Studio Technologies 50 2005 - Manual (Page 31)

Model 50/51 User Guide Issue 7, August 2005
Studio Technologies, Inc. Page 31
using two conductors to carry the signal
(pins 1 and 2) you’d get 21.8 ohms per
1000 feet of microphone cable. With our
15 ohm maximum resistance you’d be
able to use 688 feet (210 m) of this cable.
By using the numbers provided you can
select a cable, and its maximum length,
for your application.
Model 50 to Model 51 Cable Length
The Model 51 Control Console generates
system-exclusive MIDI messages which
are sent to the Model 50 Central Controller
using a 5-conductor MIDI-style intercon-
necting cable. The MIDI signal is carried
on two of the five conductors. The three
remaining conductors are for common/
shield, DC power, and communications
audio. The limiting factor in the intercon-
necting cable’s length is the transmis-
sion of the MIDI data, which has a rate of
31,250 bits-per-second. The interconduc-
tor capacitance of the cable attenuates
the data, and as the cable length increas-
es the data becomes unusable; the cable
serves as a low-pass filter. The MIDI speci-
fication calls for a maximum cable length
of 50 feet (15.3 m), which will work fine
connecting the Model 51 to the Model 50.
There is really no reason why this length
can’t be increased, as long as good low-
capacitance, shielded cable is utilized.
Runs of 100 to 250 feet (30.5 to 76.2 m)
are possible using the latest sophisticated
cable. Test to ensure that the StudioComm
system works correctly with the desired
cable length. There are too many variables
to give a simple formula—good luck!
Definition of Level—dBu and dBV
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 0 dBm is 1 milliwatt
dissipated in the known load (i.e., 0 dBm
across 600 ohms will measure 0.775 V).
In contemporary situations an output is
rarely terminated with 600 ohms; gener-
ally 10 k ohms or higher. The dBu des-
ignation is better because it refers to dB
referenced to 0.775 V, with no reference to
load impedance. 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.
StudioComm equipment is designed to
interface with audio signals that have nom-
inal signal levels of –10 dBV and +4 dBu.
You might wonder why dBV came into
the picture. Most people don’t realize that
equipment that utilizes “–10” levels usu-
ally mean –10 dBV—substantially different
from –10 dBu (–10 dBV = –7.78 dBu). The
dBV designation is simply a different way
of measuring signal level and is often used
when dealing with portable or consumer
audio equipment. The dBV designation
refers to dB referenced to 1.0 V, rather
than dBu which refers to 0.775 V.
“Hot” Disconnection of the Model 51
Control Console
Should you need to relocate the Model 51
while your StudioComm system is oper-
ating, there is no reason why you can’t
disconnect the 5-conductor cable, move
the unit, and then connect it again. If the
Model 51 is disconnected while it is op-
erating, the current operating parameters
are saved in nonvolatile memory and the
Model 50 Central Controller will continue
to operate as it did when the link was