Telephone interfacing circuits


telephone ac impedance

Impedance, which is given the letter Z, in a pure ohmic resistance is a complex number consisting only of a real part being the actual AC resistance value, (R) and a zero imaginary part, (j0). Because of this Ohm’s Law can be used in circuits containing an AC resistance to calculate these voltages and currents. Impedance is an important parameter used to characterize electronic circuits, components, and the materials used to make components. Impedance (Z) is generally defined as the total opposition a device or circuit offers to the flow of an alternating current (AC) at a given frequency, and is represented as a complex quantity which is graphically. A dc resistance less than ohms is required. This will create voltages from 3 to maybe 9V. If you want to transmit/receive audio, an ac impedance of ohms is recomended. The on-hook dc impedance should be more than 5 Mohms to V and 30 kohms to V. Notice that the .

Phone Line Basics: Revisited | News & Reviews - JK Audio

Telephone line audio interface circuits Copyright Tomi Engdahl First of all, I must officially advise against connecting anything other to the telephone line than equipment approved for the purpose by the telephone company or some other regulatery body, telephone ac impedance. Telephone company tend to be very strict about unauthorized gear hanging on their lines, and if something does go wrong with your gadget like putting dangerous voltages to telephone line you will be in deep trouble.

Index How telephone ac impedance works Network Interface in telephone Telecom hybrid circuits for other equipments than telephones Audio interfaces to telephone lines Telephone audio interfaces from Usenet news Audio interfaces without transformer isolation Simple telecom hybrid circuits More details implementing telephone line interface Problems in linking telephone hybrid to audio system Helpful tips for telephone hybrid circuit designers Connecting hybrids and telephone equipments Components for telephone line interfacing Telephone line technical details Telephone line details in different countries Interference in the telephone line signal Other related document Final words How telephone works A telephone uses an electric current to convey sound information from your home to that of a friend, telephone ac impedance.

When the two of you are talking on the telephone, telephone ac impedance, the telephone company is sending a steady electric current through your telephones. The two telephones, telephone ac impedance, yours and that of your friend, are sharing this steady current. But as you talk into your telephone's microphone, the current that your telephone draws from the telephone company fluctuates up and down.

These fluctuations are directly related to telephone ac impedance air pressure fluctuations that are the sound of your voice at the microphone. Because the telephones are sharing the total current, any change in the current through your telephone causes a change in the current through your friend's telephone.

Thus as you talk, the current through your friend's telephone fluctuates. A speaker in that telephone responds to these current fluctuations by compressing and rarefying the air. The resulting air pressure fluctuations reproduces the sound of your voice, telephone ac impedance.

Although the nature of telephones and the circuits connecting them have changed radically in the past few decades, the telephone system still functions in a manner that at least simulates this behavior.

The current which powers your telephone is generated from the 48V battery in the central office. The 48V voltage is sent to the telephone line through some resistors and indictors typically there is to ohms in series with the 48V power source, telephone ac impedance. The old ordinary offices had about ohm line relay coils in series with the line, telephone ac impedance. Just a relay with 2x ohm coils was uased for both current limiting and the coils between the line wires and the power source.

A typical off hook condition is TIP at about v and ring at about v. This means that there is about 8V voltage between the wires going to telephone in normal operation condition. The DC-resistance of typical telephone equipment is in ohm range and current flowing through the telephone is in mA range, telephone ac impedance.

Why 48V voltage is used telephone ac impedance telephone systems? The V voltage was selected because it was enough to get through kilometers of thin telephone wire and still low enough to be safe electrical safety regulations in many countries consider DC voltages lower than 50V to be safe low voltage circuits. Batteries are needed in telephone central to make sure that it operates also when mains voltage is cut and they also give very stable output voltage which is needed for reliable operation of all the circuit in the central office.

Typically the CO telephone ac impedance runs off of the battery chargers with the batteries in parallel getting a floating charge, telephone ac impedance. The line feeding voltage was selected to be negative to make the electrochemical reactions on the wet telephone wiring to be less harmful.

When the wires are at negative potential compared to the ground the metal ions go form the ground to the wire instead of the situation where positive voltage would cause metal from the wire to leave which causes quick corrosion.

