Technical Guide |
Digital & Analog Partyline Intercom Cabling Comparison
Nov2018
Page 2
Overview
Digital partyline intercom products offer some significant
feature and performance advantages over analog
partyline intercom products and in most cases using the
same cable types.
This document provides a comparison of the demands
placed on a cable by both analog and digital partyline
intercom products.
The goal of this technical guide is to provide readers with
a clear understanding of the cable characteristics
important to the performance of partyline intercom
systems. It also explains the operation of the online
Clear-Com digital partyline cable calculator:
Digital Partyline Calculator
Background
Each cable type has a number of physical and electrical
characteristics that affect its suitability for an application.
Among these are:
•
Number of cores
•
Core twist properties
•
Shielding
•
Overall diameter
•
Flexibility
•
Voltage rating
•
Flammability rating
The electrical characteristics that are of particular interest
for those transitioning from analog to digital partyline
systems are:
•
DC resistance
•
Signal attenuation
Introduction
In both analog partyline systems, which have been in use
since the 1960s, and more recently introduced digital
partyline systems, standard 3-pin mic cable is used
between a main station and beltpack to carry two things:
•
Power
•
Data (Audio and Control)
Power Limits
The one factor that affects a cable’s ability to deliver DC
power is its DC resistance which is related to its gauge.
Figure 1 shows the relationship between cable gauge and
DC resistance.
Figure 1 Cable DC Resistance vs Wire Gauge
The larger the cable (lower gauge number), the lower the
DC resistance and the better its ability to deliver power
over a longer distance. Here are some examples for both
analog and digital partyline.
Analog Partyline
In an analog partyline system, the master station delivers
around 30V DC to the line. The beltpack requires a
minimum voltage of around 12 to 20V DC (varies
dependent on type) at its input in order to operate. Most
analog beltpacks draw the same current (around 25mA)
from the line no matter what voltage they receive at their
input.
Figure 2 shows how a cable can be considered as a simple
resistance for calculation of power delivery length limits.
The resistance of both cores used in the power delivery
loop, the 30V core (R1) and the ground reference core
(R2) must be considered in the calculation.
MASTER
STATION
BELTPACK
R1
R2
25mA
30V DC
OUT
(Vpsu)
20V DC
MIN
(VLmin)
D
Figure 2 Analog Partyline Voltage Drops
In the example given in Figure 2, the maximum allowable
value for the cable DC resistance can be calculated as
follows:
(
𝑅𝑅
1 +
𝑅𝑅
2)max =
𝑉𝑉
𝑝𝑝𝑠𝑠𝑠𝑠
−𝑉𝑉
𝐿𝐿
𝑚𝑚𝑚𝑚𝑚𝑚
𝐼𝐼
𝐿𝐿
=
30−20
.
025
=
400Ω
Based on this calculation, assuming R1 and R2 are equal,
a cable with a resistance of 33 ohms/km (such as 20awg
Belden 9463F) could achieve a distance of:
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷
=
R
MAX
R
/
Km
=
400
2∗33
=
6
𝐾𝐾𝐷𝐷
Note that this is with a single beltpack. Adding nine more
to the end of the same cable, making a total of ten
beltpacks gives a maximum power distance limit of:
𝐷𝐷𝐷𝐷𝐷𝐷𝐷𝐷
=
𝑉𝑉
𝑝𝑝𝑠𝑠𝑠𝑠
−𝑉𝑉
𝐿𝐿
𝑚𝑚𝑚𝑚𝑚𝑚
𝐼𝐼
𝐿𝐿
x
R
/
Km
=
30−20
0
.
25
𝑥𝑥
66
= 0.6Km
The nature of an analog partyline system means that
when it reaches its maximum cable length for power
distribution, the user will experience a lowering of
maximum audio level available at headsets before
distortion levels increase.
0
50
100
18
19
20
21
22
23
24
DC
R
esi
sta
nc
e
(Ω
/K
m
)
Cable Gauge (AWG)
