Redundancy in Sennheiser’s Adaption of WMAS Technology
By Dr Sebastian Georgi

Dr Sebastian Georgi is one of the system engineers for theWMAS system that is currently being developed by Sennheiser.
For more than 10 years, he has been conducting research into
wireless broadband techniques and how to specifically tailor
them to professional audio applications. Georgi did his PhD
thesis at Hamburg University of Technology in the field of OFDM.
Georgi has a strong affiliation to music and plays the bassoon ina semi-professional orchestra in Hannover.

Introduction
Whenever game-changing technology is introduced, wherever unchartered technical territory isexplored, “what if” questions and comparisons with conventional technology are the rule and not
an exception. In our information sessions and talks with industry colleagues about WirelessMultichannel Audio Systems, or WMAS for short, we found “redundancy” to be a topic ofparticular interest.
Sennheiser’s WMAS technology implementation operates with one central rack-mount unit that
handles many wireless mics and IEMs at the same time – instead of many individual transmitterreceiver and receiver-transmitter links with conventional narrowband technology. What if thatcentral unit fails? Which redundancy options does Sennheiser’s WMAS implementation offer?
This paper aims to answer these questions by taking a look at various operational scenarios andhow Sennheiser’s implementation of WMAS will handle them.

The Sennheiser implementation ofWMAS: one central rack-mount unit
handles many wireless mics and IEMS
simultaneously and in the same RFchannel1

Redundancy
In our technology context, redundancy means the doubling of various subsystems to ideallyensure operation without interruption, even when one subsystem fails. Therefore, whenconsidering redundancy, all subsystems are evaluated for their probability of failure, and adecision is made to duplicate them or not.
An obvious solution to avoid a single point of failure in a technical system is to duplicate theentire system. Examples in pro audio would be duplicate mixing consoles, or a console with dual
redundant engines, for major televised live audio events, or duplicate equipment racks at a
broadcasting facility.

Redundancy in live audio: Two DiGiCo Quantum 5 consoles mirrored at the FOH position of the opera 'The Canal Ballad’ atthe opening performance of the Beijing Performing Arts Centre in December 2023.
(Image courtesy of Racpro)

In the case of a wireless audio system this would mean building the entire wireless audio system
twice, using different frequencies, separate power supply circuits, ideally generators, too, setting
up the duplicate systems at some distance and so forth. However, from an economical point ofview, this does not make sense for most practical applications.

2Encryption
The WMAS technology developed by Sennheiser is connection-oriented. This means that a mobiledevice, such as a microphone transmitter or in-ear receiver, establishes a connection to thecentral rack-mount unit. This initial connection will be called pairing from here on. When a mobile
device is paired to the central unit, an encryption key is exchanged. Then it can be coordinatedusing a permanent remote control channel within the same bidirectional RF (radio frequency)
channel that is used for audio transmission.
This implies that using a second receiver for listening in onto the same RF signal – which is often
done today as a sort of safety net – will not be possible anymore. However, it is worth noting that
this option will also disappear for legacy narrowband wireless audio systems, as a regulatory
requirement to encrypt personal data such as audio will be mandatory in the EU starting 2025.

Redundancy scenarios with Sennheiser’s WMAS technology
Let us now take a look at various fault scenarios and how they can be handled with Sennheiser’s
implementation of WMAS.

RF interference
Scenario:Interference appears on an RF frequency, for example caused by uncoordinated wireless audio
systems being switched on without checking frequency availability. Another potential cause canbe non-audio devices emitting unwanted RF signals in the TV UHF frequency range.
Countermeasure:
Sennheiser’s WMAS technology implementation will offer permanent scan functionality evenduring operation. All WMAS devices, be it mobile devices or stationary antennas, continuously
monitor and measure the used RF spectrum (so-called distributed spectrum sensing). This data isused to report detected interference to the operator and help him or her to react dynamically tothe situation. Field tests have shown that WMAS is able to work around a few interferers – thiswill give the operator crew at a live event enough time to track the source of interference andhave it switched off.
If interference in the allocated TV channel is massive, i.e. several interferers are active in thisparticular RF channel and it may not be possible to maintain operation, the RF channel can bechanged within seconds. Mobile devices do not need to be paired again. They will initiate a brief
search, and then detect the relocated carrier. This will cause a short audio interruption of a few
seconds – which is nothing if you consider how much time it would take to reprogram several
narrowband receivers and associated transmitters. Alternatively, when communication with themobile devices is still possible to some extent, a frequency change command can be issued overthe permanent remote-control channel. This procedure will reduce the time of audio interruption.

