[TenTec] 5 mV = -67 dBm

[Robert & Linda McGraw K4TAX]

Being one that lived, breathed and worked in and with professional audio and
broadcast audio for years, Paul has the best and cleanest description of
"what's what" that I've read since the writing by Ballou, Handbook for Sound
Engineers, The New Audio Cyclopedia, 1st & 2nd ed.  The book by Don Davis &
Carolyn Davis, Sound System Engineering, 2nd ed., follows very closely
along.

Yes, in pro audio we use dBu.  Instruments like the HP 400FL ACVM are
standards in the industry.  The stuff that Audio Precision produces is the
thing to have today, but don't look at the price tag.  One can buy 3 of the
ORION's for the price of a System One. And the System One requires and
external computer to run the software.

Great job Paul.

73
Bob, K4TAX


----- Original Message -----
From: "Paul Christensen Esq" <w9ac at arrl.net>
To: <tentec at contesting.com>
Sent: Saturday, July 26, 2003 8:48 PM
Subject: Re: [TenTec] 5 mV = -67 dBm


> > 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.
>
> That's right George.  With the proliferation of transformerless audio
> designs during the late '60s and '70s, it no longer became necessary to
> "power match" the source with the load.  For decades, dBm (as previously
> noted is the equivalent of 1 mW across 600 resistive ohms which nets 0.775
> volts AC RMS) was used since the majority of line-level audio circuits
> demanded a 600-ohm termination.  Due to the highly reactive nature of the
> transformer, it was (and is) necessary to properly terminate the secondary
> of the transformer for optimal transient response, but also...matching the
> designation load with the transformer's source impedance was critical in
> older power-matching circuits.
>
> When solid-state electronics began replacing transformers (e.g., discrete
> and LSI op-amps), the source impedance of audio circuits plummeted to near
> zero.  As a practical matter, even the best audio op-amps require a
nominal
> resistance at their output.
>
> Approximately twenty years ago, Richard Cabot of Audio Precision
Corporation
> drafted a white paper on the ideal output impedance of an audio circuit.
> Interestingly, it is not near zero ohms.  Rather, under long audio paths,
> the audio cabling begins to take on transmission line qualities.  And if I
> recall correctly, the optimum output impedance for most line level
> applications is...near 50-ohms.    There is a direct correspondence
between
> the optimum audio output impedance and how we derived at 50-ohm
transmission
> lines.   Another topic of discussion which is OT to this reflector is
> exactly why do we have 50, 52, 70 and 75-ohm RF transmissions lines.
AT&T
> never opted for 50-ohms on their landline audio circuits and for some
reason
> settled on 150-ohms as evidenced by their 600:150-ohm "repeat" coils.
How
> many of us remember the Western Electric 111C transformer?  I imagine
there
> is probably some history associated with the 150-ohm twisted-pair
circuit...
>
> Today, we can pretty much load the output of a solid-state device without
> much worry.  Provided that the load impedance is at least ~ 10 times
greater
> than actual source impedance, dBu is a relevant standard....we no longer
> care that it is 0.775 volts across 600 ohms...we only care that 0.775
volts
> is capable of being placed across the destination load.  Most audio studio
> equipment is NEVER terminated to the source's low output impedance.
Nearly
> everyone bridges the load with ~ 10K-ohms or greater.   In a
transfomerless
> environment, we no longer care about transient ringing and overshoot when
a
> complex wave is impressed upon a reactance as is the case with
transformers.
> Even the best Jensen transformers must be terminated for optimum transient
> response.
>
> Circling back....dBm had its roots with 600-ohm circuits.  Today dBm is
used
> universally to represent 1mW across any load.  As an example, commercial
> lasers are almost always specified in dBm, irrespective of the actual
> terminating impedance.
>
> -Paul, W9AC
>
>
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