[Boatanchors] decibels (was Microphone Output)

Fri Jan 11 14:32:04 EST 2013

     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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