dBm dBW dB dBu confusion

[KL7OF]

I have an audio deck that says it requires -20 dB at 600 ohms to drive it to full output.....I have audio  boxes that have + 22 dBu output...I have a chart from an audio book that shows outputs in dBm and dBW converted to watts...  So the question is...What is the difference between dBu, dBm, dBW and just plain dB?   If they all relate to an output in watts, why do we use the dB terms?   My chart says +10dBm =10 milliwatts  but +10 dBw = 10 watts....No definition for just plain dB or dBu....seems like a lot of terms from the department of redundancy department....Is there a simple explanation?  
 

[N8ETQ]

 Kinda like MPG   I guess

 http://www.sengpielaudio.com/calculator-volt.htm

73
/Dan

[KA2DZT]

I have an audio deck that says it requires -20 dB at 600 ohms to drive it to full output.....I have audio  boxes that have + 22 dBu output...I have a chart from an audio book that shows outputs in dBm and dBW converted to watts...  So the question is...What is the difference between dBu, dBm, dBW and just plain dB?   If they all relate to an output in watts, why do we use the dB terms?   My chart says +10dBm =10 milliwatts  but +10 dBw = 10 watts....No definition for just plain dB or dBu....seems like a lot of terms from the department of redundancy department....Is there a simple explanation?  
 

Steve,

dB  refers to increase or decrease in signal level but does not tell you at what level you're at.  Much the same if I said the temperature went up 20 degrees.  All you know is the temp went up 20 degrees but you don't know what the temp is.

If an amplifier had a gain of 20dB, the output would be 20dB greater than whatever signal level is injected into the input.

When you have notation as dBm or dBw and others, this notation refers to an exact signal level.  0 dBm refers to 1mw at 600 ohms.  Your audio deck requires -20dB at 600 ohms (which is -20dBm).  -20dBm is equal to .01 milliwatts or .077 volts at 600 ohms (V squared/R).  There are dBm scales that can be at different loads (not 600 ohms).  There were dBm scales that 0 dbm was not 1mw at 600 ohms, 6 mw was also used to refer to 0 dBM, meaning that 0 dBm was equal to 6 mw at 600 ohms.  This old 6mw notation is not use in more modern equipment, but could have been used with old equipment.

TV antenna work we use meters that measure tv signal in dBmv  0 dBmv refers to 1000 microvolts at 75 ohms.  TV signal amplifiers will have two ratings, one in dB gain and another in dBmv output, which is the amp's max output in millivolts.  Example, a TV amp might have a gain of 30dB and a max output of +60dBmv (+60dBmv refers to 1 volt at 75 ohms).  Notice the TV signal meter is calibrated in volts and not in watts.

I not really sure what dBu refers to, unless you mean dBv.  There is a dBv notation.

Hope some of this helps,
Fred

[W3SLK]

Fred that was an excellent dissertation! I have reviewed those calculations over a period of years and never seem to remember them, (a mind is a terrible thing when it is wasted).

[WD8BIL]

dBm= dB difference from a milliwatt
dBuv= db difference from a microvolt
dBW= dB      "           "      1 Watt

It can be used to reference a change in voltage, current, power, sound and a bunch more.

[W2VW]

I thought the same thing while reading Fred's post.

He has a good way of explaining stuff in written form.

Fred that was an excellent dissertation! I have reviewed those calculations over a period of years and never seem to remember them, (a mind is a terrible thing when it is wasted).

Edit. IIRC Tom Vu has posted a nice conversion for aligning Asain rigs somewhere in this board.

dBVu?

[WBear2GCR]

I dunno, I found some other information probably on WikiGridLeaks.com...

dBu = dBuser, or whatever you measure
dBm = dBmerde, from the French for s**t... as in "oh s**t, the dB level is way too high - TURN IT DOWN!"
dBv = dBvamoose, from the Spanish (incompatable with dBm!!), from "get that s**t out of here!"
dB = aka "dog biscuits" - as in what you give to poochie.

Hope this clears things up for once and for all...

