Xref: cantaloupe.srv.cs.cmu.edu rec.audio.car:9691 rec.audio:63217 sci.electronics:52758 Newsgroups: rec.audio.car,rec.audio,sci.electronics Path: cantaloupe.srv.cs.cmu.edu!crabapple.srv.cs.cmu.edu!fs7.ece.cmu.edu!europa.eng.gtefsd.com!emory!swrinde!cs.utexas.edu!uunet!infonode!ingr!b30news!catbyte.b30.ingr.com!dtmedin From: dtmedin@catbyte.b30.ingr.com (Dave Medin) Subject: Re: What is Zero dB???? Message-ID: <1993Apr5.214549.23508@b30news.b30.ingr.com> Sender: usenet@b30news.b30.ingr.com (Usenet Feed) Reply-To: dtmedin@catbyte.b30.ingr.com Organization: Intergraph Corporation, Huntsville AL References: Date: Mon, 5 Apr 1993 21:45:49 GMT Lines: 81 In article , sehari@iastate.edu (Babak Sehari) writes: |> In msunde01@mik.uky.edu (Mark Underwood) writes: |> |> >I am somewhat familiar with the dB measurements as they apply to |> >electrical circuits - i.e. it is the gain of (for example) an amplifier |> >measured on a logarithmic scale. However, this requires that you have a |> >reference value: i.e the ouput is +20dB (e.g.) with respect to the input |> >signal. |> What you actually talking about here is dBm and not dB. However, the terms |> are used loosely by most people. ``dBm'' is power with respact to 1mW, whereas |> dB is a ratio. Now, like these two English statement: |> |> 1. I am doing well. |> 2. I am doing good. |> |> Similarly, people usually use dB for dBm. Another common mistake is spelling |> ``db'' instead of ``dB'' as you did in your article. See the ``B'' is for |> ``Bell'' company, the mother of AT&T and should be capitalized. |> |> With highest regards, |> Babak Sehari. Good gravy! Decibels are all *ratios.* The question that remains in any ratio is the reference unit used. Sometimes, this will be a reference power, such as 1 milliwatt (given a certain circuit impedance which should also be included in the fine print or known, like 50 ohms in an RF circuit of that impedance), leading to an accepted notation of dBm. Maybe it might be dBV, disregarding the impedance of the circuit and power developed, using 1 volt as reference *amplitude* (rather than reference power). Or, it might have an arbitrary or omitted reference that is not included in the notation, leading to just plain dB. So. look at it this way--'dB' has an implied reference while notation such as 'dBm' has an explicit reference. For power: dB = 10*log( P(measured)/P(reference) ) For amplitude dB = 20*log( A(measured)/A(reference) ) 'B' is for bel, which is a standard term for a log ratio to the base 10, named after Alexander Graham Bell. A 'deci' Bel is 1/10 of a bel. It has nothing to do with the Bell Telephone company except for the common founder's name. The small 'd'/large 'B' is per SI notation convention. I don't know anyone that's been crucified for messing it up. Common references for audio are: 0 dBm = 1 milliwatt across 600 ohms 0 dBV = 1 volt 0 VU (a zero on the VU meter) = +4 dBm (pro gear line level) 0 VU = -10dBV (consumer gear line level) Often times, a power amp VU meter will be aligned using the rated power of the amp as the 0 dB point. It is all done to whatever reference is reasonable for the application or moment. Note that in a circuit with a given (and maybe unknown) linear impedance, if the amplitude goes up so many decibels, the power will also increase the same amount. This proof can be done with the above two identities and ohm's law. -- -------------------------------------------------------------------- Dave Medin Phone: (205) 730-3169 (w) SSD--Networking (205) 837-1174 (h) Intergraph Corp. M/S GD3004 Internet: dtmedin@catbyte.b30.ingr.com Huntsville, AL 35894 UUCP: ...uunet!ingr!b30!catbyte!dtmedin ******* Everywhere You Look (at least around my office) ******* * The opinions expressed here are mine (or those of my machine)