Mixed Signals - AvNetwork.com

Mixed Signals

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Although wireless microphone systems have been in use since the 1950s, it wasn't until the early 1970s that the first companded systems were employed. Greg Nady determined that the way to improve signal-to-noise with FM wireless systems was to compress the audio before transmission and expand it after reception, in a similar manner as to what was done by Ray Dolby and his patented method of noise reduction on analog tape (we all remember that stuff, right?).

The improvement in noise performance by using a compandor system was dramatic. But even more remarkable was the fact that other manufacturers did not adopt this system until nearly a decade later. It wasn't until the 1980s that wireless systems came to be more of a "standard" in touring and theater productions, and improved noise performance was one of the reasons why.

Companded wireless systems have a big drawback, though. Being a compression/expansion scheme, it is impossible for the designer to select the perfect attack/release and compression ratios for all types of audio content. As it turns out, if you choose too fast an attack, distortion occurs. If the attack is too long, transients cause clipping. If the decay is too short, there is distortion. And if the decay is too long, you get the infamous "pumping and breathing" effect only appreciated in the adult film industry.

One solution employed by several manufacturers is to carefully tune the compandor to the type of source expected by the system. Most often, the tuning scheme chosen will work well with voice, but the drawbacks become apparent when the vocalist picks up a tambourine and bad audio is the result (not to mention potentially bad music). Similarly, stage leakage from cymbals can cause the same effect. In other words, the compandor can't cope with the abundant high-frequency transient information.

With these problems in mind, Lectrosonics introduced the CR195/UM195 UHF wireless system in the early 1990s, employing a dual-band compandor scheme. The advantage of the dual-band scheme is that for the lower frequencies, a slower attack and decay time can be used; while for higher frequencies, faster attack and release times are specified. The result is a more natural-sounding audio for a wider spectrum of audio sources.

Advances in digital audio systems in the late 1990s led to efforts in developing digital wireless systems to avoid the compandor altogether. Some of the first systems commercially available came from Xwire (later purchased by Sennheiser). Musicians, especially guitar and bass players, praised their system for the natural sound when compared with typical analog companded systems. However, this version only offered four frequencies, had reliability problems, and is now off the market.

In 2002, Lectrosonics filed for a patent on a new technology employing analog FM UHF transmission but without the compandor used in standard analog systems. In the Lectrosonics 400 Series equipment employing Digital Hybrid Wireless technology, the audio is converted to digital and encoded via DSP into a proprietary analog signal format, which is then transmitted over an analog FM carrier. At the receiver, a DSP decodes this signal and outputs the original audio, retaining a very wide dynamic range and a quality faithful to the source.

This system has the advantages of both analog FM UHF (well-understood designs, long-range, graceful signal decay and good battery life) with digital signal handling (freedom from noise, no compandor artifacts and the ability to process the signal via DSP).

This last feature (i.e, DSP capability native to the 400 Series wireless products) allows for some very interesting possibilities. Indeed, Lectrosonics has chosen to use this platform to enable the 400 Series products to be backward-compatible with older, analog equipment such as the Lectrosonics 100 and 200 Series transmitters and receivers. In addition, the 400 Series can be programmed to emulate systems from other top wireless manufacturers, allowing transmitters from Lectrosonics (such as the SM and UM400 body pack units) to be used with receivers already installed at theaters or in touring systems.

If artists need to use specific transmitter/capsule combinations, the Lectrosonics 400 Series receivers (such as R400 or the Venue modular system) can be programmed to accept the signal from these transmitters. Overall, this platform gives the user a great deal of flexibility in choosing and configuring their wireless systems without investing in completely new equipment all at once.

With new products on the market such as the SM "super-mini" transmitter and the Venue modular receiver system, Lectrosonics will be approaching markets traditionally held by other manufacturers. At the same time, intensive R&D is being conducted on the next generation of innovative wireless platforms in order to "stay ahead of the curve."

One of the great concerns for all wireless manufacturers and users is the erosion of RF spectrum due to DTV and potentially from wireless networked computers. Since the UCR200D in 1997, Lectrosonics has incorporated the use of "tracking" front-end filters in order to use frequency-agile receivers while still rejecting out-of-band RF energy. The employment of these filters in the Venue system will certainly aid users of this receiver to locate clean frequencies despite ever-increasing broadband RF noise.

And looking further ahead, clearly there will be a need for sophisticated "smart" digital wireless systems in order to maintain a high level of audio quality while retaining the mobility to which we have all grown accustomed in this golden age of wireless microphone systems.

Lectrosonics...
www.lectrosonics.com

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