Tutto sulla Telefonia
General details of telephone line
Plain old telephone systems (POTS) telephone line consits of one
wire pair which carries full duplex audio and the operating current for the telephone. The
telephone connected to line is powered from current limited 48V power source, so phones
on-hook, should measure around 48 volts DC. Practically the opearating voltages of
telephone systems can vary from 24V to 60V depending on the application, although 48V
nominal voltage is the most commonly used. Telephone applications often require and use
positive grounding in the central office, where the positive conductor of the 48V power
supply is connected to earth ground. The telecommunication industry began the positive
ground convention in the 1940s and many telecomm companies still employ the traditionally
positive grounded system.
This means that generally when telephone is on-hook, one telephone
line wire is quite near to the ground potential and other one carries -48V. When telephone
is put off-hook the voltage beween wires going to telephone drops down to the 3 to 9 volt
range and typically acurrent of 20-60 mA will flow through the telephone. The remaining
voltage drop occurs over the copper wire path and in the telephone central electronics.
Typical telephone line bandwidth is around 200-3200 Hz, which was
what phone companies decided years ago be sufficient for speech intelligibility while
allowing them to multiplex many calls over coax and twisted pair. The low end is rolled
off early to stay away from the mains frequency (50 or 60 Hz) interference. The high end
cut off is caused by the telephone transmission system (nowadays the audio is digitized at
8 kHz). The typical signal to noise ratio of a telephone line is approximately 45 dB or
somewhat less. The average signal levels on telephone line is -9 dBm average speech.
The voice on a tip/ring pair is full duplex balanced audio which
requires a two wire to four wire hybrid circuit or transformer to convert it into separate
transmit and receive audio paths. The hybrid circuit makes it possible to transmit two
channels of information in opposite directions on a single pair of wires. Historically
hybirid circuits have used one or two transformers. Bulky and expensive hybrid
transformers have been replaced in most telephones by ICs which perform the same function.
In a telephone, the biggest contributor to poor audio quality is the handset microphone
(it has be be cheap and withstand very hard use).
Telephone standards world is fragmented. Typically each country has
its own standards because of both the historical roots of the phone service and the desire
to protect the local phone market from outside competition.
Basically the telephone systems work in the same way in different
countries, but there are are some differences which can mean that a devices designed for
one country does not meet the regulations of other country and work poorly or not at all.
The differences in local technical standards range from minor to severe and affect many of
the signaling conditions on local loops.
The biggest differences are different wiring practices and
connectors, different line impedances, different nominal loop currents, different
signaling tones and different electrical safety regulations. Unfortunately nowadays many
countries are harmonizing many standards across their boundaries so it is nowadays
possible to designe devices which work well and meet the regulations in more than one
country at the time. For example in Europe the European Commission adopted CTR 21 standard
covers nonvoice equipments (for example FAX and MODEM) in more than 20 countries.
It would be a really good thing if telephone tones were standardised
throughout the world. Unfrotunately they are not and will propably never be fully
standardized to be same all around world. Telephone ringivers have differences between
countries. The ring signal is much the same, worldwide. It is around 80V at a frequency
between 16 2/3 Hz and 50Hz (20-25 Hz quite common). But its timings are wildly different,
as are the return tones it generates. Also the "dial tone", "busy
tone" etc. are different in different countries.
General information links
Business telephone systems very often take use of technologies like
PABX, multiline telephones and digital telephones.
PABX (Private Automatic Branch eXchange) is a telephone exchange
operated within an organisation, used for switching calls between internal lines and
between internal and PSTN lines. A PABX can route calls without manual intervention, based
entirely on the number dialed. In some old companies the might still be PMBX (Private
Manual Branch eXchange), which involves company employed operators manually switching each
call using a manual switchboard.
Alexander Graham Bell (1847-1922) is most famous for his invention
of the telephone. As a teenager of 18, Bell had been experimenting with the idea of
transmitting speech. In 1874, while working on a multiple telegraph he developed the basic
ideas for the telephone. He and his assistant Thomas Watson finally proved successful on
March 10, 1876, when the first telephone message was transmitted: "Watson, come here;
I want you.". This led eventually to the establishment of the Bell Telephone Company,
still in existence today, which introduced the telephone to the world.
- Antique Telephone Collectors
Association - The Antique Telephone Collectors Association, or ATCA, is the largest
telephone collectors organization in the world.