Some countries use other voltages in typically 36V to 60V range, telephone ac impedance. PBXes may use as low as 24 Volts and can possibly use positive feeding voltage instead of the negative one used in normal telephone ac impedance network. Positive voltage is more commonly used in many electronics circuits, so it is easier to generate and electrolysis in telecommunications wiring is not a problem in typical environment inside office buildings.

Some older offices employ battery reversal swap DC feed to tip and ring telephone ac impedance signal off-hook at the remote end.

What is sealing current? The current sent to telephone line as an another advantage besides that it supplies the telephone ac impedance power for your telephone. Telephone practice uses or did use twisted splices.

These splices did not always make good connections. Placing a small DC bias on a long transmission pair is often done by telecommuncation carriers to reduce poor connections, and noisy lines, telephone ac impedance. The DC bias is often refered to as a "sealing current". So putting DC current through the cable sealed the connection and so improved the transmission.

Why full duplex operation in single wire pair? Full-Duplex is a term used to describe a communications channel which is capable of both receiving and sending information simultaneously.

Telephone sets ordinary analog ones have only 2 wires, which carry both speaker and microphone signals. The signal path between two telephones, telephone ac impedance, involving a call other than a local one, requires amplification using a 4-wire circuit. The cost and cabling required ruled out the idea of running a 4-wire circuit out to the subscribers' premises from the local exchange and an alternative solution had to be found.

Hence, the 4-wire trunk circuits were converted to 2-wire local cabling, using a device called a "hybrid". This function can send and receive audio signals at the same time is accomplished by designing the system so that there is a well balanced circuit in both ends of the wire which are capable or separating incoming audio from outgoing signal.

This function is done by telephone hybrid circuit contained in the network interface of the telephone. What is the bandwidth of the telephone line? The frequency response is limited by the telephone transmission system the actual wire telephone ac impedance central office to your wall can usually do telephone ac impedance more. Nowadays POTS is sharply bandlimited due to the fact that the line almost always is digitally sampled at 8kHz at some point in the circuit. The absolute, theoretical limit with perfect filters is therefore 4kHz - but this isn't reality, 3.

The bass frequency response is limted because of the limitations in telephone system components: transfromers and capacitors can be smaller if they don't have to deal with lowest frequencies. Other reason to drop out the lowest frequencies is to keep the possibly strong mains frequency 50 or 60 Hz and it's harmonics hummign away from the audio signal you will hear. Network interface details The telephone has a circuit called network interface also called voice network or telephone hybrid which connects the microphone and speaker to the telephone line.

Network interface circuitry is designed so that it sends only the current changes the other telephone causes to the speaker.

The current changes which the telephone's own microphone generates are not send to the speaker. All this is accomplished using quite ingenious transformer circuitry, telephone ac impedance. Telephone ac impedance theory the hybrid circuit can separate all incoming oudion from the audio sent out at the same time if all the impedances in the circuitry hybrids on both ends and the wire impedance in between are well matched, telephone ac impedance.

Unfortunately, the hybrid is by its very nature a "leaky" device. As voice signals pass from the 4-wire to the telephone ac impedance portion of the network, the higher energy level in the 4-wire section is also reflected back on itself, creating the echoed speech.

The because circuit does not work perfectly and you can still hear some of your own voice in the speaker The actual amount of signal which is reflected back depends on how well the balance circuit of the hybrid matches the 2-wire line.

In the vast majority of cases, the match is quite poor, resulting in a considerable level of signal being reflected back.

The signal which is reflected back is not always bad and in normal telephone some if it is really intentional by the design. The separation of the received and transmitted audio could be done much better with modern electronics than with old phones, telephone ac impedance, but but people who use the telephone ac impedance prefer to hear some of their own voice back.

Radio Shack's "Understanding Telephone Electronics" copyrighted around I think calls this effect sidetone and gives the impression that this was indeed intentional in order for the speaker to determine how loud they were speaking with reference to the called party.

Signaling Ringing signals When the central office want telephone ac impedance make your telephone ring it will send an AC telephone ac impedance voltage to the line which will ring the bell in your telephone. Most of the world uses frequencies in The ringer is built so that it will no pass any DC current when it is connected to telephone line traditionally there has been a capacitor in series with the bell coil.

So only the AC ring singal can go though the bell and make it ring. The bell circuit is either designed so that it has high impedance in audio frequencies or it is disconnected from line when phone is picked off-hook.

