented with a type of thin CRT known as

the Z eus display which contained CRT -like functionality in a flat panel . T he devices were demonstrated but never marketed .

V ideo Am plifier .

R112 in parallel with R3 in the ti mebase gives approxi mately the right input i m pedance of 75 . R301adjusts the contrast .

IC301, R302 and R303 invert the video signal so that it is the right way up with positive sync pulses and peak white most

negative . An EF85 pentode valve is used for the output for i m proved linearity to that of a triode. IC302 and associated resistors set the gain and DC biasing . R304and C102 are

for high frequency sta bility . T he output is taken from the

valve anode . V oltage gain from input to output is about

100.

C om posite V ideo S ignal

A signal that contains all three of these com ponents intensity infor mation, horizontal-retrace signals , and vertical- retrace signals -- is called a com posite video signal . Acom posite-video input on a VCR is nor mally a yellow RCA


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jack . One line of a typical com posite video signal looks something like this:

T he horizontal-retrace signals are 5 -microsecond ( abbreviated as " us" in the figure ) pulses at zero volts. E lectronics inside

the TV can detect these pulses and use them to trigger the beam's horizontal retrace . T he actual signal for the line is a varying wave between 0.5 volts and 2.0 volts, with 0.5 volts representing black and 2 volts representing white. T his signal drives the intensity circuit for the electron beam. I n a black-and-white TV,this signal can consu me about 3.5 megahertz ( M H z ) of bandwidth, while in a color set the li mit is about 3.0 M H z . A vertical-retrace pulse is si milar to a horizontal-retrace pulse but is 400 to 500 microseconds long. T he vertical-retrace pulse is serrated with horizontal-retrace pulses in order to keep the horizontal-retrace circuit in the TV synchronized .

Video Door Phone For Home Security

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