Simple
Sometimes, the most revolutionary ideas are those that in hindsight
appear quite obvious and simple.The technology behind the SpectraPulse®
ultra wideband wireless microphone system is just that – revolutionary
and simple in concept.
Consider that something as simple as a miniature switch is part of what
was behind the technology revolution of integrated circuits.The engineering
skill and effort it took to implement this simple idea was extremely
difficult, but this simple idea changed the world. Once it worked, it
became ubiquitous.
The Audio-Technica U.S., Inc. SpectraPulse® development has been such
a project. Many companies have tried to implement ultra wideband (UWB)
wireless into products, but we have been the first and only company to
succeed in incorporating UWB into a commercially viable audio product.
Ultra wideband wireless is a naturally simple idea. It consists of
very rapidly pulsed signals, each one a short burst over a very wide
instantaneous bandwidth of frequencies. In early conception dating
back to Marconi in the late 1800s, a UWB signal was a spark-gap
transmission that was used for long range communications. These early
communications systems were replaced by more efficient, longer pulse
duration, narrowband techniques.
Short pulse technology was only revisited nearly 80 years later in the
mid 1960s. At this time, as technologies, methods and materials allowed
engineers to work in much higher frequency ranges, UWB began to
emerge as a novel form of communications due to its low probability
of detection characteristics. The successful use of UWB by the U.S.
government and military led to the FCC's groundbreaking rule-making
that permitted unlicensed commercial use of the technology for the very
first time. They set the definition of an ultra wideband signal as having
an instantaneous bandwidth of at least 500 MHz (where the spectrum is
defined as the 10 dB down points from the center of the transmission) –
or – a signal with a >20% fractional instantaneous bandwidth. (500 MHz
is approximately 20% of a 2.4 GHz center frequency signal. 2.4 GHz
is a popular wireless frequency with many available parts, and many
engineers have experience working at this frequency.)
Secure
The FCC has mandated both peak and average output power limits for
unlicensed UWB products in order to ensure that they will not interfere
with other wireless and security devices. The UWB signal must be lower
than the spectral power density emission limit of - 41.3 dBm/MHz, which
is also the limit applied to unintentional radiators in Part 15 of the FCC
codes. We, however, are intentionally radiating at these very low levels
and must still maintain reliable operation and range. Although higher
power has advantages, the method in which the SpectraPulse® system
works allows us to accomplish extremely reliable and safe communication
that is also inherently secure.
The nanosecond duration burst signal is one of the keys to UWB
operation. The pulse for the SpectraPulse® system is approximately
two nanoseconds in duration and occupies a bandwidth of about 500
MHz during the limited transmit duration. The detection of this limited-
duration signal is extremely difficult even with expensive test equipment,
since these very short pulses are so close to the ambient noise floor. In
comparison, traditional FM signals are always on, meaning that the carrier
is always present and signals are easily detected and decoded. Traditional
FM microphone systems transmit about 50 milliwatts of average power.
The SpectraPulse® system transmits 40 nanowatts average power. This is
approximately one million times lower than an FM system.
Although SpectraPulse® is inherently secure and basically “invisible” to
anyone scanning the environment looking for communications, some
highly sensitive applications may require added assurance to prevent
detection and decoding of the signal. For applications such as this, A-T
has developed an optional 128-bit encryption feature. This feature permits
the microphone and audio control interface to be encrypted and keyed
to each other so that even someone with another SpectraPulse®
system
would not be able to decode the signals. The computer-controlled
encryption software allows the user to check the status of encryption
and to change the encryption code as often as desired.
Revolutionary
Sometimes it is easier to describe why something is revolutionary by
comparing it to more traditionally used technology.
Those familiar with traditional FM wireless use are also familiar with
a host of problems that accompany this method. What makes
SpectraPulse®
revolutionary is also part of what makes it simple. For
example, there are no traditional wireless set-up issues to worry about.
SpectraPulse®
does not require frequency selection or coordination,
there is no intermodulation distortion, no multi-path, no varying signal
strength resulting in pops, clicks, fades or drop-outs. The system is
fully digital, has under 2ms latency, and does not use a compander, so
the sound quality of the microphones with a SpectraPulse®
wireless is
comparable to a wired condenser microphone. The system does not have
to use squelch or pilot tones to keep out interference and signals cannot
break in to an “open” receiver. You can use two transmitters adjacent
to each other without mutual interference. The diversity used in the
SpectraPulse®
system is digital and it is optimized for maintaining a
selected bit error rate. This novel diversity feature has nothing to do with
the audio signal, and as such we have eliminated diversity switching
noise, pops, fades and level differences.