For more information about telephone ringing take a look at my telephone ringing circuits web page. Dialing There are two types of dials in use around the world: pulse dialing and tone dialling. The most common one is called pulse dialing also called loop disconnect or rotary dialing. Pulse dialling is oldest form of dialing, it's telephone ac impedance with us since the 's.

Pulse dialing is traditionally accomplished with a rotary dial, which is a speed governed wheel with a cam that opens and closes a switch telephone ac impedance series with your phone and the line.

It works by actually disconnecting or "hanging up" the telephone at telephone ac impedance intervals. The mostly used standard is one disconnect per digit so if you dial a "1," your telephone is "disconnected" once and if you dial "2" your telephone is "disconnected" twice and for zero the line is "disconnected" ten times but there are also other systems used in some countries. Touch tone is fast and less prone to error than pulse dialing.

Bell Labs developed DTMF in order to have a dialing system that could travel across microwave links and work rapidly with computer controlled exchanges. Touch-tone can therefore send signals around the world via the telephone lines, and can be used to control phone answering machines and computers this is used in many automatic telephone services which you operate using your telephone keypad.

Each transmitted digit consists of two separate audio tones that are mixed together the four vertical columns on the keypad are known as the high group and the four horizontal rows as the low group. Standard DTMF dials will produce a tone as long as a key is depressed, telephone ac impedance. No matter how long you press, the tone will be decoded as the appropriate digit. The shortest duration in which a digit telephone ac impedance be sent and decoded telephone ac impedance about milliseconds ms, telephone ac impedance.

Other signals The telephone central can send any different types of signals to the caller telling the status of telephone call.

Those signals are typically audio tones generated by the central office. Typical this kind of tones are dialing tone typically constant tone of around Hztelephone ac impedance tone tone telling that the telephone in other end is ringing or busy tone usually like quickly on and off switched dialing tone. The exact tones used vary from country to country. Detecting end of call There is no guaranteed single way to determine when a call was terminated at the far end.

If you want to do something for unknown lines on unknown switches then you will need a combination of the above.

Safety issues of telephones The telephones should be designed so that they do not cause danger to the telephone ac impedance. Telephone wires are also exposed to any different environmental effects nearby lightning, ground potential differences in buildings, interference from power lines which can cause that there are sometimes high voltage spikes on the telephone wires. Normal telephones are designed to be fully enclosed in insulating plastic case which provides telephone ac impedance. The plastic case telephone ac impedance nicely as isolation if there are no metal contacts in the telephone which are somehow connected to telephone line.

If the equipment has touchable metal surfaces or connections for power from other sources the equipment must provide proper electrical isolation between the telephone line. You have to provide volts between the telephone line and rest telephone ac impedance your electronics. Typically computer modems does this isolation using transformers, optoisolators and relays.

The telephone company can't know what kind of foolishly designed gadgets their customer has hanging on the end of line, telephone ac impedance, but it does specifically tell them, that at any time without warning and at their convenience they might just put a variety of voltages and currents on any given loop. If the device is not designed to meet the regulations it can cause dangers telephone ac impedance problems in those situations.

Equipments must be also designed to meet the safety rules so that they do not led dangerous voltages to enter telephone lines and causing a danger of being electrocuted to the telephone company workerd which do the wiring. Lamps With the exception of the lamps, all of the above are a safety hazard in addition to being very likely to make the phone line malfunction, telephone ac impedance.

In particular polarized capacitors or any cap rated at less than volts DC working voltsand batteries of any kind, should be avoided because of the potential for a an explosion.



telephone ac impedance


Impedance is an important parameter used to characterize electronic circuits, components, and the materials used to make components. Impedance (Z) is generally defined as the total opposition a device or circuit offers to the flow of an alternating current (AC) at a given frequency, and is represented as a complex quantity which is graphically. A dc resistance less than ohms is required. This will create voltages from 3 to maybe 9V. If you want to transmit/receive audio, an ac impedance of ohms is recomended. The on-hook dc impedance should be more than 5 Mohms to V and 30 kohms to V. Notice that the . The DC-resistance of typical telephone equipment is in ohm range and current flowing through the telephone is in mA range. When the central office want to make your telephone ring it will send an AC ringing voltage to the line which will ring the bell in your telephone. Most of the world uses frequencies in Hz range and.