I showed an early interest in radio, and got my first radio job when I was Before I finished high school, I had already worked at four radio stations and one TV station: I liked radio much better than TV.
General technical details of telephone line Surprisingly, a telephone is one of the simplest devices you have in your house. It is so simple because the telephone connection to your house has not changed in nearly a century. If you have an antique phone from the s, you could connect it Telephone bill format the wall jack in your house and it would work fine!
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 s and many telecomm companies still employ the traditionally positive grounded system.
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.
This means that generally when telephone is on-hook, one telephone line wire is quite near to the ground potential and other one carries V. When telephone is put off-hook the voltage beween wires going to telephone drops down to the 3 to 9 volt range and typically a current of mA will flow through the telephone.
The typical operating current range is mA. Any more than 55 or 60 mA and it might harm the phones. So the telephone equipment itself does nto need any high voltages to operate. The remaining voltage drop from 48V to volts occurs over the copper wire path and in the telephone central electronics.
This high voltage is needed in the beginning because the length of the telephone line can be many kilometers, which means lots of wire resistance on the way to drop the voltage.
Those names originate from plugboard terminology! Typical telephone DC resistance around ohms and AC impedance is typically somewhere around ohms. Typically the telephone central provide from to ohms of series resistance to protect from short circuits and decouple the audio signals. Typical telephone line bandwidth is around 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 and keep telephone transformer size small. The high end cut off is caused by the telephone transmission system nowadays the audio is digitized at 8 kHz.
The high end cut off is critical. Voice on the modern telephone network is digitized at the central office at 8 kHz sampling rate which means that any signal above 4 kHz will be aliased back as noise in the voice band. The phone company uses 8 bit mulaw nonlinear coding which yields about 12 bits of dynamic range.
The bottom line is that you can never count on more than about 45 dB signal to noise ratio. The typical signal to noise ratio of a telephone line is approximately 45 dB or somewhat less. Telephone line signal to noise ratio is not as easy to quantify because noise comes in many forms, such as electrical interference from fluorescent fixtures or hiss from the many amplifier stages in the voice path.
Speech correlated noise can be introduced from non-linear speech coding and compression algorithms. Crosstalk from other conversations is another form of noise. The FCC requires that all telephone audio interconnect equipment limit speech to -9dBm, averaged over 3 seconds.
Consult FCC Part 68 requirements for all the details. Telephone line is balanced voice path. Balanced voice path refers to the fact that the two wires tip and ring are only referenced to each other.
This design allows signals to travel for miles without expensive shielding by using common mode rejection to remove noise that is induced onto both wires.
In a balanced pair of wires, your voice travels on both wires at the same time, but one wire is electrically degrees out of phase with the other.
At both ends of the connection your telephone and the phone companythe signal on one of the wires is inverted and then added with the other or the voltage difference is otherwise receved. If electrical noise from an appliance leaked on to the wires somewhere between your phone and the phone company.
The noise would appear equally and in phase on both wires. When the signal on both wires was inverted and then added together degrees out of phase, the electrical noise would cancel itself out while the voice signal gets stronger.telephone bill format in pfmlures.comg-invoice-template_png[/caption] Related For 5+ telephone bill format in excel.
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FCC Chairman Ajit Pai discusses FCC efforts to help protect consumers from the illegal and harmful practices of slamming - the unauthorized change of a consumer's preferred telephone company - and cramming - the placement of unauthorized charges on a consumer's phone bill.
Telephone technology page. General info. General technical details of telephone line. Surprisingly, a telephone is one of the simplest devices you have in your house.
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