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Information about phone2

Published on October 13, 2007

Author: Aric85


Basic Telephone Components:  Transmitter (microphone) Receiver Power Supply (originally a battery in each phone, now at central office) All elements were originally connected in series. This arrangement worked, but had many problems. It did not transfer energy well to the line (because of impedance mismatching) - resulting in low received speech signal volume Basic Telephone Components BATTERY TRANSMITTER RECEIVER TELEPHONE LINE “Optional” Components or Improvements:  “Optional” Components or Improvements Induction Coil (transformer between transmitter and receiver patented by Edison in 1878 which provided sidetone and better impedance matching) see Figure Switch Hook (on/off switch) Ringer Hand-cranked magneto (now obsolete, formerly used to signal the operator) Dialing mechanism (Rotary or TouchTone) Balance Network (1918 AT&T, to provide balanced sidetone so you can hear yourself at about the same volume as the party you are calling) see Figure Hearing Yourself Better:  Hearing Yourself Better The induction coil, or transformer, was used to separate electrically the transmitter and receiver. The number of turns in the secondary winding was greater than in the primary so that the transmitter voltage was stepped-up before being sent down the line. (ref 2) A simple telephone Receiver Secondary Coil Battery Transmitter Transformer (Induction Coil) Primary Coil To Telephone Line Slide4:  A telephone with improved sidetone A center-tapped transformer is used in the anti-sidetone circuitry invented to reduce electrical sidetone to an acceptable level. The receiver is connected to the secondary winding of the transformer, C. The balance network attempts to duplicate the impedance of the telephone line as seen from the telephone. The current generated by the transmitter divides equally through the primary windings of the transformer, A and B. The effects of the two divided currents cancel in the secondary winding, and no signal is heard from the receiver. (ref 2) Balance Network Transmitter Receiver C A B Telephone Line A more complete version (circa 1910):  A more complete version (circa 1910) “Candlestick” phone (ref. 3) Bell’s Microphone:  Basic principle: Voice input causes change in resistance which then modulates the current in the circuit. Bell’s original microphone used acid. Bell’s Microphone Bell’s liquid transmitter consisted of a metal cup filled with a mixture of acid and water. A sound wave causes the diaphragm to move thereby forcing the wire to move up and down in the liquid. The electrical resistance between the wire and the cup is inversely proportional to the amount of wire submerged. Therefore the resistance across the two terminals varies in response to the sound. Cup Wire Funnel to concentrate sound Diaphragm Acid Output Terminals The Edison Microphone:  The Edison Microphone Most common type (invented by Edison and still in use) uses carbon granules - when vibrated by attached diaphragm, the overall resistance alternately decreases (when particles are compacted) and increases (when particles are loosened) Loosely-packed carbon granules Metal cavity Diaphragm Flexible edges Output Incoming Sound Waves Microphone - continued:  Microphone - continued nonlinear input-output characteristics suppress background noise (microphone does not respond to low level sounds) limits maximum output (so you can’t hurt someone’s ears by screaming!) range of normal voice levels (faithfully reproduced) background room noises (are attenuated) loud sounds (are attenuated) Input Sound Level (dB SPL) Output Electrical Level (dBV) -70 -55 -40 -25 70 100 130 Receiver:  Receiver similar to a loudspeaker we still use Bell’s basic design ref van derPuie A cross section of the moving iron telephone receiver Magnetic Flux Path Permanent Magnet Iron Alloy Diaphragm Electromagnet Coil Electromagnet alternately adds to and substracts from the permanent magnet’s attraction of the diaphragm, causing it to oscillate Sound Output S N AC Electrical Signal Input Rotary Dial:  Rotary Dial produces 10 pulses/sec (pause longer than 1/10 sec is interpreted by central office as inter-digit separator) ingenious governor and clutch mechanism regulates the rotational speed Cup When finger wheel is released, wings rotate and fly apart due to centrifugal force, contact cup and generate friction, limiting the speed of rotation Finger Plate Wheel Gears Clutch (only engages the governor when the finger wheel is recoiling) Wings Governor Return Spring Touchtone Dial:  Touchtone Dial Pressing a button produces a Dual Tone, Multiple Frequency (DTMF) signal - so you can’t imitate it by whistling 1 2 3 4 5 6 7 8 9 * 0 # 897 770 852 941 1209 1336 1477 Column Frequencies (Hz) Row Frequencies (Hz) ABC DEF GHI JKL MNO PRS TUV WXY OPER Electro-Mechanical Ringer:  Electro-Mechanical Ringer Brass gongs struck by a clapper Mechanism resonates at 20 Hz Ringing signal is ~75V at 20 Hz Modern phones use an electronic ringer (similar to a small speaker) OPERATION OF ELECTRO-MAGNETIC RINGER N S N S What happens when you call home?:  What happens when you call home? HANDSET LIFTED SWITCHOOK CONTACTS CLOSE CALLER CENTRAL OFFICE A DIRECT CURRENT FLOWS IN LINE CURRENT DETECTED DIAL TONE GENERATOR CONNECTED TO LINE DIAL TONE HEARD NUMBER DIALED NUMBER STORED Calling Home - continued:  Calling Home - continued NUMBER STORED FIRST THREE