- Antique
Telephone Collector's News
- Cyber Telephone Museum
- An Exhibit of Common & Rare Antique Telephones
- Historic Telephony -
telephony history and old telephones
- Memorial
to the Bell System
- Telephony history pages
from Mike Sandman - old telephone related ads and articles
- The Strowger Telecomms Page
- information about old electromechanical telephone exchanges
- Tribute to the
Telephone - history, schematics, trivia, photos, links and other telephony related
pages
- Telephone
Schematics for Western Electric Phones - schematics, drawings, tables, photos, and
other information pertaining to telephones produced by Western Electric for the Bell
System prior to 1984
Computer can be used to perform lots of fuctions in modern telephony
systems. This link collectains mostly information on linking computers and traditional
telephone systems. There is a separate link section for
Internet telephony.
General information
Resource pages
Necessary techniques
Accessory circuits
- Circuit
eliminates PC echoes - long-distance-telephone services available via the Internet
often require the PC user to wear headphones of a headset to prevent echo caused by the
microphone's picking up the loudspeaker outputs, this circuit eliminates the echo while
using the existing PC microphone and speakers
Programming
Technical characteristics of tones for the telephone service are
listed in ITU-T Recommendation Q.35 (1988).
Unfortunately that document is not freely available (can be ordered from ITU-T if you are willing to pay), so the information
available in the following documents might be useful if you are looking for free
information on telephone line signaling.
Basic signaling
DTMF dialling
Touch-tone dialing, also know as DTMF dialling, is a method of
sending signals from telephone customer's premises to central offices and beyond. The idea
of touch-tone dialing was first introduced in 1964. Today, most of the telephones in the
in developed countries use touch-tone dialling. The advantage of touch tone signaling is
that the signaling is voice band signal and the dialing can be done faster than with pulse
dialing. The fact that DTMF signaling energy is in the voice frequency band, makes it
possible to transmit signaling information (12 distinct signals) to any point in the
telephone network to which voice can be transmitted. This makes it possible to use DTMF
signals for remote control functions in additions to normal dialling. DTMF tones are used
for controlling many modern automated telephone answering services.
Called ID
Caller ID system is a feature of digital telephone network, which
allows the telephone number of the caller to be transferred to the called person before
he/she answers the phone. This feature can be useful for example for storing numbers of
the people who have tried to call you when you were not home. In completely digital
systems (like ISDN and GSM) send the caller ID information as part of normal completely
digitla signaling. After digitialization of PSTN network (all parts except last mile are
digital) this caller ID functionality is become available in PSTN network also.
There are two different commonly used caller ID signalling systems
for sending caller ID informationg using normal telephone line (PSTN). Bellcore system
(which is used in USA) send the numer in data packet which is sent at 1200 bps speed using
FSK-modem. Eropean system send the same information using standard DTMF (Dual Tone Multi
Frequency) signalling.
Audio signal coding
Misc
- Ensuring Voice
Quality with Adequate Tail Length - Designers who worked on the POTS never dreamed of
the challenges facing those working on next generation telecommunications equipment.
Today's engineers live in the digital world where equipment designs are required to
codify, compress, cancel echo, control jitter and loss, packetize, switch, route, and
bill, and do it all more quickly and efficiently than their competitors. It is easy to
lose sight of the need to provide a quality voice connection when the principal design
goal is to maximize the volume of data the device can handle. Given these concerns,
worrying about the length of the circuit tail delay in an echo-cancellation algorithm
seems downright old fashioned and low on the design requirements list. But failure to
appreciate tail delay will adversely affect voice quality.
- E-Series
Recommendations Excerpts
- How message waiting
lights work - description of different message waiting light systems
The following telephone circuits are mainly ment to give ideas for
your own desigs. Many of the circuits are quite propably do not meet your local telephone
network terminal equipment specs (they vary from country to country) and are quite
propably poorly designed (this applies as well to many circuits in books and hobby
magazines). Connecting non-approved equipment to public telephone network is illegal in
many countries. If you connect non-approved circuit which do not meet the specs to the
public telephone network the telephone do not work properly with them and they also cause
safety hazards.
For information what is allowed to be connecte to telephone line the
reader form USA should check FCC part 68 regulations and European reader should check NET4
(ETS 300 001) regulations. Good information on telephone equipment regulations can be
found from Compliance Engineering Magazine website.
- 1
line status indicator - circuit gives you a visual indication when a line is in
operation, line polarity insensitive and operates from two AA batteries for one year, pdf
file
- 2
line telephone line status indicator - circuit gives you a visual indication when a
line is in operation, line polarity insensitive and operates from two AA batteries for one
year, pdf file
- 4
line telephone line status indicator - circuit gives you a visual indication when a
line is in operation, line polarity insensitive and operates from two AA batteries for one
year, pdf file
- Audio
Visual Ringer - This is an extra telephone ringer circuit. All that needs to be done
is to connect the given circuit in parallel with the existing telephone lines using twin
flexible wires. This circuit does not require any external power source for its operation.
- Free-line
indicator stops interruptions - A simple circuit lights an LED, which indicates
whether the line is free (Figure 1a). Batteries are unnecessary; the phone line powers the
circuit, and an accumulator saves energy for an "in use" indication.
