What's a dBu in terms of voltage?
ron newman
1) When dealing with -10dBu and +4dBu levels in audio, what are we really
talking about in terms of RMS voltage? (is dBu the correct term?)
2) What is the formula for dBu? The standard decibel formula relates to a
ratio, (20log A'/A), how does 0 dBu relate to actual voltage on a line?
*****************************************************************
Ron Newman
Troubadour Technology Music
http://www.imt.net/~ronman/animal.htm
*****************************************************************
Mike Rivers
In article <66b30l$im0...@bzn4-03.imt.net> ron...@imt.net writes:
> 1) When dealing with -10dBu and +4dBu levels in audio, what are we really
> talking about in terms of RMS voltage? (is dBu the correct term?)
Today, we commonly talk about +4dBu and -10 dBV. In the old days we
used to talk about +4dBm.
dBV is dB referenced to 1 volt. The standard dB voltage formula
applies:
dB = 20 x log(voltage ratio)
x (the voltage = -10 dBV)
so, for -10 dBV, we have: -10 = 20 log(----------------------------)
1 (the reference voltage)
or, 10 dB below 1 volt = .316 volts.
> 2) What is the formula for dBu?
dBu is a little more complicated - not mathematically, but
historically. The "u" is really an invention based on the old
standard that 0 dB was one milliwatt of power. In order to translate
this into a voltage, it was necessary to know the impedance that power
was working into. The audio (and telephone company, from which most
of our old standards come) standard line impedance was 600 ohms, and
in the interest of maximum power transfer, everything had a 600 ohm
output impedance and a 600 ohm input impedance.
Power = V^2/R, so the voltage necessary to pump 1 mW into 600 ohms is:
.001 = V^2/600 or V=.775 volts.
It's incorrect to say that 0 dBm equals .775V because that's saying
that power equals a voltage, so they made up the term dBu.
Today we don't care about matching input and output impedances for
maximum power transfer. Solid state equipment tends to have a fairly
low output impedance and fairly high input impedance, so essentially
the output voltage doesn't change when an output is hung across an
input. dBu is the voltage relative to .775 volts at any reasonable
impedance.
+4 dBu is 4 dB higher than .775 volts. Plug those values into the
formula above and try calculating it yourself. (as they say, "left as
an exercise for the student")
Why +4? That's another good story. The short version was that the
manufacturers couldn't get a meter that, when hung across a 600 ohm
line with a large enough resistor in series with it so that it didn't
present a significant load, was sensitive enough to go to about 3/4
scale (where the 0 dB mark would be). So they cranked up the level
to get the meter to read 0 dB, and the combination of a 3.9 k ohm
resistor and 4 dB higher than the voltage producing 1 mW was what they
ended up with.
--
I'm really Mike Rivers (mri...@d-and-d.com)
Eric Haney
ron newman wrote:
>
> 1) When dealing with -10dBu and +4dBu levels in audio, what are we really
> talking about in terms of RMS voltage? (is dBu the correct term?)
"Pro" operating level is +4dBu, "pro-sumer" operating level is
-10dBV. A classic case of "apples and oranges", lets investigate why.
Standardization is paramount in measurements - a universal reference
is imperative for standardization. The universal reference is 0VU,
which is 1 mW into a 600ohm resistor.
> 2) What is the formula for dBu? The standard decibel formula relates to a
> ratio, (20log A'/A), how does 0 dBu relate to actual voltage on a line?
>
Lets try this: dBu=20*log(Voltage/Reference) where Reference is the
voltage needed to produce 1mW in a 600ohm resistor.
P=V^2/R
0.001w=Reference^2/600ohm
Reference=SQRT(0.001w*600ohm)
Reference=0.775V
Therefore: dBu=20*log(Voltage/0.775V)
dBV has a reference voltage of 1V, so:
dBV=20*log(Voltage/1V)
--
Eric Haney
............................................
mghaney...@worldnet.att.net
ron newman
Thanks for John Burne for the following:
>I found the following at
>"http://aix1.uottawa.ca/~gmartin/electronics_text/3_5.html"
>
>-----
>[dBu] used to be called dBv, but people kept mixing up dBv with dBV and
>that couldn't continue, so they changed the dBv to dBu instead. You'll
>still see dBv occasionally - it is exactly the same as dBu. Just
>different names for the same thing. [dBV is relative to 1V RMS; dBu (aka
>dBv) is relative to 1 milliwatt into a 600ohm load]
>Once upon a time you learned that "professional" gear ran at a nominal
>operating level of +4 dB compared to "consumer" gear at only -10 dB.
>(Nowdays, this seems to be the only distinction between the two.....)