DIGITS INDICATE CENTRAL OFFICE DESTINATION OR AREA CODE FIND BEST AVAILABLE ROUTE (TRUNK) AND SEIZE IT TRANSFER NUMBER (4 DIGITS) STORE NUMBER CONNECT TO MOM’S LINE CHECK IF BUSY SEND RINGING SIGNAL PICK UP PHONE CONNECT SWITCHHOOK DC CURRENT FLOWS ANSWER DETECTED STOP RINGING CONNECT PARTIES CENTRAL OFFICE A CENTRAL OFFICE B MOM’S HOUSE “Hello mom? I need money” AT&T Recent History:  AT&T Recent History Prior to 1980, AT&T was a regulated monopoly - (consisting of AT&T Long LInes, Western Electric, Operating Companies and Bell Labs) It provided 80% of the local telephone service in U.S. These local companies used only Western Electric equipment and only AT&T long distance - thereby excluding all competitors All consumer premises equipment (like your home telephones) were leased from AT&T for a monthly fee - it was difficult if not impossible to purchase your own equipment AT&T was prohibited from selling computer equipment and anything not directly connected to telephone service (also barred from cellular phones-because it was radio) Non-telephone inventions from Bell Labs were licensed for $1 Some Bell Labs Non-Telephone Inventions and Milestones:  Some Bell Labs Non-Telephone Inventions and Milestones First TV transmission (Wash DC demo, April 1927) Wave nature of matter (1937 Nobel Phyics prize, C.J. Davisson) Quality Control (1931 book by W. A. Short “Economic control of quality of manufactured products”) Nyquist diagram 1932 Radio Astronomy 1933 Green salt wood treatment 1934 First speech synthesizer 1938 Microwave radio 1947 Transistor 1947 (Bardeen, Brattain, Shockley - 1956 Nobel prize) First solar cell 1955 Lasers 1958 ECHO satellite 1960 Cellular radio 1960 Cosmic background noise 1965 (1978 Nobel prize - Penzias and Wilson) Unix operating system 1969 C language 1972 Optical fibers 1974 Through 1975, Bell Labs held over 18,000 patents Deregulation:  Deregulation In 1974, the U.S. Justice Dept. (at request of several companies like MCI) sued AT&T for anti-trust violations - for monopolizing interstate communication services and the market for telecommunications equipment In 1982, the Justice department and AT&T came to an agreement, in exchange for dropping the lawsuit, AT&T would among other things: by 1984, divest its 23 operating companies (the Baby Bells) into 7 regional companies, be allowed to keep most of Bell Labs and Western Electric could not do electronic publishing for 5 years Net Effect of Deregulation on AT&T:  Net Effect of Deregulation on AT&T Lost monopoly on long distance services (enter MCI, Sprint, GTE, ...) Lost the operating companies Operating companies could sell consumer premises equipment but not manufacture it Customer equipment could now be purchased from anyone (phones for sale at Kmart) One bright side - AT&T could enter businesses previously forbidden by a 1956 court ruling, including the computer business (but it failed miserably) The finest research lab in the world was gutted. Basic research became “directed” research. All projects must have a direct benefit to AT&T business and have high probability of success How Telephones WERE Designed:  How Telephones WERE Designed Reliability was the most important factor (30 year life goal), manufactured cost was secondary, because the cost to service a phone was so high (if something ever failed, they had to send a union repairman out with donuts in a green truck) The design philosophy then : “The telephone is designed to meet functional requirements and is then cost-reduced to maximize the cost saving associated with the large production volumes... Cost considerations are, however, not limited to manufacturing. Since operating companies own the sets, reliability, maintenance, and ease of installation are important factors in the design” - Engineering and Operations in the Bell System, 1977 Using a SERIAL design process which could take years Slide20:  So when deregulation hit, AT&T was stuck with highly reliable, but costly to produce designs, and a cumbersome design process. The competition was ready with low cost equipment, much of it of lesser quality. The American consumer, accustomed to AT&T quality, thought a phone was a phone, and bought up the cheap competitors like crazy. It took AT&T years to learn about concurrent engineering and product design teams - engineers and support personnel from all relevant disciplines working together in parallel ELECTRICAL DESIGN MECHANICAL DESIGN INDUSTRIAL DESIGN (styling) MANUFACTURING PROCESS DESIGN FINAL PRODUCT Telephone as a Consumer Product:  Telephone as a Consumer Product Obvious things which consumers base their purchases on: low cost good looking lots of features low cost Hidden qualities of a good design (and things the designer cares about) good voice quality reliability (must survive “drop test”) ease of use easy to cradle handset filters out room noise An Example of Bad Design:  An Example of Bad Design Plate mounted over typical office phones at Penn State - there are lots of features, but they are hard to use, see “Design of Everyday Things” by Donald Norman, pg 7 Dissection exercise:  Dissection exercise Session I - Dissect rotary dial phone (pre-deregulation) Session II - Dissect electronic phone (post-deregulation) Compare them: electrical and mechanical technology manufacturing ease other attributes Session III - Consumer reports style testing: Voice quality Handset preference Buyers survey Ringer volume

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