FOH has written a lot of information over the past year regarding FCC reallocation of the UHF band and how it will affect pro audio wireless. We’re not going to rehash the problems facing the pro audio industry in our efforts to continue using the UHF spectrum for wireless operation. If you missed it, revisit “Bleeding Edge” in the February, June, July, November, December 2007 and October 2008 issues.
The RF problems remain for pro audio, but at least one audio manufacturer, Music Sciences (www.musicsciences.com), has taken a decidedly different approach by avoiding the UHF band entirely and moving wireless audio into the microwave band. That’s not the only story here. Music Sciences has also introduced a product the likes of which we have never seen: The Mongoose Microwave Snake System (we’ll get to that in a minute).
Pre-Heat to 10 GHz
As the name implies, microwaves are very high frequencies possessing very short wavelengths. The microwave band occupies space in the 10-GHz range, which (at the moment) is relatively uninhabited when compared to the crowded UHF band and is capable of carrying both audio and video data. Initially, microwaves were used to broadcast network television across the country or for links between studios and their transmitters, but small portable transmitters and receivers can be used in the field to relay signals.
You can think of a microwave transmission akin to a flashlight shining a beam of light from one point to another: Microwaves are essentially transmitted in a straight line from point A to point B. Unlike UHF, microwave transmission requires relatively unobstructed line of sight because solid obstructions (including human beings) can block the signal.
According to the manufacturer, the band and power/spread in the Mongoose system is well above 20 GHz, and is relatively unaffected by anything other than solid metal and concrete objects — i.e., simply blocking the path with people or drapes or even a torrential downpour at 1,000 or so feet does not reduce the signal enough to impede the connection and signal flow. The system as specified has 24 dB of RF headroom so these obstructions (anything weather-related in the specified user distance) would not interrupt transmission.
Why Transmit Audio Via Microwave?
As mentioned, the microwave band at present is relatively empty compared to UHF, and therefore, less prone to interference from local television, cell phone, 2.4 GHz WiFi and other consumer activity. Microwave channels provide a wide bandwidth. When used for audio, microwave transmission has the ability to maintain a wide dynamic range without FM modulation, compression or companding (the bandwidth of the Mongoose system is such that the signal is a pure, uncompressed digital stream at 48 kHz/24-bit).
While the real estate available for UHF transmission of pro audio is shrinking at an alarming rate, the microwave band currently may be used without licensing under FCC regulations. Beamwidth of a microwave transmission is narrow enough to avoid interference from other RF technologies yet wide enough to allow easy alignment of antenna using an RF signal strength meter, or even by sight.
Let Loose the Mongoose
The Mongoose is a wireless, digital multichannel snake operating in the microwave range. It is a modular system with 16 channels per single-space rack unit and is expandable up to 96 channels of audio (or data) between stage and front of house in a 64-x-32 configuration. The basic concept behind the Mongoose is that audio signals are patched into a stage box providing A/D converters as well as a CAT5 link to a microwave transmitter situated near the stage. The CAT5 link also carries power for the transmitter and can be up to 50 feet in length. Analog audio signals are converted to digital data with 24-bit/48-kHz resolution, yielding a frequency response of 10 Hz to 20 kHz (+/- 0.5 dB) and dynamic range upward of 110 dB (resolution up to 96 kHz/24-bit is supported). Latency is spec’d at 0.160 milliseconds, any analog in to analog out.
The transmitter beams multichannel audio to a microwave receiver (typically located at front of house) where the signal is downlinked to a front of house rack with D/A conversion — thus the Mongoose can be used with existing pro audio consoles. The system also has the ability to transmit audio back to the stage box where it can be connected to a drive rack for distribution throughout the venue.
Mongoose can accommodate a variety of connections including analog line-level audio I/O and microphone level inputs. AES/EBU and MADI interfaces are currently under development. Systems incorporating microphone level inputs employ a hardware control panel for remote control and monitoring of preamp parameters including gain adjust, mic/line selection, phantom power on/off, channel level and channel grouping for up to 64 microphone inputs. The system also allows an engineer to perform a line check without a console, monitor audio of any stage or return channel, and eyeball levels for all channels. Setups may be stored and recalled for later use.
The Mongoose transmission system can be configured in a variety of manners including one-way transmission without diversity, one-way transmission with diversity, or two-way “X-shaped” transmission with diversity. Several antenna options are available for varying distance requirements, and all antennas are equipped with standard hardware for tripod or mic stand mounting. As far as the accuracy required in aiming one antenna to another, the size of the “target” area is one-tenth of the distance between the antenna divided by two. If, for example, the antennas are 500 feet apart, then the target from one antenna to the other is 25 feet in diameter — so it should not be too difficult to “hit” for accurate transmission.
The fact that the microwave beamwidth is relatively narrow also means that it is less subject to unwanted interference. Transmission distance varies depending upon the type of antenna used with the system. A 15-inch parabolic antenna is easily capable of covering distances over 1,000 feet — plenty for just about any pro audio application. Smaller antenna can be used for shorter transmissions.
What does this mean to audio engineers? Well, for one thing, we won’t need to rent a backhoe for outdoor live events where the copper multipair has to be buried below ground under the audience. It also means that we won’t have to worry about multipair cable being permanently damaged when it is run over by a truck. If we could have a direct downlink to a digital console then there’d be no need for any copper on the receiving end. Mongoose price and availability TBA.
Steve “Woody” La Cerra is still out on tour mixing front-of-house for Blue Öyster Cult. He can be reached via email at Woody@fohonline.com.
