What Spectrum is Right for You?
Shure UK Pro Audio Group Manager, Tuomo Tolonen walks us through the most common bands of spectrum used for wireless microphones to help you determine what best suits your application.
For radio microphones to work reliably, we need access to clean spectrum - this much is well known and documented. What's less well known, is which bands of spectrum are best suited to each given application. Unfortunately, we can't just use any portion of spectrum; and while in theory there's a lot of spectrum available, the parts useful for the reliable operation of wireless microphone or in-ear monitor systems are limited.
Why?
Different parts of spectrum have different performance characteristics, of which, propagation is arguably the most important to understand.
What do we mean by propagation?
In very simple terms, RF propagation refers to how well a given frequency can travel over distance and penetrate through surfaces such as walls or other obstacles. Strong propagation characteristics are critical to obtaining a solid, stable signal across a wide operating area in applications such as live performance.
Here's how it works:
One of the shared characteristics between radio frequencies and sound is wavelength. Just like sound waves, lower frequency radio waves have larger wavelengths while higher frequencies are smaller in wavelength. To fully understand this, we must also understand the wave formula (C = L x F), which we can use to calculate the wavelength of any given frequency. Yes, we know, it's maths, but bear with us....
In the case of radio waves, we are, of course, dealing with the speed of light, which in this case forms our constant, or the letter C in our equation. (The speed of light is 300, 000, 000 metres per second, for your reference). To calculate the wavelength (L), we would take our frequency and divide it by the speed of light. See the example using 600MHz below:
300, 000, 000 ÷ 600, 000, 000 = .5 metres
In other words, one cycle of 600MHz is half a metre long, which makes for a sizeable enough wavelength to transmit over a healthy operating range while easily penetrating many surfaces and walls. In contrast, if we take a higher frequency, such as 10GHz, then the wavelength would be so short it wouldn't even be able to travel through lighter surfaces such as curtains or thin walls. In professional applications, this level of performance is simply too prohibitive.
Understanding UHF & VHF
Over the last 50 years, the vast majority of wireless microphones and in-ear monitor systems have operated in the VHF and UHF bands of spectrum. VHF stands for Very High Frequency while UHF stands for Ultra High Frequency.
Looking at the chart below we can see how each frequency varies in wavelength based on the wave equation we just covered. Some of the previous systems operating in the UK occupied the VHF space (around 300MHz and below). As you can see, these systems had a nice, long wavelength allowing for exceptional operating range and propagation; what was missing, however, was enough quantity of spectrum to cater for larger events. As a result, the bulk of our industry today make wireless systems that operate in the UHF bands (from 470MHz and up).
In the UHF bands, which start at 470MHz and go up to the mid 800MHz, we still retain excellent wavelength – only this time, there's a much greater quantity of space from which to operate. Historically, though, users of wireless microphones have always shared this space with terrestrial television, which requires some coordination to avoid interference. The operation of TV channels will vary from place-to-place, with occasional gaps known as 'white spaces'. It's important to remember that we operate as a secondary user – next to TV – meaning you must programme your wireless systems into these clear white space gaps. This arrangement makes for an operating environment where interference is predictable; we understand where the other users are, and we can safely coordinate a large number of channels for large professional events.
1.8 GHz, 2.4 GHz, and DECT
While the UHF bands are the preferred operating space for professional live events, not all applications require such a high level of performance for vast channels of wireless systems. If your channel counts are smaller, there are other bands available. Here's a brief explanation of each option:
The 1.8 band (1786-1800 MHz) was recently announced in the UK as available for wireless microphone operation. The wavelengths at these frequencies are smaller than our ideal UHF bands, however, for many smaller applications they remain more than adequate.
2.4 GHz is commonly used for WI-FI, making for a challenging operating environment shared with lots of unlicensed devices. Smart 2.4GHz systems combat the challenge of WI-FI interference with intelligent frequency scanning and interference avoidance; Shure's GLX-D system is a perfect example. Perhaps the greatest advantage of 2.4GHz is that it's globally licence free, and you don't need to worry about whether or not your system will work when touring abroad.
Finally, we have the DECT (Digital Enhanced Cordless Telecommunications) bands. Most systems occupying this space are designed for installed applications, again where intelligent frequency scanning and allocation is required. The Shure Microflex Wireless system is a great example of DECT applied to corporate conference room applications.
[video=HCDGnkQUGqA]
