The SpectraPulse wireless microphone system from Audio-Technica is one of the more interesting products to enter the market.
It was announced about two years ago and appears aimed mostly at installed sound applications, specifically boardrooms and courtrooms where operating range is not so much the issue, but data security and ease of use are important.
The system represents a fresh approach in its form factor, technology and performance, when compared to “traditional” wireless microphone systems.
It is my understanding that when the DTV transition became imminent, Audio-Technica decided to investigate alternatives to standard systems based on UHF frequencies.
Only Sabine, X2, RevoLabs, and some of Lectrosonics’ more specialized products have gone out of the standard TV band in recent years, with products in the ISM (902-928MHz), 1.9 GHz and 2.4 GHz ranges.
System Basics
The SpectraPulse system is made up of the following main components: drm141 antenna/digital receiver module, aci707 audio control interface, mtu101 boundary microphone/transmitter, and the mtu201 desk stand/transmitter that can be used with gooseneck microphones.
There is also a charging station for up to seven transmitters and an optional software package to add data encryption to the system.
The drm141 antenna/receiver unit is usually mounted on the wall in the conference room, and then connected to the aci707 control interface via a standard shielded CAT5 cable.
Technology
In contrast to all these other offerings, SpectraPulse operates with a center frequency of 6.35 GHz and a bandwidth of 500 MHz, and thus has an UWB (Ultra Wide Band) designation. UWB is a “carrier free” wireless method, while even spread spectrum still relies upon a carrier, which, if disrupted, means that signal will be lost.
There are some advantages to operating at such a high frequency – most importantly; there are no TV or other wireless microphone transmissions in the range.
In addition, the SpectraPulse design involves the use of very rapidly pulsed signals, each one a short burst over a very wide instantaneous bandwidth of frequencies. While the technology concept is by no means new (it dates back to Marconi in the late 1800s), A-T is the first to implement it with a commercially viable product.
In the course of writing this article, I also received some additional input from Jackie Green, Vice President of R&D/Engineering at Audio-Technica, who notes that the 6 GHz band was targeted because it’s one of the most “open” ranges of spectrum.
A look at the FCC web site shows only fixed communications, fixed satellite communications and mobile communications operating in the area of 5.925 - 7.075 GHz.
Jackie also notes, “We have eliminated most possible conflicting frequency applications by being centered at 6.350 GHz, and in addition, even if we operate a SpectraPulse system physically next to the broadcast satellite up and down links (not random, but specific fixed frequencies between 4 GHz and 6.425 GHz), we do not experience interference or degradation of our signals.
“This is because SpectraPulse is not a carrier-based technology. There is no signal to ‘mix’ or ‘fade’ or ‘drop.’ If a competing wireless system comes in on exactly our frequency of operation at exactly our point in time (very unlikely) it’s not that our system won’t work - the problem is that the DRM (Digital Receiver Module) simply cannot decide which signal is the one it wants in the TDMA (Time Division Multiple Access) time slot ,and starts bouncing back and forth between signals in time or experiencing collisions. This sounds like talking through a ‘fan,’ but it doesn’t sound like a hit, drop, pop, fade, or disappear entirely, as users experience with traditional FM signals.”
Specifications
The SpectraPulse system supports the use of up to 14 channels in one location, which is more than enough for most conferences. It appears to be designed around two seven-channel blocks, in that the aci707 supports seven channels.
Audio frequency response is 100 Hz – 12 kHz, plenty for speech applications but not optimum for full-range program material.
But again, the system is designed for conference applications rather than for theater or live music, and by eliminating the need for a compander, audio quality is comparable to traditional FM systems.
Range is quoted as up to 75 feet, which is more than enough in this application.
Here’s an interesting specification: the average RF output from the transmitters is 40 nanowatts. This is several orders of magnitude less than the typical analog FM or even digital modulation systems that have outputs in the range of 10 to 100 milliwatts.
Part of the key to understanding the A-T spec is that this is the average of a pulsed system. The peak output of any pulse is undoubtedly much higher than this but only for a very short duration (2 ns).
A/D and D/A conversion are 16 bit, and overall latency is an impressively low 1.1 ms. Normally, this would indicate a high sample rate, but in this case, the sample rate is 24 kHz, thus the audio frequency response extending “only” to 12 kHz.
The link between transmitters and receiver is bi-directional, allowing for muting if synch is lost, and reacquisition occurs in only 3 ms after sync loss.
Data encryption is offered as an optional feature by adding the sep128 software package. AES – Level 3 standard 128-bit encryption gives this system the edge over some other digital wireless products. If you need data security, this is a very robust way to go.
Cost
I found that the mtu101 transmitter has a street price of about $890, which is right in there with what AKG and Lectrosonics offer for digital transmitters. The drm141 antenna/receiver seems to have a street price of about $8,600, which seems expensive until you consider that it can work with up to 14 channels – about $615 per channel.
The aci707 carries a price of about $4,600 and handles up to seven channels, or about $660 per channel. All in all, if you were to purchase a complete 14-channel system, this works out to about $2,100 per channel. This is not inexpensive, but there are several systems that cost significantly more.
Conclusions
All in all, this is a very impressive piece of technology and shows that Audio-Technica has vision and was willing to take a substantial risk with large R&D project outside the “normal” realm of wireless microphones.
The result really shows promise, particularly for installed applications where very long range is not needed, but data security, ease of setup and basically invisible operation are required.
It will be interesting to see if A-T is able to refine this technology even further to provide products applicable for use in touring, theater, and other types of productions.
Signing off for now…
Mike Wireless
Mike Wireless is the nom de plume of a long-time RF geek devoted to better entertainment wireless system practices the world over.
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