[TenTec] 5 mV = -67 dBm

George, W5YR w5yr at att.net
Sat Jul 26 19:36:52 EDT 2003


Ken, it sounds like you might be thinking of the dBu, which is a dB
reference widely used in the professional audio world. Its definition is a
product of the National Association of Broadcasters.

At first one would expect that the "u" represented "mu"and that the
reference was one microvolt, but such is not the case. It actually stands
for "unloaded" for reasons to become apparent.

Since the "u" stands for "unloaded," or loaded with such a high resistance
that the circuit is essentially unloaded, under such special conditions, it
makes sense to use a voltage reference independent of a specified
resistance.

The meaning of the dBu is not exactly intuitive. It is one of those
constructs developed especially for use in a field where conventional dB
references were found inadequate, confusing and, due to the nature of the
systems, unnecessary.

Formally, and bear with me as this is harder to explain that it is to just
use the darned thing, 0 dBu represents a voltage of 0.775 across a
resistance of 600 ohms. 0 dBu is equal to 0 dBm under those conditions since
both represent a power level of one milliwatt.

Now the plot thickens, as dBu usage allows describing a voltage level in
even though the resistance involved in the circuit is NOT 600 ohms! However,
it must be "high enough" as to not seriously if at all affect the voltage
level at that point in the system.

A typical use might be to describe the output of a mixing console as +20
dBu, provided that the load on the console is 10K or more ohms.

That means that as long as the load is 10K ohms or more, the mixer output
circuitry can output a maximum voltage of 10 times 0.775 volts or 7.75 volts
into that load.

The factor of 10 is derived by translating the +20 dB using the implied
factor of 20 in the actual dB equation since a voltage is being considered,
rather than a power. That is, the multiplier of 10 is numerically the log
base of 10 raised to a power consisting of the dBu value of  20 divided by
20 or 10^1 = 10. Thus, +20 dBu = 10 x 0.775 volts or 7.75 volts as measured.
across a load resistance of at least 10K ohms.

It should be noted that if the mixer is loaded with a lower resistance, such
as a 600 ohm line circuit, its output circuitry might be stressed and
damaged and certainly its maximum rms output voltage would decrease from the
specified 7.75 volts.

So, here is a case where an industry found it desirable to describe signal
voltage in decibel terms without requiring that the voltage be measured
across the same resistance as the implied reference: 600 ohms.

As long as the resistance is 600 ohms, then one dBu equals one dBm
numerically and mathematically. But regardless of the resistance, any
voltage expressed in dBu can be found in actual volts by merely applying the
basic dB equation with the "20" factor in it to the value in dBu, as in the
example above, keeping mind that the reference voltage is 0.775 volts.

Most non-audio engineers cringe at this"perversion" of the decibel, but it
can be justified on the basis that much of the time, 600 ohm circuits are
involved or loads with such high impedance that the circuitry behaves as a
constant-voltage source, producing the same voltage output as long as some
minimum load resistance is provided.

The problems arise when folks with that background or mindset attempt to do
the same thing with voltage ratios in environments where the equipment
design is heavily dependent upon load resistance as in the transmitter and
antenna areas.


Now, to confuse matters further, while the pro-audio world uses the dBu, we
find that the commercial or consumer audio world largely uses the dBV where
one volt is the voltage reference instead of 0.775 volts as with the dBu.

Conversion between the two is facilitated by noting that the difference
between the two voltage levels in a 600 -ohm circuit is 2.2 dB. Thus a value
in dBV is converted to the corresponding value in dBu by adding 2.2 dB to
the dBV value. Similarly dBu is converted to dBV by subtracting 2.2 dB.

One can go on and on with examples, Ken, but I have probably already bored
you enough with detail. The decibel as we use it in r-f work in amateur and
commercial work is usually the genuine decibel with a requirement for either
specified load resistances or for equal resistance values that cancel out in
the equation allowing voltage ratios to be used directly for convenience.

But, stray into the professional audio field, and you find the "dBu" widely
used to describe voltage levels without regard to resistance, other than the
implicit understanding that it is high enough to not enter into the issue at
hand.

I trust that this has suitably raised your level of confusion as well as
aided your memory, Ken.  Incidentally, apart from Mark's original question
that started all this, and his subsequent responses, I have seen only your
response and one other posted on this topic. While this speaks to the
general lack of interest in this entire subject, I feel that the reflector
bandwidth usage is justified on the basis of  the confusion and erroneous
concepts that exist over  decibel measure.

73/72, George
Amateur Radio W5YR -  the Yellow Rose of Texas
Fairview, TX 30 mi NE of Dallas in Collin county EM13QE
"In the 57th year and it just keeps getting better!"
<mailto:w5yr at att.net>





----- Original Message -----
From: "Ken Brown" <ken.d.brown at verizon.net>
To: <tentec at contesting.com>
Sent: Saturday, July 26, 2003 4:52 PM
Subject: Re: [TenTec] 5 mV = -67 dBm


> There is a standard used in telephony and analog multiplexing wherin the
> value of an audio level in dB(with a suffix I cannot remember) is
> expressed as it would be read on an AC RMS voltmeter with a scale
> calibrated in dBm at 600 ohms, regardless of the actual impedance. In
> other words an AC RMS voltmeter which has a scale that is calibrated in
> decibles relative to 1 milliwatt at 600 ohms is used. A reading is taken
> and the level is called dB(something). The impedance of the source or
> load may be thousands of ohms or only a few ohms. So the dB(something)
> value is really not equal to dBm, except in the special case where the
> impedance happens to be 600 ohms. Nevertheless this way of specifiing a
> level is often used. I wish I could remember what letter they follow dB
> with.
>
> Ken N6KB
>
>
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