Studio Technologies 50 2000 - Manual (Page 33)

Model 50/51 User Guide Issue 4, July 2000
Studio Technologies, Inc. Page 33
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 inter-
connecting cable. The MIDI signal is
carried on two of the five conductors.
The three remaining conductors are for
common/shield, DC power, and communi-
cations audio. The limiting factor in the
interconnecting cable’s length is the trans-
mission of the MIDI data, which has a
rate of 31,250 bits-per-second. The inter-
conductor capacitance of the cable attenu-
ates the data, and as the cable length
increases the data becomes unusable; the
cable serves as a low-pass filter. The MIDI
specification calls for a maximum cable
length of 50 feet (15.3m), 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
76.2m) are possible using the latest so-
phisticated 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!
¼-Inch Plugs versus EIA RS-453
An incompatibility problem lurks between
some ¼-inch 2-conductor and 3-conduc-
tor phone plugs and the jacks found on
professional audio equipment. While all
the plugs seem to “look” the same, some
do not comply with the industry standard,
called EIA RS-453. This standard defines
the physical dimensions, including the
shape of the plug’s tip. It seems that many
plug manufacturers don’t bother to make
the tip comply with the standard. Why is
this relevant to you? Because the phone
jacks used on the Model 50 Central Con-
troller, Model 35 Talent Amplifier, and
Model 38 Talent Amplifier do comply with
the standard. They expect to be mated
with plugs that also meet the specification.
When interfacing your line inputs, line
outputs, or headphones be careful with
the plugs you utilize. Should a connection
appear “flaky,” sound noisy, or make an
intermittent contact, the most likely prob-
lem is a nonstandard phone plug. Replace
the plug if this is the case. You should find
that all plugs from Switchcraft or Neutrik
will work correctly, specifically Switchcraft
No. 297 or Neutrik NP3C. In our experi-
ence, headphones are the most likely
place to find nonstandard plugs—we even
found them on some “world-class” head-
phones that we use in our lab!
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 0dBm is 1 milliwatt
dissipated in the known load (i.e., 0dBm
across 600 ohms will measure 0.7747V).
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 0.7747V, 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.