Studio Technologies 55 2005 - Manual (Page 24)

Issue 5, August 2005 Model 55/56 User Guide
Page 24 Studio Technologies, Inc.
Model 55’s MIDI system-exclusive format
will the LED light. Refer to Appendix A,
located at the end of this guide, for details
on how data must be sent to the Model 55.
Clicks in the Audio
As covered in the Confi guration section
of the manual, the four stereo line inputs
can be confi gured for –10 dBV or +4 dBu
operation. Setting an input for –10 dBV,
while connecting an audio source with a
+4 dBu nominal level will lead to distortion
(“clipping”) of the signal. In this fault condi-
tion the user would hear a harsh “clicking”
sounds in the audio, especially when peak
levels occur in the program material. To
remedy this problem simply use the Model
56 Control Console to confi gure the input for
+4 dBu operation. The distortion will go away
and the gain structure of the StudioComm
system will be correctly established.
Technical Notes
Talent Amplifi er Cable Length
There are no hard and fast rules defi ning
the maximum cable length when connect-
ing Model 35 Talent Amplifi ers to the Model
55 Central Controller. The maximum cable
length is directly related to the amount of
resistance in the connecting cable; the lower
the resistance per foot (or meter), the longer
the cable can be. (Although cable capaci-
tance affects high frequency performance,
resistance is the limiting factor is this case.)
To lay out the facts in grammar-school
story-problem format: for correct operation,
a Model 35 needs to see at least +20 Vdc
between pins 1 and 2 of their input connec-
tor. The Model 55’s talent amplifi er output
voltage across pins 1 and 2 is +23 Vdc,
with a maximum current draw of 0.2 A
(200 mA). This difference between the
voltage supplied and the voltage required
results in a maximum voltage drop of 3 V
over the interconnecting cables. Since cable
is rated in ohms per 1000 feet (or ohms per
1000 meters), you need to know what the
maximum cable resistance is. This can be
easily calculated by dividing the maximum
voltage drop by the maximum current fl ow:
3 V divided by 0.2 A = 15 ohms. For exam-
ple, a standard 20 AWG microphone cable
is Belden 8412, which has 10.9 ohms resis-
tance per conductor per 1000 feet. Since
we’re 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 55 to Model 56 Cable Length
The Model 56 Control Console generates
system-exclusive MIDI messages which
are sent to the Model 55 Central Controller
using a 5-conductor MIDI-style intercon-
necting cable. The MIDI signal is carried
on two of the fi ve 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 transmission
of the MIDI data, which has a rate of 31,250
bits-per-second. The inter-conductor
capacitance of the cable attenuates the
data, and as the cable length increases the
data becomes unusable; the cable serves
as a low-pass fi lter. The MIDI specifi cation
calls for a maximum cable length of 50 feet
(15.3 m), which will work fi ne connecting the
Model 56 to the Model 55. There is