- Telephone
partyline blocker - blocks other phones coming to telephone line when one is using it
- Phone Busy
Indicator - simple busy indicator, does not meet regulations of many countries because
this circuit draws excessive continuous current from the phone line even when not active
and thi can cause problems with telephone system
- Phone-In-Use
indicator - simple design idea for indicator
- Phone
in use circuits - two designs
- Phone In
Use Indicator
- Phone-line
privacy circuit - circuit uses three HS20s, bilateral silicon trigger switches, to
form a phone-line privacy circuit with three telephone outputs
- Ringing Phone
Light Flasher - when the phone rings light flashes, good ring detector example circuit
- Telephone In-Use
Indicator - versions for LED indicator and relay output
- Telephone
Line Indicator design - information on the characteristics of telephone lines and
circuit which connects to a telephone line and indicates whether or not the line is in use
(seems to be quite good design)
- Telephone privacy
adapter - revents other telephones from hearing your call and disturbing your modem
connection
- Audio
Visual Ringer - extra ring detector with LED and sound output
- Chip
recorder customizes phone ringer - By placing a circuit in series with the telephone,
you can customize a phone ringer without modifying the phone. The heart of the ringer is a
chip-recorder IC, which can play as much as 10 seconds of telephone-quality recorded
sound.
- Ringing Phone
Light Flasher - when the phone rings, lights connected to circuit flash
- Phone
rang indicator light - will detect the ring signal, energize the relay which latches
up, and the LED comes on and stays on till you push a switch
- Smart
Phone light - The circuit shown here is used to switch on a lamp when the telephone
rings, if the ambient light is insufficient.
Dialler circuits
Test equipments
- Bi-Directional
Telephone Line Simulator - with Dialtone and Ring signals
- Cut Phone
Line Detector - can be used to alarm security system
- Digital/Standard
Phone Line Tester - make sure that the line is suitable for modem use and your PCMCIA
does not damage if you plug it in
- Digital/Standard
Phone Line Tester - telephone line polarity and current detector
- Off line
Telephone tester - Here is a circuit of an off-line telephone tester which does not
require any telephone line for testing a telephone instrument. The circuit is so simple
that it can be easily assembled even by a novice having very little knowledge of
electronics.
- Off
line Telephone tester - Here is a circuit of an off-line telephone tester which does
not require any telephone line for testing a telephone instrument. The circuit is so
simple that it can be easily assembled even by a novice having very little knowledge of
electronics. This circuit provice the line current, sign signal and audio interface.
- Telephone
line monitor - detects if there is another telephone connected to the line, if there
is a short or an open line
- Phone
Remote System - a telephone interface that allows the householder to give instructions
to appliances from outside, application note from SGS-Thomson
- Remote
control using telephone - Here is a teleremote circuit which enables switching
"on" and "off" of appliances through telephone lines. The circuit
described here can be used to switch up to nine appliances (corresponding to the digits 1
through 9 of DTMF the telephone key-pad). The telephone can be used to switch on or switch
off the appliances also while being used for normal conversation.
- Remote
control using telephone - This is a teleremote circuit which enables switching
‘on’ and ‘off’ of appliances through telephone lines. It can be used
to switch appliances from any distance, overcoming the limited range of infrared and radio
remote controls.
- Remote
control using telephone - Here is a teleremote circuit which enables switching
'on’ and 'off’ of appliances through telephone lines. The circuit described here
can be used to switch up to nine appliances (corresponding to the digits 1 through 9 of
the telephone key-pad). The DTMF signals on telephone instrument are used as control
signals.
- Telephone Line
Relay Switch - Control 4 relays over your phone line. Dial up your phone from anywhere
in the world, input your pin number then turn on or off any of 4 relays. Use the relays to
control any devices you have in your house or office. The pin number is set by 4 on-board
BCD rotary switches. The circuit has telephone line protection circuitry built-in (but not
approved). Standard telephone line impedance termination can be customised by the user for
their own particular country if they want to get better impedance matching.
Misc telephone accessories
- 9-Line Telephone
Sharer - This circuit is able to handle nine independent telephones (using a single
telephone line pair) located at nine different locations, say, up to a distance of 100m
from each other, for receiving and making outgoing calls, while maintaining conversation
secrecy. This circuit is useful when a single telephone line is to be shared by more
members residing in different rooms/apartments.
- Building a
telephone answering system - laboratory report
- Having
secrecy in parallel telephones - Often a need arises for connection of two telephone
instruments in parallel to one line. But it creates quite a few problems in their proper
performance, such as overloading and overhearing of the conversation by an undesired
person. In order to eliminate all such problems and get a clear reception, a simple scheme
is presented here.