SpectraPulse®
is designed with high speed digital components and solid-
state RF devices. Therefore, SpectraPulse®
does not need “alignment.”
It works reliably because of how it is designed and built. It cannot go out
of alignment, and it does not need adjustment over time or use by either
the end-user or the factory.
There are a few more issues that are common concerns for most
audio product users: battery life and “Blackberry” noise (noise due to
picking up RF interference from Blackberries or other forms of wireless
communication tools).
Despite being packed with advanced-technology devices, the SpectraPulse®
battery is only used during the nanosecond data bursts. The control
systems of the SpectraPulse®
are designed to use battery power only
when needed and then are placed in a “sleep” mode. Battery life with
rechargeable batteries is a solid 9 hours.This is very difficult to accomplish
in current FM wireless products.
Those familiar with recent “Blackberry” noise issues may know that
Audio-Technica leads the field in developing RF immunity for condenser
microphones. Naturally, that immunity applies to this product as well with
careful filtering, grounding and shielding playing very important roles in
protecting the performance of our high-quality microphone.
The Details
The concept of UWB is simple – send a known fast pulse sequence,
identify the sequence by looking for the sequence and known pulse
characteristics, synchronize to the pulse train and decode the audio
information for a complete communication transaction. In practice,
however, it is difficult to implement the jobs of detecting and coordinating
the UWB pulses for use in a high-quality audio product.The SpectraPulse®
system required careful design and coordination. Working in the 6 GHz
range meant learning new methods and materials.This included everything
from the physical microphone material dimensions, circuit board
materials and grounding techniques needed to be re-defined by audio
standards.
At the heart (or brains) of the system is the drm141. This unit contains 14
complete channels of digital receivers (one for each possible microphone
transmitter address), a transmitter to communicate with each available
microphone transmitter, (the communication of the SpectraPulse®
system
is bi-directional) and a full digital circuit for the detection, analysis,
conditioning, and coordination of all the information that is coming and
going through the system. The drm also transmits logic states that are
set on the back of the audio control interface (aci707) via dip switches
that define the function of the push-button on the top of the microphone
transmitters.
Ultra Wideband (UWB) Wireless Microphone System
SpectraPulse®
White Paper
SpectraPulse®
White Paper
To accomplish the silent and seamless coordination of 14 channels of
digital wireless without collisions, the SpectraPulse®
employs a TDMA
method, and advanced forward error correction with proprietary bit
error rate and audio muting logic. The TDMA along with the bi-directional
communication sync method allow us to operate with negligible latency
and near instantaneous re-acquisition time in the event of a lost signal.
The SpectraPulse®
operates with an audio bandwidth of 100–12,000 Hz.
The system uses 16 bit digital audio with a 24 kHz sampling rate and a
UWB data rate of 8 megabits per second. ATDMA architecture containing
15 total time slots per frame is employed with a frame duration of 1ms.
The drm transmits during the first time slot of the frame to synchronize
the microphones and to send status and control. Each of the 14 mics
transmit sequentially on their respective time slot to send their audio
stream to the drm. Audio data is queued up by each microphone over the
1ms duration immediately prior to transmitting on its respective time slot,
thus minimizing audio latency to under 1.5ms.
Another advantage of the SpectraPulse®
system over current wireless
methods is a very sophisticated, fast and robust bi-directional linking
system. The SpectraPulse®
will silently mute if system sync is lost, but
a very robust communication link process is designed to avoid sync loss
and to quickly re-acquire. This process is typically transparent to the user
as reacquisition occurs in only 3ms after sync loss. A spread spectrum
system that is working well may take 1-2 seconds. Many wireless
microphone systems take much longer than this, and some exhibit a
very noisy artifact during sync loss or re-sync. A traditional FM wireless
system may make noise when it loses communication connection, and
typical re-acquisition of audio time is around 4ms (some are longer with
slower audio ramp-up times to prevent audible noises).
