[TenTec] dbm and VU units of audio measurement

[Paul Christensen, Esq.]

> Per ITU and ISO standards, which have legal and regulatory binding force
in
> all member countries, "dBm" is more than a mere label - it is the legal
> definition of dB with reference to an absolute power level of one
milliwatt.

Today, dBm represents one milliwatt of power irrespective of individual
voltage, current or impedance parameters.  But I believe that today's
international definition of dBm is different than it was when it was used in
the early Bell Labs development era during the 1930s.  I suspect that dBm
evolved from a standard consisting of 1mW across 600-ohms (and hence the
corresponding 0.775 Vrms), to a uniform measurement of power regardless of
the voltage, current and impedance involved.  But...it's just a hunch on my
part based on my readings through the years.

-Paul, W9AC