Studio Technologies 55 2002 - Manual (Page 28)

Issue 4, June 2002 Model 55/56 User Guide
Page 28 Studio Technologies, Inc.
nominal level will lead to distortion (“clip-
ping”) of the signal. In this fault condition
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 configure the input
for +4dBu operation. The distortion will
go away and the gain structure of the
StudioComm system will be correctly
established.
Technical Notes
Talent Amplifier Cable Length
There are no hard and fast rules defining
the maximum cable length when connect-
ing Model 35 or Model 38 Talent Amplifi-
ers to the Model 55 Central Controller. The
maximum cable length is directly related
to the amount of resistance in the connect-
ing cable; the lower the resistance per foot
(or meter), the longer the cable can be.
(Although cable capacitance 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 opera-
tion, a Model 35 or Model 38 needs to see
at least +20Vdc between pins 1 and 2 of
their input connector. The Model 55’s
talent amplifier output voltage across pins
1 and 2 is +23Vdc, with a maximum cur-
rent draw of 0.2A (200mA). This difference
between the voltage supplied and the
voltage required results in a maximum
voltage drop of 3V over the interconnect-
ing 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 volt-
age drop by the maximum current flow: 3V
divided by 0.2A = 15 ohms. For example,
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 (210m) 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 Control-
ler 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 com-
mon/shield, DC power, and communica-
tions 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 56 to the Model
55. 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 sophisticated
cable. Test to ensure that the Studio-
Comm system works correctly with the