Technical Guide |
Digital & Analog Partyline Intercom Cabling Comparison
Nov2018
Page 3
Digital Partyline
In a Clear-Com digital partyline system, the master
station delivers around 59V DC to the line. The beltpack
requires a minimum of around 24V DC in order to
operate. Clear-Com digital partyline beltpacks take the
same amount of power (around 3.5W) from the line no
matter what voltage (within a specified range) they
receive at their input.
MASTER
STATION
BELTPACK
R1
R2
I
L
60V
DC
OUT
24V
DC
MIN
3.5W
LOAD
Figure 3 Digital Partyline Voltage Drop
This means that the calculation of maximum cable length
is different to that for an analog beltpack. First it is
necessary to calculate the current (I
L
) that would flow in
the circuit with the lowest acceptable voltage (
𝑉𝑉
𝐿𝐿
)
available at the load:
𝐼𝐼
𝐿𝐿
𝐷𝐷𝐷𝐷𝐷𝐷
=
𝑃𝑃
𝐿𝐿
𝑉𝑉
𝐿𝐿
𝐼𝐼
𝐿𝐿
𝐷𝐷𝐷𝐷𝐷𝐷
=
3
.
5
24
=
146
𝐷𝐷𝑚𝑚
then calculate the maximum resistance:
𝑅𝑅
1 +
𝑅𝑅
2 =
𝑉𝑉
𝑝𝑝𝑠𝑠𝑠𝑠
−𝑉𝑉
𝐿𝐿
(
𝑚𝑚𝑚𝑚𝑚𝑚
)
𝐼𝐼
𝐿𝐿
𝑅𝑅
1 +
𝑅𝑅
2
=
59−24
.
146
=
240Ω
and maximum distance allowed as before:
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷
=
R
MAX
R
/
Km
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷
=
240
2
x
33
=
3.6
𝐾𝐾𝐷𝐷
Again, if the number of units at the end of the line
increases from one to ten beltpacks, the maximum
cable length allowable (with Belden 9463F) drops to:
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷
=
𝑉𝑉
𝑝𝑝𝑠𝑠𝑠𝑠
−𝑉𝑉
𝐿𝐿
𝑚𝑚𝑚𝑚𝑚𝑚
(
𝑃𝑃𝐿𝐿
𝑉𝑉𝐿𝐿
)
x
R
/
Km
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷
=
59−24
(
35
24
)
x
66
= 0.36Km
The nature of Clear-Com’s digital partyline system means
that when it reaches its maximum cable length for power
distribution, units towards the end of the line will cease
to operate until the cable connecting them to their main
station is brought back within the required specification.
Data Limits
In addition to delivering power, in both analog and digital
party line systems it is also necessary to ensure that the
required data signal can be passed from source to
destination.
Figure 4 Comparison of Cable Attenuation
As shown in Figure 4, the attenuation of a cable over a
given distance increases as transmission frequency
increases. Figure 4 also shows us that some cable types
are designed to be more suitable for transmitting higher
frequency data whilst other cable types are designed to
be suitable for lower frequency transmission.
Analog Partyline
In analog partyline, the maximum data frequency of
interest is the analog audio signal, a frequency of less
than 20 KHz. Figure 4 shows us that at frequencies below
1 MHz, all of the cable choices shown have a very low
attenuation (<30dB/Km).
The nature of the analog partyline system is also such
that as cable attenuation between nodes increases, the
effect observed by the user is a decrease in audio level
heard at the headset with a low distortion audio signal
still being received.
Take our previous analog partyline system as an
example:
BELTPACK 10
BELTPACK 2
MASTER
STATION
BELTPACK 1
250mA
30V
DC
OUT
20V
DC
MIN
0.6Km
Belden 9463F
20
Ω
20
Ω
Figure 5 Ten Beltpack Analog Partyline Example
0
50
100
150
200
250
300
350
1
10
100
A
tt
e
n
u
a
ti
o
n
(
d
B
/K
m
)
Frequency (MHz)
1813A
1800F
8412
9207
9463F
Belden Cable Part
Numbers