A versatile control unit is linked via a single shielded CAT5 cable to the
drm141. The aci707 audio control interface sends signals to, and receives
signals from, the drm141. It also supplies power (24VDC) to the drm141
via the cable. The aci707 takes the multiplexed 14 channels of digital
audio data from the 14 receivers in the drm141 and demultiplexes them
in 7 channels of analog audio. Switches on the back of the aci707 allow
the user to set the audio output of the channels to 1-7 or 8-14. For 14
full analog audio outputs, you simply link two aci707’s via a supplied
cable. The aci707 acts as an interface into the site audio system (mixer,
speakers, conferencing equipment, etc,) and also as a gateway for
control of the microphones from the rest of the audio equipment. System
settings (set via DIP switches on the rear of the aci707) and control
information are converted to digital data and passed back to the drm141
via the shielded CAT5 cable. The drm141 includes this information when
transmitting instructions to each microphone transmitter for seamless
control.
Despite complex audio circuitry and control in the aci707, and the
many advanced digital audio and control features of the drm141,
the microphone transmitters are the most impressive technical
accomplishment of the SpectraPulse®
system. These units are designed
for performance using a technology previously only associated with
RFID or lower-audio-quality military applications. One of the challenges of
the SpectraPulse®
project was making a 6 GHz digital wireless system
work in such a small enclosure; an enclosure that was also shared with
sophisticated digital and analog audio circuits. The impressive solid zinc
die-cast microphone body and hardened steel grille house a power supply
(with DC-DC converter), RF transmit and receive circuitry, transmit and
receive antennas, digital control, digital audio, ground plane, analog
audio, circuitry for encryption and charging contacts and circuits for use
with 2 x AA high-performance NiMH rechargeable batteries.
The Spectrum
Those working with wireless today cannot avoid the “White Spaces”
issue. All of the world's regulatory bodies have a challenge ahead of
them. People like the freedom of “wireless.” However, the available
wireless spectrum is becoming overcrowded with increasing interference,
and future allocations are becoming hotly contested. This is happening
around the world, not just in the United States. Audio-Technica is
dedicated to providing high-quality wireless audio, and it became obvious
in the late 1990s that spectrum would become an issue if we continued
to rely on traditional methods of wireless communication. Our desire was
to employ digital wireless, but applying digital conversion to existing
methods did not give us the improvements we needed or a solution
to impending frequency allocation problems. We investigated spread
spectrum extensively and ruled it out due to some performance
issues. We did not want to invest in a new technology unless it truly
accomplished our goals and also met our high standards without
compromise. UWB technology clearly met our needs and we elected to
invest in development even prior to the FCC rule-making on UWB.
UWB is a simple concept that holds great promise for a wide variety of
applications. The SpectraPulse®
method is a unique method that allows
us to gain the range and audio performance necessary for a multi-channel
wireless system. It is important to point out where UWB fits in the
frequency spectrum relative to other technologies and their specific
allocations. In the U.S., the FCC has authorized a very large band for
unlicensed UWB use (3.1 GHz –10.6 GHz). It appears that UWB will be
permitted in Europe in a similar frequency band. There is room to expand
into additional frequency channels and since UWB is immune to most
forms of interference and inherently not likely to interfere with other
devices, it is a very secure and reliable choice.
Specifications Overall system
6.100 GHz–6.600 GHz
6.350 GHz
16 bits
24.576 MHz
24 kHz
2 nanoseconds
1 ms
15
8 mbps
None
None
1.1 ms
40 nanowatts
<3 ms
23 m (75')
14
100-240 V AC, 50/60 Hz
100 – 12,000 Hz
100–240V, 50/60 Hz
RoHs-compliant power supply
Transmitter units
Fixed-charge back plate, permanently
polarized condenser (mtu101)
Unidirectional (mtu101)
Approximately 9 hours (mtu101, mtu201),
depending on battery type and use pattern
135 mA
100 – 12,000 Hz (mtu101)
AES level 3, 128 bit
Frequency range
Center frequency
AD/DA
Clock
Sampling rate
Pulse duration
Frame length
Time slots per frame
UWB rate
Compression
Companding
Latency
Average RF power
Sync/Re-acquisition time
Range
Simultaneous channels
Power input
System frequency response
Mains (aci707)
Microphone element
Polar pattern
Battery life
Current consumption
Audio response
Optional encryption
Audio-Technica U.S., Inc., 1221 Commerce Drive, Stow, Ohio 44224
Audio-Technica Limited, Old Lane, Leeds LS11 8AG England
©2010 Audio-Technica U.S., Inc. audio-technica.com 0001-0008-00
Specifications are subject to change without notice.
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