>What few people ever notice is that this is not a 14 dB difference in
>level. If you take a piece of consumer gear outputting what it thinks is
>0 dB VU, and you plug it into a piece of pro gear, you'll find that the
>level is not -14 dB but -11.8 dB VU....... The reason for this is that
>the professional level is +4 dBu and the consumer level -10 dBV.
>Therefore we have two seperate reference voltages for each measurement.
>
>0 dB VU on a piece of pro gear is +4 dBu which in turn translates to an
>actual voltage level of 1.228 volts. In comparasion, 0 dB VU on a piece
>of consumer gear is -10 dBV, or 0.316 volts. If we compare these two
>voltages in terms of decibels, the result is a difference of 11.79 dB.
Malcolm Leech
ron newman <ron...@imt.net> wrote in article
<66kc4j$b0s...@bzn4-48.imt.net>...
> Thanks for John Burne for the following:
>
> >I found the following at
> >"http://aix1.uottawa.ca/~gmartin/electronics_text/3_5.html"
> >
> >-----
> >[dBu] used to be called dBv, but people kept mixing up dBv with dBV and
> >that couldn't continue, so they changed the dBv to dBu instead. You'll
> >still see dBv occasionally - it is exactly the same as dBu. Just
> >different names for the same thing. [dBV is relative to 1V RMS; dBu (aka
> >dBv) is relative to 1 milliwatt into a 600ohm load]
Sorry, but dBm is rlative to 1 milliwatt into a 600 ohm load, dBu is
relative 0.775 volts into a 600 ohm load. If this is wrong my lecturer LIED
to me.
My two cents worth.
Michael K. Poimboeuf
Malcolm Leech wrote:
>
...
>
> Sorry, but dBm is rlative to 1 milliwatt into a 600 ohm load, dBu is
> relative 0.775 volts into a 600 ohm load.
Almost. dBu is relative to SQRT(0.6) Volts (which is 0.775V
to three significant digits) _independent_ of load impedance.
Being independent of load impedance is the whole point.
--
Michael K. Poimboeuf m...@sgi.com
Advanced Media Products Division
Silicon Graphics Inc. Mtn View California
---------------------------------------------------------
John T. Cotton
I have in front of me "The Sound Reinforcement Handbook" by Gary Davis
and Ralph Jones, as well as "Sound System Engineering" by Don &
Carolyn Davis. On page 22 "The SR Handbook" states that "0 dBm=1
milliwatt. dBm has no direct relationship to voltage or impedance".
On the same page it says "0 dBu is always 0.775 volts." and also has
no reference to impedance. "SSE" makes similar assertions about the
dBm and curiously does not mention the dBu at all.
Messrs. Davis & Jones make reference to the Proceedings of the
Intstitute of Radio Engineers, volume 28, January 1940, in which the
dBm was first established. In any case, the dBm and dBu are only
equivalent when they both are measured at a 600ohm load. NEITHER of
them actually specifies such a termination.
We would all be well advised to consult a library every now and then
so that we might share the same language and standards.
Happy Holidays to all,
JTC
"Malcolm Leech" <mle...@ctcc.gov.za> wrote:
>ron newman <ron...@imt.net> wrote in article
><66kc4j$b0s...@bzn4-48.imt.net>...
>> Thanks for John Burne for the following:
>>
>> >I found the following at
>> >"http://aix1.uottawa.ca/~gmartin/electronics_text/3_5.html"
>> >
>> >-----
>> >[dBu] used to be called dBv, but people kept mixing up dBv with dBV and
>> >that couldn't continue, so they changed the dBv to dBu instead. You'll
>> >still see dBv occasionally - it is exactly the same as dBu. Just
>> >different names for the same thing. [dBV is relative to 1V RMS; dBu (aka
>> >dBv) is relative to 1 milliwatt into a 600ohm load]
>Sorry, but dBm is rlative to 1 milliwatt into a 600 ohm load, dBu is
Mark McQuilken
John T. Cotton wrote:
>
> I have in front of me "The Sound Reinforcement Handbook" by Gary Davis
> and Ralph Jones, as well as "Sound System Engineering" by Don &
> Carolyn Davis. On page 22 "The SR Handbook" states that "0 dBm=1
> milliwatt. dBm has no direct relationship to voltage or impedance".
> On the same page it says "0 dBu is always 0.775 volts." and also has
> no reference to impedance. "SSE" makes similar assertions about the
> dBm and curiously does not mention the dBu at all.
> Messrs. Davis & Jones make reference to the Proceedings of the
> Intstitute of Radio Engineers, volume 28, January 1940, in which the
> dBm was first established. In any case, the dBm and dBu are only
> equivalent when they both are measured at a 600ohm load. NEITHER of
> them actually specifies such a termination.