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Left: A single 200-kHz interferer can be harmful to a dense multichannel narrowband deployment. At least one audio
channel is lost; a device operating on a backup frequency is required. If more interferers occur, more channels are lost.
Right: The same interferer hits a WMAs deployment. Thanks to the system’s frequency diversity and advanced signal
processing, the WMAS is robust against this co-channel interferer. If more interferers occur, distributed spectrum sensing
and remote control of all devices support the operator in taking informed action.

Broken cable
Scenario:
In general, wireless audio requires many cables on the rack-mount side. Besides the obvious
power cords and cables for remote antennas, there are cables for audio networks such as Dante,
or for connections over MADI (Multichannel Audio Digital Interfaces). Each of these cables could
be damaged or get accidentally unplugged.
Countermeasure:
All audio connections can be made redundant in Sennheiser’s implementation of WMAS
technology.
Multiple antennas operating on the same RF channel would not only serve to extend the coveragearea, but could also compensate for a “lost” antenna: Should an antenna cable get disconnected,
another antenna could take over without any audible audio artifacts, provided that the mobile
devices are within its coverage area. Therefore, overlapping antenna zones will offer redundancy,and although WMAS only needs a single antenna for the central unit in many scenarios, RFmanagers might consider adding an extra one anyway.
A temporary loss of control network connection to the central unit would not cause any audio
interruption. Regarding the power cable, separate power supply units and separate power cables
could be an option.

Control PC failure
Scenario:4

A laptop or desktop computer will be necessary for configuring and monitoring the system before
and during a production. This PC could fail.
Countermeasure:
Like with the cable break scenario, the technology would allow for a production to continue when
the control PC fails. The operator could connect a replacement computer to the WMAS central
rack-mount unit and upload the production data onto a new control PC. This would take over all
functions of the failed device.

Highly energetic stage shows can take their toll on a microphone. Spare devices remain
a must.

Faulty Mic
Scenario:
Mobile devices can get damaged during rough stage use. Lavalier mic capsules can get clogged
up by sweat or makeup.
Countermeasure:
Just as today with conventional narrowband systems, spare devices would be paired to thecentral unit. These can replace any faulty mobile devices.

Failure of the WMAS central rack-mount unit
Scenario:
What to do in the unlikely case that a central unit fails? Although a fault is highly improbable,
there may be critical applications, where system redundancy is key.

5Countermeasure:
The technology would allow for the configuration of a second central unit, which could take overall connections from its sister central unit. When implemented, the operator would copy the fullconfiguration of the first central unit to a second one before a high-profile event. This wouldinclude the encryption key and all pairing information. If a need arises to switch to the backup
central unit, this will be able to take over operation from the first unit. As described under “RFinterference”, the mobile devices need to re-establish a connection to the new central unit, which
would cause an interruption of several seconds. However, no pairing or reconfiguration will benecessary.

Conclusion
We will see products based on Sennheiser’s WMAS technology implementing redundancy
options step by step. For many typical scenarios that require redundancy, the technology wouldbe able to offer a solution that ensures operation without audible interruptions. In the unlikely
event that an entire RF channel (which is 6 or 8 MHz wide) becomes inoperable due tointerference, or a central rack-mount unit is failing, production could continue after a shortinterruption. If even short interruption is unacceptable, mirroring of the wireless audio system
would always be an option, and with WMAS just using a compact central unit rather than a rack
full of microphone receivers and in-ear monitoring transmitters, would be much easier toaccomplish than with today’s narrowband systems.

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