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Audio Transducers
Understanding audio transducers is essential to understanding communications
circuits. You are expected to review
Audio Transduces in
Analog circuits (particularly microphones and speakers) as part of your
study of Communications Circuits. The invention of the telephone was
essentially the invention of audio transducers.
The original telephone system
The telephone system that has been in use for over 100 years is often
called:
PSTN - Public Switched Telephone Network POTS - Plain Old Telephone
System
This system emerged for the original telephone developed by Alexander Bell
and Thomas Edison.
Bell had developed what we now call a dynamic speaker. This was essentially
a paper diaphragm suspended such that it was able to vibrate. This diaphragm
was connected to a coil of wire such that the coil was suspended in the
field of a permanent magnet. When an electric current is passed through the
coil the resulting magnetic field interacts with the field of the permanent
magnet, causing the coil and paper diaphragm to move. When the current
varied, the coil and diaphragm vibrate as the current varies. If this
vibration is fast enough the diaphragm produces audible sound waves.
To produce a working telephone, Bell needed a way to convert sound waves
into a varying electrical current. Thomas Watson, Bell's assistant
discovered that the speaker they had already invented also worked in
reverse; it also worked as a microphone. The dynamic speaker and the dynamic
microphone are essentially the same device, but optimized to work in
"opposite directions."
An actor portraying Bell speaking into his telephone
Bell and Watson made a working telephone by simply wiring two of the
speakers together.
Schematic of Bell's Original Telephone
On February 14, 1876, the same day Bell's attorney filed a patent
application for Bell's telephone, Elisha Gray filed a patent caveat
(essentially an incomplete patent application) for a device that varied its
electrical resistance in response to sound waves. It used a metal diaphragm
connected to a needle. This was suspended such that the needle dipped into a
metal container of water mixed with acid. There is strong evidence that
Bell's attorney persuaded a clerk at the U. S. Patent Office to copy details
of Gray's caveat to Bell's application. Despite the resulting lawsuits and
conspiracy theories, Gray's device was not used in Bell's telephone
(although Bell did test it as a proof of concept).
Bell's telephone was no more than two speaker/microphone transducers
connected together. The system was self-powered, meaning there was no
external power source. When speaking into one end it generated a signal that
could be heard in the other end. You can make a modern version by wiring two
small speakers or dynamic microphones together. Simply connect two speakers
together with a length of double wire (such as speaker wire or lamp cord) to
make a loop.
A modern version of Bell's telephone you can make yourself
Have a friend take one of the speakers into another room and close the door.
Talk into one speaker and your friend will faintly hear you in the other.
Sound-powered telephones work the same but with improved transducers
optimized for the purpose.
Bell's telephone was almost unusable. The dynamic speaker/microphone was not sensitive
enough as a microphone. The person talking had to yell into the transducer
to be heard at the other end.
Edison added a battery and carbon microphone to make the system that was in
use for over 100 years.
A person speaking into either microphone could be heard clearly in both
speakers.[1]
Until the late 1960s the basic telephone system remained unchanged. A
basic telephone circult (a connection between two customers) consists of
two dynamic speakers, two carbon microphones (one speaker and microphone
per telephone) and a battery to power the system. Early systems contained
batteries in the phones themselves but these were replaced by batteries
in the central telephone switch buildings. Telephones also has a
hand-cranked magneto to produce the apporximately 90 volts AC required to
ring a bell. Early telephones had a separate earpiece and hand-held
microphone, requiring two hands to operate. When carbon microphones were
developed that were insensitive to gravity they were placed in a handset
with the speaker.
The Bell/Edison Telephone minus the bell-ringing magneto.
To make a call the call the customer turned the crank on the magneto to ring
a bell or clicker at the central switchboard. This got the attention of the
operator who used a plugboard to complete a circuit between one customer and
another.
The early
system required operators to connect customers.
the switchboard used 1/4-inch three-conductor plugs and jacks
Parts
of the plug were called the tip, ring and sleeve.
Sleeve: ground
Ring:
negative, red wire (primary line) or black wire (secondary line)
Tip:
positive, green wire (primary line) or yellow wire (secondary line)
Insulating rings
The terms "tip" and "ring" are still used to denote the positive and negative
connections to the POTS network.
The Strowger switch
system
The Strowger switch
served to route calls until electronic switches were invented in the late 1950s.
A Strowger switch had ten levels and ten rotary positions. To place a call,
the customer lifter the receiver. This closes the hook switch (the phone is
now off-hook) causing the voltage on that line to drop from 48 volts to
between six and 10 volts. This triggered the system to connect the phone to a
Strowger switch and to an AC voltage producing the dial tone.