> We would all be well advised to consult a library every now and then
> so that we might share the same language and standards.
> Happy Holidays to all,
> JTC
>
Hi John!
I'm sorry to say that even though common usage would say otherwise,
YOU'RE CORRECT about the definition of 0 dBm with two misleading points:
(1) Since the decibel is defined as the ration of two powers, the
reference power is 1mW for the 0dBm point ("0 dBm = 1mW" is a
mathematical bastard, but does get the point across), and (2) "dBm has
no direct relationship to voltage or impedance" is an inaccurate
OVERSTATEMENT, I presume, to make the point that the ratio of power in a
system can be evaluated independently from voltage/impedance. I call it
"inaccurate" because the quantity POWER has a direct [mathematical]
relationship to voltage/impedance and, if transitivity holds, then so
too does the decibel. I understand why Davis/Jones overstated the
point...this thread is proof of the on-going confusion. BTW, here's
proof (once again) that even our venerable 0 dBm is suject to the "when
a standard isn't a standard" problem: it is convention in telephony to
assume that when we talk about terminations, we're talking about 600
Ohms. This has sort of crept into the "definition" of the 1 mW reference
level, but as you pointed out, is not truly part of the definition of 0
dBm proper.
Regards,
Mark McQuilken
Formerly of RCA
Advanced Technologies Lab
Mark McQuilken
jblac...@vlint.com
In article <01bd0552$4d132780$9450...@mleech.ctcc.gov.za>,
"Malcolm Leech" <mle...@ctcc.gov.za> wrote:
>
>
> ron newman <ron...@imt.net> wrote in article
> <66kc4j$b0s...@bzn4-48.imt.net>...
> > Thanks for John Burne for the following:
> >
> > >I found the following at
> > >"http://aix1.uottawa.ca/~gmartin/electronics_text/3_5.html"
> > >
> > >-----
> > >[dBu] used to be called dBv, but people kept mixing up dBv with dBV and
> > >that couldn't continue, so they changed the dBv to dBu instead. You'll
> > >still see dBv occasionally - it is exactly the same as dBu. Just
> > >different names for the same thing. [dBV is relative to 1V RMS; dBu (aka
> > >dBv) is relative to 1 milliwatt into a 600ohm load]
>
> Sorry, but dBm is rlative to 1 milliwatt into a 600 ohm load, dBu is
> relative 0.775 volts into a 600 ohm load. If this is wrong my lecturer LIED
> to me.
>
According to Ohms Law and the power theorem, you just said that they are
equal. The difference is that dBv(u) is .775v not relative to any load,
and dBm is 1mW. The 600 ohms is an old telecom standard and when 1mW is
dissapated into a 600 ohm load yields a .775v result.
JB
-------------------==== Posted via Deja News ====-----------------------
http://www.dejanews.com/ Search, Read, Post to Usenet
Bill
jblac...@vlint.com wrote:
>
> According to Ohms Law and the power theorem, you just said that they are
> equal. The difference is that dBv(u) is .775v not relative to any load,
> and dBm is 1mW. The 600 ohms is an old telecom standard and when 1mW is
> dissapated into a 600 ohm load yields a .775v result.
And, if I'm remembering correctly, 600 ohms can also be described as "0
dBr" (not that I've ever been called upon to express something in terms
of dBr).
Mike Rivers
In article <66nvcj$d...@dfw-ixnews10.ix.netcom.com> XSpam_J...@ix.netcom.com writes:
> On page 22 "The SR Handbook" states that "0 dBm=1
> milliwatt. dBm has no direct relationship to voltage or impedance".
That's correct. It's 0 dB referenced to 1 milliwatt. dBm is a power
measurement and we don't talk about power in audio any more except for
power amplifiers. But it would sound funny to say "We recommend a
53 dBm amplifier to power these montitors". We're more used to saying
"200 watt". dBm is still a common way of expressing RF power, but
there the impedance is generally matched source to load and is 50 or
75 ohms, not the 600 that we came to love in audio.
> On the same page it says "0 dBu is always 0.775 volts." and also has
> no reference to impedance.
That's where the part about 600 ohms comes in. It takes 1 milliwatt
of power to pump 0.775 volts into 600 ohms. dBu is that voltage, but
it's only that voltage. It doesn't have to be across 600 ohms.
We like to know what the output source impedance is (generally it's
very low in modern "voltage driven" equipment) so that we'll know that
if it puts out .775 volts into an open circuit, it will put out very
close to that voltage into whatever we connect it to.
> In any case, the dBm and dBu are only
> equivalent when they both are measured at a 600ohm load.
Slight correction - they're numerically equivalent. But one is still
a voltage measurement and the other is still a power measurement.
