Studio Technologies 81 - Manual (Page 13)

Model 81 User Guide Issue 1, January 2000
Studio Technologies, Inc. Page 13
Technical Notes
Definition of Level—dBu
Whenever possible, Studio Technologies
has opted to use the dBu designation as
it seems to be quite rational. Using dBm
was fine when all audio line outputs were
terminated with 600 ohm loads. In this way
it was easy to say that 0dBm is 1 milliwatt
dissipated in the known load (i.e., 0dBm
across 600 ohms will measure 0.775V). In
contemporary situations an output is rarely
terminated with 600 ohms; generally 10k
ohms or higher. The dBu designation is
better because it refers to dB referenced
to 0.775V, with no reference to load im-
pedance. This takes into account today’s
audio scene where signals have a low
source impedance, and a high input
impedance. The dBu designation is be-
coming the standard for the professional
audio industry.
–10dBV Outputs
Note that a user monitoring, by means of
a level meter (e.g., a VU meter), one of the
Model 81’s outputs might expect to see a
14dB drop when changing from the +4 to
the –10 DIP switch position. In fact, the
output level will drop by only 11.78dB.
This is because the output level is not only
switching between "+4" and "–10," but it's
changing from a dBu to a dBV reference
as well. As –10dBV is the same as
–7.78dBu, the expected 14dB drop never
occurs!