Rotating the telephone dial opened and closed a switch on
the phone. This caused the system to disconnect the dial tone and to operate the
Strowger switch. For example, if you dialed with the number five hole, the switch
would open and close five times. This opened and closed the circuit, causing a solenoid to
step the switch through the five levels. Each level had 10 contacts that
connected to other Strowger switches. When the switch stopped at a particular
level it automatically rotated to connect to the next switch that was not in
use. Dialing the next digit caused the second switch to operate like the
first. This continued until the last switch made a connection to the
receiving customer. The final switch connected to up to 100 customers. The
next to last number dialed took the switch to the corresponding level. That
level connects to 10 customers. The final digit dialed caused the switch to
rotate to the corresponding customer. Once a connection was made from one customer to another, a "line
finder" connected the customers directly, freeing the Strowger switches to be
used by other customers. If the final switch could not make the connection,
because the receiving customer was already using his/her phone, the system
directed the caller to a "busy signal".
A Strowger switch
As the telephone network developed, an amplifying unit was invented using a
speaker physically coupled to a carbon microphone, allowing long
distance calls. These were later replaced with vacuume tube and then
transistor amplifiers. Radio techniques were developed to send multiple calls over single coaxial cables.
Microwave links were used to send multiple calls over radio links. More recently the system has been augmented with fiber optic cables.
Early
television networks used specially optimized telephone lines to distribute
signals across the country.
Modern system
The switch is the building
where telephone calls are routed. This is often called the exchange by people
outside the industry.
A trunk is multiple lines
between switches. These may be 900-pair wire bundles, coaxial cables, microwave
links or fiber optics
The Local Loop
or Subscriber Loop is the
connection between the switch and the customer. Today, many customers are
connected by fiber optics instead of copper wire. Customers connected by fiber
optics have a device (an OLT) that converts the fiber optic signal into a POTS
signal for the home telephones.
Local loop connections
start as 900-pair cables. 900-pair cables are divided into 25-pair cables in a
B-box.
A
telephone B-box
Commercial customers are
served by 25-pair cables. Residential customers are usually served by a 2-pair
cable where only one pair is used unless the customer orders a second line.
25-pair cables terminate at
the commercial customer at a 66-block. Not all commercial customers get all 25
pair. These are often shared by several customers (each with a partially filled
66-block). The wires are color coded where each wire has a colored vinyl coating
with a painted stripe of a different color. The color of the vinyl coating is
the major color and the stripe is the minor color.
A
66-block and color code
The proper way to wire a
66-block is to use a punch-down tool to wire the 25 wires to the left side of
the block. Individual telephones are wired to the opposite side of the block.
Jumper bars are used to connect the sides. The jumper bars can be removed to
isolate telephones from the telephone system. 66-blocks are often wired whatever
way the technician finds convenient.
A 110-block is an
alternative to the 66-block that is compatible with high-speed networking.
Part
of two 110-blocks
A
punch-down tool with blades for both the 66-block and the 110-block.
The POTS system continues
to work even if there is a power failure since it is powered by 48-volt
batteries at the telephone switch. However, wireless telephones need the mains
power to work and still won't operate during a power failure.
Residential customers of
all ages are largely abandoning "land lines" in favor of cell phones. Commercial
customers are switching to Voice Over Internet Protocol (VOIP). VOIP telephones
are essentially computers connected to an Ethernet network. Some VOIP systems
use a POTS system throughout the office suite connected to a central VOIP switch
in the telecom closet. This switch connects to the Internet. It is now popular
for VOIP phones to connect individually to a VOIP provider via the Internet.
With such systems, a phone call to the next cubical goes out to the Internet,
through the VOIP provider's system and
back in to the neighboring phone.
Most cable TV providers
also provide telephone service. This is usually a type of VOIP or similar
technology that is routed through the cable providers system rather than the
Internet (the Internet is inherently unreliable so this makes cable TV phone
systems independent of Internet interruptions). Telephone companies are also
bundling television and Internet service with their telephone service, often via
fiber optics. One system provided by telephone companies connects via fiber
optic cable to an intermediate box. From there, a DSL signal (Digital Subscriber
Line, a high-speed digital signal sent over a POTS line) is sent to the customer
(this is called Fiber To The Curb or FTTC). The DSL signal is compatible with
the POTS system. Customers get television and internet through their traditional
telephone connection using DSL.
Some residential customers
may use VOIP services such as Vonage, Ooma or MagicJack. For these systems a
customer may connect a single phone to a VOIP router. Alternatively, the
residence's wiring my be disconnected form the telephone company where the local
loop enters the residence. A VOIP router connects to the customers telephone
wiring from there.
Cell Phones
Cell phone technology is
not related to traditional telephone technology (POTS/PSTN). Cell phones are
compact computer-controlled radio transceivers. They connect by radio to nearby
towers. Calls are then routed to the POTS network and then to traditional
telephones or through the cellular tower network to other cell phones.
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