What is a Bell (telecom term)

[tom baker]

I am helping with some research for Mike Holt on a history of grounding.
I read once the Bell (unit) was orginally the resistance of one mile of telephone wire.
Now its a power unit, or sound unit.
Any references to the orginal Bell defintion would be appreciated.

 

[ron]

Re: What is a Bell (telecom term)

The unit Bel, is actually a dimensionless unit. It is more like a ratio (similar to a percentage).
I can't think of any reason that it would associated with resistance except that it was a term identifying a relative resistance (ratio) to something else (like a mile of telephone wire).
Look in some of the on-line encyclopedias for for information on the bel.
We are more used to seeing 0.1 bel or 1 decibel.

 

[hbiss]

Re: What is a Bell (telecom term)

I don't think "bel" and "Bell" are the same thing. As ron points out "bel" is what decibel is derived from. I have never heard of the "Bell" term.

-Hal

 

[charlie b]

Re: What is a Bell (telecom term)

A ?Bel? is defined as the ?common logarithm of the ratio of ?power out? divided by ?power in.? ? A ?decibel? is defined as ten times this value. Using the logarithm function makes the numbers easier to work with. For example, if there is 100 times more power out than in, then the number of ?Bels? is 2. If you are amplifying a signal, and if the power gain is a factor of 2, then the number of decibels works out to 3.

Please note that we are not talking about creating power from nowhere. This is describing a power signal that is being amplified, and there is some external power source that is supplying the energy to make it happen.

I don't know whether there is an unrelated unit of measure called the "Bell," having to do with resistance.

 

[dereckbc]

Re: What is a Bell (telecom term)

Tom in the telco buisness we measure all signals in reference to dBm = decibels referenced to 1 milli-watt. The number can be positive of negative.

For example test tone level (TTL) on a POT's (plain old telephone) is generated in the CO @ 0 dBm, meaning 1 milli-watt generated into 600 ohms balanced line. With a test set connected to a line say in your home some distance away from the CO you then measure the line loss in db as a result of the copper wire resistance. So at your home you might see something like -6 dBm or 1/4 the power level or .25 milli-watts

However your question confuses me because you made a reference to grounding, which has nothing to do with what has been discussed so far. In the old days of analog telephone when party lines still existed, the telco's applied ring voltage between the ring conductor and ground. So it was important at that time the telephone office had a very low impedance ground electrode system (5-ohms or less) to facilitate the ring signal.

Look here for some more info:It will define Bels. To get decibels you multiply by 10. So in Charlie's example you would have a power gain of 20 dB with out a refference. http://www.tpub.com/content/neets/14188/css/14188_50.htm

[ May 13, 2005, 03:44 PM: Message edited by: dereckbc ]

 

[mc5w]

Re: What is a Bell (telecom term)

On 2-party lines 1 party had the ringing current from the tip conductor to the water mains. The other party had the ringer from the ring conductor to the water mains. Party identification for outgoing calls was to divert different amounts of talk current to ground - ground fault relays at the CO would then respond to the differnet amounts of current. However, usually 1 of the parties diverted ZERO talk current to ground and they would get somebody else's long distance charges if somebody else's telephone plug was not plugged in enough or whatever.

On 4 party lines biased ringing would be used. Direct current was placed in series with the 20 Hertz or so ringing current generator. The ringer had a permanent magnet that could be turned around and adjusted to respond to different amounts of direct current bias.

If there were more than 4 parties or if a good ground could not be obtained, there were 2 other schemes. One of them was to use different frequencies of ringing current and resonant ringers. The other was to key the ringing current using different letters of International Morse, what was known as ringdown and now known as distinctive ringing. Outgoing ID was in some cases effected by a special dial that interrupted the deliberate ground fault at a certain number of dial pulses.

Starting back in the 1970s a Japenese company came out with pair gain units that worked on the analog carrier principle. The first party would have their audio and ringing current at the usual frequencies. The other parties got to use low frequency AM radio signals over the same twisted pair. Pair gain units are still being installed. RCA makes a 4 line telephone that works without a key service unit and implements the intercom using the pair gain principle.

 

[rattus]

Re: What is a Bell (telecom term)

Let's simplify the answer to this question.

Strictly speaking, the bel and decibel are not units of power. They are instead dimensionless units of power gain (or loss).

Power gain (or loss) in bels is:

A = log(P2/P1)

The bel is hardly used, instead the decibel, defined below, is used:

Power gain (or loss) in decibels (db) is:

A = 10log(P2/P1)

where "log" is the common logarithm.

For perspective, a change in sound level of one db is discernible by a normal ear, and three db indicates a power gain of two.

Its main advantage is that gains in db can be added algebraically.

As has been said already, "dBm" indicates power gain relative to one milliwatt.

Another logarithmic unit, the Neper, using natural logarithms is sometimes used, but we will save that for another day.