[WA1GFZ]

dBuA = dBuV - 34 dB
120 dBuA = 1 amp

[WD8BIL]

AND:
0dBm= 107 dBuv in 50ohm systems

[WA1GFZ]

and 0 dBuV = -107 dBM

[k4kyv]

 There were dBm scales that 0 dbm was not 1mw at 600 ohms, 6 mw was also used to refer to 0 dBM, meaning that 0 dBm was equal to 6 mw at 600 ohms.  This old 6mw notation is not use in more modern equipment, but could have been used with old equipment.

dBm has always been used in reference to 1mw, never 6mw.  The old 6mw reference standard was simply notated as "dB".  The reference impedance for line level audio used to be 500 ohms and later changed to 600, but this has nothing to do with dB or dBm, which refer to power levels at any impedance.  Audio transformers are often rated in terms of either dB or dBm, and their impedances may be hundreds of thousands of ohms, as in the case of low level interstage transformers. My early 50s UTC catalogue lists the "line" impedance of its audio input and output transformers as 500/600 ohms. In most applications the difference can be ignored.

I have many transformers in my collection that are rated in terms of "dB",  referenced to 6mw.  That standard originated with the landline telephone. Sometimes the 6 mw reference is stamped on the name plate of the transformer. I think they started using the dBm standard sometime after WW2, probably in the late 40s or early 50s.

I would guess that KL7OF's audio deck requires -20 dB at the 6 mw reference.  If it were at the 1 mw reference, it would have been stated as dBm. But if it is of recent manufacture it is unlikely they would be using the old 6 mw standard. You might just have to try it at various levels and see what works, if there is no documentation that states what the db reference is.

[WA1GFZ]

-20 db audio is the level you can shoot down a phone line remote without having cross talk problems

[KL7OF]

The deck I am working on is a CBE-1 TMC  late 50's to mid 60's technology....It has balanced and unbalanced audio inputs at 600 ohms..  -20dB required for full output.....I have found that I can drive it with a Shure 444 mic (-24dBV) ...It also responds well to any of the usual Behringer type boxes (+22dBu) as long as the  levels are kept low.  Quite a difference in power output between the plain mic and the audio boxes but it seems to be within the range of the input gain control on the TMC deck! Thanks for the help.....Steve --...  ...--

[KA2DZT]

 There were dBm scales that 0 dbm was not 1mw at 600 ohms, 6 mw was also used to refer to 0 dBM, meaning that 0 dBm was equal to 6 mw at 600 ohms.  This old 6mw notation is not use in more modern equipment, but could have been used with old equipment.

dBm has always been used in reference to 1mw, never 6mw.  The old 6mw reference standard was simply notated as "dB".  The reference impedance for line level audio used to be 500 ohms and later changed to 600, but this has nothing to do with dB or dBm, which refer to power levels at any impedance.  Audio transformers are often rated in terms of either dB or dBm, and their impedances may be hundreds of thousands of ohms, as in the case of low level interstage transformers. My early 50s UTC catalogue lists the "line" impedance of its audio input and output transformers as 500/600 ohms. In most applications the difference can be ignored.

I have many transformers in my collection that are rated in terms of "dB",  referenced to 6mw.  That standard originated with the landline telephone. Sometimes the 6 mw reference is stamped on the name plate of the transformer. I think they started using the dBm standard sometime after WW2, probably in the late 40s or early 50s.

I would guess that KL7OF's audio deck requires -20 dB at the 6 mw reference.  If it were at the 1 mw reference, it would have been stated as dBm. But if it is of recent manufacture it is unlikely they would be using the old 6 mw standard. You might just have to try it at various levels and see what works, if there is no documentation that states what the db reference is.

Don,

You're probably right about the 6 mw notation.  I have an old mod iron that is rated at +43 dB.  Judging from its size I had concluded that the +43 dB had to be the 6mw reference.  At first I thought it was mislabeled and that it should have read +43dBm.

You're probably also correct about KL7OF's audio deck, judging from what he says it reads, -20dB and not -20dBm.

I just rechecked the dB charts in my Radiotron Handbook and saw something I had not before noticed.  The 6mw reference charts show the power levels as just "dB" and the 1mw reference charts show the power levels as "dBm"

Thanks for helping to clear up that confusion.

[k4kyv]

I believe standard VU meters reference 0 dB as 1 mw at 600 ohms, the same as dBm.  I have some older non-VU "line level meters" that use the 6 dB reference, some at 500 ohms and some at 600 ohms. The 0 dB reference and line impedance is written below the meter scale on most of them.

[w1vtp]

Here's a worksheet that Benson Chan ginned up years ago

[K5WLF]

Here's a discussion of dB related topics from Rohde & Schwarz. Got a bunch of info in it.

ldb
K5WLF