John McCullough
mri...@d-and-d.com (Mike Rivers) wrote:
>
>> On the same page it says "0 dBu is always 0.775 volts." and also has
>> no reference to impedance.
Table 3-3 in the SR Hanbook only converts dBV down to -20.0 dBV
unterminated and dBu down to -17.8 dBu terminated. I would be
interested in seeing a chart that converts signals down to -60.0 dBV
and the logarithm and formula necessary to calculate it.
Schuyler Hupp
.775 volts or 1 volt? Pin 2, or pin 3 'hot'? 50Hz or 60Hz? 110VAC,
117VAC, or 120VAC? Yikes!
Mike Rivers
In article <3490b35e...@news.mindspring.com> john_mc...@mindspring.com writes:
> Table 3-3 in the SR Hanbook only converts dBV down to -20.0 dBV
> unterminated and dBu down to -17.8 dBu terminated. I would be
> interested in seeing a chart that converts signals down to -60.0 dBV
> and the logarithm and formula necessary to calculate it.
When working with voltages, the formula is:
dB = 20 log (voltage ratio)
That works whether you're talking dB of gain or dBV or dBu of voltage.
When working with power (dBm), replace the 20 with 10.
Since you're looking for a voltage relative to .775 volts:
.775
dBv = 20 log ---------
voltage
The algebra is left as an exercise to the student. A calculator that
has log, antilog (10-to-the-X) and reciprocal (1/X) functions is
handy. Look at the $7 Casios at your local discount store.
John McCullough
mri...@d-and-d.com (Mike Rivers) wrote:
>
>In article <66nvcj$d...@dfw-ixnews10.ix.netcom.com> XSpam_J...@ix.netcom.com writes:
>
>> On page 22 "The SR Handbook" states that "0 dBm=1
>> milliwatt. dBm has no direct relationship to voltage or impedance".
Nigel Orr- Underwater Acoustics Group wrote:
>
> At 11:19 12/12/97 -0600, you wrote:
> >> In both cases, the dB value is found by dividing the measured value by
> >> the reference value, taking log to base 10 and multiplying the result
> >> by 20.
> >
> >Could you apply this to -50dBV = ???Volts(RMS)
>
> Yes. Taking 0dB as 1Vrms, +6 dBV (shown in your table) is
> (10^(6/20))*Vref, where Vref is 1V for dBV
> =(10^0.3)*1
> =1.995V
>
> -50dBV is (10^(-50/20))*Vref
> =0.0031, ie 3.1mV
>
> For any dBV value, divide it by 20 and raise 10 to that power
> For any dBu value, as seen in your table, add 2.2dB to the dBV!!
>
> This is because the reference for dBu is 0.775V, so working out 0dBV in
> terms of dBu, we get
>
> 1V=20log(1/0.775) (log is log to base 10)
> =20log(1.290)
> =20(0.1106)
> =2.212
>
> >I would be interested in seeing a chart that converts signals down to
> >-60.0 dBV
>
> Get a calculator with a log button and work them out!! (but I can tell you
> off the top of my head that -60dBV is 1/1000V (multiples of 20dB are easy
> as they convert to x10, so 60 is 3*20, so the drop is 10^3)
Thanks Nigel
Here is the chart thanks to your information. Unfortunately I was
unable
to figure out a way to insert that formula in Corel's Quattro Pro 8 So
that it might do the math for me, so I had to use my calculator. If
you
or someone you know is a little more familiar with that particular
program that could help me to plug that formula in I would appreciate
it.
Relating dBV, dBu and dBm Specifications
Level in dBu or dBm
Level in dBV (0 dBV=1V
(0dBu=0.775V unterminated)
Without Reference to Impedance,
Voltage 0 dBm=0.775V across
Which is Usually High)
(RMS) a 600 ohm impedance)
+6.0
1.995 +8.2
+4.0
1.585 +6.2
+1.8
1.227 +4
00.00
1.000 +2.2
-2.2
0.776 00.00
-6.0
0.501 -3.8
-8.2
0.389 -6.0
-10.0
0.316 -7.8
-12.0
0.251 -9.8
-12.2
0.245 -10.0
-20.0
0.100 -17.8
-22.2
0.078 -20.0
-24.0
0.063 -21.8
-26.2
0.049 -24.0
-30.0
0.032 -27.8
-32.2
0.025 -30.0
-36.0
0.016 -33.8
-38.2
0.012 -36.0
-42.0
0.0079 -39.8
-44.2
0.0062 -42.0
-48.0
0.0040 -45.8
-50.0
0.0032 -47.8
-50.2
0.0031 -48.0
-54.0
0.0020 -51.8
-56.2
0.0015 -54.0
-60.0
0.0010 -57.8