[Boatanchors] decibels (was Microphone Output)

Fri Jan 11 14:41:58 EST 2013

Great description.  Thanks.  Various industries and individual companies have used a variety of definitions for line level also. Sometimes +4dbm instead of +8 dbm.  I think a few folks even used +10 dbm at times.

YMMV
Bry,  AF4K 

Richard Knoppow <1oldlens1 at ix.netcom.com> wrote:

>     FWIW, the Bel was originally called the Transmission 
>Unit or TU. There were a couple of articles in the BSTJ 
>about it and I will try to find them.  The entire BSTJ is 
>one the web for free. The TU was derived from the loss of 
>one mile of "standard" wire, at the time open wire #19 line 
>as used for "long line" telephone transmission. Previous to 
>the TU loses were stated as being in miles of standard wire.
>     Of course TU or Db is a ratio so is not meaningful for 
>absolute levels without a reference level being given.  The 
>reference level used in broadcasting was not standardized 
>for many years leading to confusion in determining 
>transmission levels on network lines. The level of 6 mW in 
>500 ohms was common but RCA used 12.5 mW and both 500 and 
>600 ohm impedances were used.  In addition the meters used 
>for measurement had different characteristics.  In 1939 a 
>co-operative effort by AT&T/Western Electric and the three 
>broadcasting networks arrived at a standard unit and 
>standard instrument to measure it. The unit was called The 
>Volume Unit and was based on a zero level of +8 db with 
>reference to 1 milliwatt in 600 ohms.  This was essentially 
>the same as the previously used level of 6 milliwatts in 500 
>ohms so that the new instrument could be used in place of 
>all others with no significant change in level. The VU meter 
>also had carefully designed ballistic characteristics so 
>that it would produce meaningful readings from speech and 
>other audio signals as well as steady tones.  They work 
>quite well if you understand them.  In addition the term dbm 
>was introduced to indicate a reference of 1 mW in 600 ohms.
>     The reference level of +8 dbm was based on the original 
>level of 6 mw because it was about 10db below the overload 
>point of the repeaters used at AT&T at the time. Also, while 
>the new VU meter will give a full-scale reading from lower 
>signals it contains an oxide bridge rectifier which 
>generates distortion, so when used on high quality lines, 
>the meter must be isolated by means of a resistive loss 
>network. 4 db loss is sufficient to bring the distortion 
>down to an acceptable level. Modern meters are usually 
>electronic so do not have this problem.
>     All sorts of reference levels for decibel measurements 
>are in existence, one finds levels for RF based on power in 
>50 ohm lines and also voltage references that relate to the 
>common power levels.
>     I also should point out that while the VU meter is 
>still available it has fallen out of favor, many preferring 
>peak reading meters of various sorts. In fact, the VU meter 
>is probably a more satisfactory instrument for setting audio 
>levels than some of the newer meters.  The best sort of 
>meter is one that indicates VU based on the original meter 
>but also indicates instantaneous peak levels.  There are 
>several of these on the market but I think the first one was 
>made by Mike Dorrough, who did quite extensive research on 
>it.
>
>
>--
>Richard Knoppow
>Los Angeles
>WB6KBL
>dickburk at ix.netcom.com
>----- Original Message ----- 
>From: "Al Klase" <ark at ar88.net>
>To: <boatanchors at mailman.qth.net>
>Sent: Friday, January 11, 2013 10:02 AM
>Subject: Re: [Boatanchors] decibels (was Microphone Output)
>
>
>> "Logarithmic" is the important word.  In audio and radio 
>> we're generally dealing with large dynamic ranges.
>>
>> Trivia: A decibel (dB) in one tenth of a Bell (one order 
>> of magnitude, a factor of 10 times) named for Alexander 
>> Graham Bell who researched hearing before moving on to the 
>> telephone.
>> (Your ears have a dynamic range of about 120 dB.  That a 
>> trillion to one ratio!)
>>
>> Power ratio in dB = 10 * log (P1/P2)
>>
>> Voltage ration in dB = 20 * log (V1/V2)
>>
>> Change of 10 dB = 10 * power, 20 dB = 100 * power, 30 db = 
>> 1000 * power etc.
>>
>> 1 dB is a barely audible change.
>>
>> 3 dB is twice the power.  -3db is half the power,  Not a 
>> big change in audibility.
>>
>> 6 dB is twice the voltage etc.
>>
>> ENOUGH ALREADY!  Sorry, I freaked out.
>>
>> Al
>>
>>
>> On 1/11/2013 11:39 AM, David Stinson wrote:
>>> "dB" is the unit of measure of a relative logarithmic 
>>> ratio,
>>> usually involving power or amplitude.
>>> It needs a suffix to define a point of reference to which 
>>> the reading is relative.
>>>
>>> A few examples of these units are:
>>> dBm = power relative to 1 mW dissipated in 600 ohms.
>>> dBW = power relative to 1 W regardless of load impedance.
>>> dBi =  gain of an antenna relative to an isotropic 
>>> radiator.
>>> dBd = gain of an antenna relative to a dipole in free 
>>> space.
>>> dBV =  voltage relative to 1 volt, regardless of 
>>> impedance.
>>>
>>> A chart of the ratios and more info than you want is at:
>>> http://en.wikipedia.org/wiki/Decibel
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>>
>> -- 
>> Al Klase - N3FRQ
>> Jersey City, NJ
>> http://www.skywaves.ar88.net/
>>
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regards,
Bry Carling

-- 
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