In Broadcast - CABSAT 2018 - 16

www.inbroadcast.com | CABSAT InShow Guide | 14 - 16 January 2018

IP Convergence Brings Solidarity
to Co-Channel Audiences
Two Los Angeles area FM stations long hindered by mutual interference join forces through
a signal-frequency network, with GatesAir Intraplex IP networking technology bridging three
transmitters, writes Michael Worrall, Technical Director and Chief Engineer, KCSN(FM)...

he challenges of broadcasting
at low power run deep,
from managing tight budgets
to balancing light engineering
resources. However, there is
little more frustrating than when
the cards are stacked against
maximising audience penetration.
The latter has been a challenge
for KCSN(FM) for nearly 40 years.
Broadcasting on 88.5 MHz from
the campus of California State
University in Northridge, KCSN is
perhaps the lowest powered station
serving the Los Angeles market. The
station's grand total of 370 watts
offers fairly limited coverage, even
from a high transmitter location.
The greatest challenge to signal
coverage comes from co-channel
interference. Located on the
northern fringes of the market, KCSN's
signal has historically been clear and
robust. Things went south quickly
as listeners travelled south. This is
because FCC rules for low-power
broadcasting in the U.S. below 92MHz
have long permitted co-channel
broadcasting in the same market, so
long as those stations are separated by
predicted interfering contours.
This has been the case with 88.5 in
Los Angeles since KSBR(FM) signed
on from Saddleback College in Orange
County in the 1970s. As KCSN listeners
drove south, the broadcast would
turn into a horrible mishmash of two
signals. KSBR listeners were met with
the same fate as they drove north.
Consumer radios simply could not
differentiate between the two signals.
The only solution was to join forces
with a common format, and have the
same programme broadcasting from all
transmitters at the exact same time.
Come Together
Traditionally, bringing two signals
together has been managed with a
main/booster scenario, where the
latter broadcasts at a far lower power
than the main transmitter. This was
a more unusual situation where two
main transmitters would join forces,
and a third booster in Beverly Hills that
had long been on the air to fill in local
coverage gaps for KCSN.

Efficiency: GatesAir Intraplex

Once the deal was struck with KSBR,
the focus quickly turned to overcoming
the obstacles that have historically
hindered single-frequency networks
(SFNs). We consulted with some of the
best minds in the business - including
John Kean, formerly of NPR Labs; and
consulting engineer/SFN specialist Bert
Goldman - to find the right technical
solution. The results of these studies
led us to GatesAir, whom we also
consulted with to effectively transition
us away from a T1 architecture.
For many years, KCSN had operated
an Intraplex T1 STL system from
GatesAir, which incorporated the
company's SynchroCast software to
time-lock signals between the KCSN
main and booster transmitters. We first
considered moving programme audio
from studios to all three transmitters
in the same way. To make the SFN work
properly, the modulation between all
three transmitters had to be identical
- preferably less than a one-quarter
difference in dB. Various studies have
shown that if the modulating signals
offset above that range, the noise
floor dramatically rises. In addition,
we needed to control the timing offsets
to the microsecond level so that it
was possible to manipulate timings in
an organised manner, as opposed to
having the timing wander around on its
own.

This would have been a danger
over any transport architecture
considering the network delay
possibilities as audio left our
Northridge studios to their
transmitter
destinations
in
Northridge, Mission Viejo and
Beverly Hills. It was certainly
possible to achieve with T1, but the
upgrade work would have been
significant. First was the expense
of adding another T1 frame; second,
we would have to break out the
mini-screwdrivers to adjust audio
levels in the existing frames. All
of this would have been quite a
gamble to maintaining identical
modulating signals.
It quickly became clear that
a transition from a legacy TDM
architecture to an IP networking
solution was in order - and GatesAir
had exactly the kind of system we
sought.
All About Technique
The complete system brings three
transmitters - KSBR, KCSN main and a
KCSN booster in Beverly Hills - together
using an Intraplex IP Link MPXp codec
with integrated Intraplex SynchroCast®
technology. The latter simulcasts and
time-locks the signals via an integrated
GPS receiver that is key to its timing
functions. This technique ensures that
the three overlapping transmitters
operated on the same frequency,
and avoided interference with each
other while broadening overall signal
coverage.
The IP Link MPXp brought several
new efficiencies to the surface,
including lower monthly costs,
reduced capital investment, and
support for a transparent AES192
digital multiplex composite signal. A
competitive analysis of MPX codecs on
the market proved that the GatesAir
solution excelled in dual-domain
design, network reliability and built-in
dynamic SFN capability. The migration
from T1 to IP has also saved the
operation about US$1700 each month,
which uses a very wide-bandwidth,
private Ethernet service from AT&T to
accommodate all signal networking.
That network also provides plenty of

bandwidth for monitoring, which we
achieve by remotely logging into the
MPXp via its internal web interface.
Using this simple approach, we can
access the GUI and attain the readings
and measurements needed.
The MPXp allows us to originate the
baseband signal at our Northridge
studios, and duplicate that signal
within the codec. This ensures that
strict control of signal levels and
phase are maintained across the
network. While the goal of identical
AES192 audio delivery to all three
transmitters was achieved, the MPXp
offers an additional transport benefit:
simultaneous analogue MPX composite
output to route programme audio
to our backup transmitters. This
was important because our backup
transmitters are not yet capable of
accepting an AES192 digital stream,
and we can instead duplicate the same
signal using analogue BNC jacks.
From an operational efficiency
standpoint, the MPXp allows us to
achieve all of this from a single codec,
and without the burden of adjusting
audio levels in T1 frames to match
modulation levels. The MPXp keeps
those modulation levels identical,
which eliminates the noise floor
concerns we originally had. The signal
is clean from north to south, and we're
doing this all for about US$330 per
month. That is a significant savings
compared to the US$2,000 a month we
anticipated had we remained on T1.
It's impossible to predict how a
new system or service will operate
long-term once brought to life. The
reliability of this system has so far
been on par with our T1 service, with
no effect on signal quality - all while
providing a greater coverage, and
a more enjoyable service, to 88.5
listeners throughout the Los Angeles
area.
www.gatesair.com

Single codec: GatesAir Intraplex MPXp

http://www.inbroadcast.com http://www.gatesair.com/

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