- Parallel Telephones
with Secrecy - enable the incoming ring to be heard at both the ends and any one time,
only one telephone is connected to the line
- Phone
"Hold" With Music - interface to a radio or CD player headphone jack
- Tape Recorder
Switch for Telephone - This kit uses a FET to switch on a tape recorder when the phone
is picked up. It will work with low voltage tape recorders as low as 1.5V. It requires the
REMOTE & MIC jacks of the tape recorder to be available.
- Telephone
call meter using calculator & COB - In this circuit, a simple calculator, in
conjunction with a COB (chip-on-board) from an analogue quartz clock, is used to make a
telephone call meter.
- Telephone
"distinctive ring" switcher - zipper demo file from Dunfield Devellopment System
- Telephone
Headgear - A compact, inexpensive and low component count telecom head- set can be
constructed using two readily available transistors and a few other electronic components.
. Since the circuit draws very little current, it is ideal for parallel operation with
electronic telephone set.
- Telephone
Hold Button
- Telephone
In-Use Indicator
- Telephone
In-Use Indicator, Telephone Ringer and The Surfers Preserver
- Telephone Line Based
Audio Muting and Light-On Circuit - circuit would automatically light a bulb on
arrival of a telephone ring and simultaneously mute the music system/TV audio for the
duration the telephone handset is off-hook
- Telephone
line based audio muting and light on circuit
- Telephone
to radio patch
- Telephone
Recorder - lets you record your phone conversations automatically, notice that this
circuit does not provide isolation between telephone line and the recorder
- Telephone recording
circuits - how they work and some example circuis
- The
telephone ring monitor - this circuit is very simple and can be mounted into a big
telephone plug
Surge protection
Telephone transmitters
Telephone bugging is an illegal practice. The following circuits are
for educational use. They are illagal in many countries.
Misc telephone circuits
Signaling
Line interfacing
- 3750 V of Isolation
Not Required at the Network Termination - technical note in pdf format from Midcom
- Approximation
problem of telephone-line characteristi impedance - article in pdf format
- dBm to RMS voltage
chart - in pdf format, also available in Microsoft Excel 4.0 format, from Midcom Technical Note library
- DC
Holding Circuits - features several DC Loop Holding Circuits, one of them meets the UK
voltage/current template as specified by NET4
- Gyrator circuit -
simulate large coils electronically
- Handy Line Interface Design
Hints!
- How to measure Total
Harmonic Distortion (THD) of a transformer or other passive device - none page Word
file from Midcom Technical Note
library
- Impedance
measurements of telephone lines - two page technical note shows the impedances
measured on the telephone line, for more information read the whole article in Microsoft Word format
- Low
cost, CTR 21 compliand DAA for Europe - application note from Midcom
- Midcom
Technical Note Technical Note Index - tech notes about telephone transformers
- Modem Line Interface
Solutions using Vishay SSRs
- Notes on the
Telephone-To-Sound-Card Interface Project - The purpose of this circuit is to allow a
sound card (a SoundBlaster AWE 64 in my case) to interface with the phone line so that
output from the sound card that normally goes to the speakers goes instead to the phone
and can be heard by a remote caller or local phone, and also speech on the phone line will
be sent to the sound card via the sound card's microphone interface.
- Optimum
Hybrid Design - nice application note in pdf format
- Optocouplers
Isolate Modem Data Access Arrangement - application note in pdf format
- Overview of IEC 950
- EN 60950 (BABT), UL 1950 / CSA 950 - A Brief Overview of what it is and what is
important when dealing with transformers in pdf format from Midcom Technical Notes
- Return Loss
Calculations - four page note explains how return loss can be determined and contains
a MathCad analysis, an Excel spreadsheet analysis and a QuickBASIC program listing, zipped
file from Midcom Technical Note
library
- Siemens
Optical DAA2000 Circuit Variations for International Operation Application Note - pdf
file
- Siemens
Optical DAA2000 Theory of Operation Application Note - pdf file
- Transformer
Equivalent Circuit - he 'complete' equivalent circuit as used in Midcom's Transformer
Analysis Program (TAP)
- Transformer Modeling
Tips - technical note in pdf format from Midcom
- Transformer Seminar
Notes - companion to the seminar entitled "The Telecommunications Transformer:
low-cost solution to product reliability and continuing regulatory compliance" by
Dave LeVasseur, in pdf format, from Midcom
- Universal
DAA Reference Schematic from Infineon - pdf file
- USA
DAA Reference Schematic from Infineon - pdf file
- Wiring
inside telephones
Automatic terminal equipment
Telephone ringing
Telephone ICs
Other ideas
The following telephone hacking circuits are here only for
educational purposes. Most of them don't work anymore and connecting them to telephone
network is illegal.
Related pages
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