Low Bit-Rate Audio Coding in Multi-channel Digital Wireless Microphone Systems

White Paper








Low Bit-Rate Audio Coding in Multi-channel
Digital Wireless Microphone Systems
a technical white paper
by APTX
www.aptx.com


















1 | © APT Licensing Ltd., 2009 White Paper
ABSTRACT
Despite advances in voice and data communications in other domains, sound production for live events
(concerts, theatre, conferences, sports, worship, etc) still largely depends on spectrum-inefficient forms of
analogue wireless microphone technology. In these live scenarios, low-latency transmission of high-quality
audio is mission critical.

However, while demand increases for wireless audio channels (for microphones, in-ear monitoring and
talkback systems), some of the radio bands available for "Programme Making and Special Events" are to be
re-assigned for new wireless mobile telephony and Internet connectivity services: the FCC recently decided
to permit so-called White Space Devices to operate in sections of UHF spectrum previously reserved for
shared use by analogue TV and wireless microphones. This paper examines the key performance aspects of
low bit rate audio codecs for the next generation of bandwidth-efficient digital wireless microphone systems
that meet the future needs of live events.

DIGITAL SWITCHOVER
SPECTRUM-SHAKE UP: The contribution that wireless microphone systems make to audience enjoyment of
cultural, entertainment and sporting events has largely been taken for granted until recent and profound
changes to spectrum allocation for such radio-based systems were proposed in the USA and elsewhere
which seemed to threaten the imminent end-of-life of existing analogue equipment.

The sometimes impassioned response of vested interests to the proposals put forward by government
regulatory agencies for the expected "digital dividend" - UHF spectrum released following the migration of
NTSC/PAL/SECAM based terrestrial TV broadcasting to digital and HD standards - shows how protective
certain user groups and industries are towards the possible loss or compromised use of wireless
microphone, in-ear monitoring (IEM) and talkback (intercom) systems.
WHITE SPACE DEVICES WIN: In the USA, and after much political lobbying, the Federal Communications
Commission (FCC) [1] has now bowed to pressure from industry consortia to move forward with measures to
reallocate multiple channels in the UHF spectrum for use by new types of shortrange radio communications
equipment and license exempt cognitive radio modems called White Space Devices.

As a result, specific 8MHz wide channels in the UHF band previously exclusively available for wireless
microphones are to be lost to or shared with new, possibly interfering, digital wireless services. Meanwhile
in the UK, the Office of Communication (Ofcom) [2] is asking interested parties to comment on proposals
2 | © APT Licensing Ltd., 2009 White Paper
that could see channel 69 in the UHF band proscribed for wireless microphone use. Despite previous
assurances to the contrary, one suggestion from Ofcom is that users of channel 69, a highly popular
frequency for wireless microphone operation, should re-tune equipment to operate on channel 38 in
order to comply with tentative pan-European plans for harmonized use of channel 69 [3].

DESIGN PARADIGM SHIFT: Leaving aside the barrage of new legislation dealing with the UHF radio
spectrum, the bottom line is essentially this: existing (mainly analogue) wireless microphone systems that
make use of the scarce number of shared or exclusive channels in the UHF spectrum affected by the digital
switchover (DSO) will become redundant in just a few years for now. The designers of the next generation of
wireless microphone systems will have to work with a new set of engineering constraints in relation to
frequency allocation and spectrum efficiency. The la... [download for more]