How Long Has This Been Going On?
As early as 1917, there was already a serious effort underway towards giving stage performers and actors freedom by introducing a radio microphone system into the picture. Then, as now, many of the same issues applied. First of all, it has always been important for the audience, whether in a theater, watching a film, or at a religious service to hear the speech of the person or persons they are watching. Secondly, freedom of movement for the performers, actors, pastors, etc. allows for a greater range of expression. Thus, considerable efforts have been undertaken during the last half century towards meeting these two needs with reliable, good-sounding, and most recently, affordable solutions in the form of wireless microphone systems.
The Radio Side
Ever since Marconi invented practical radio transmission methods in the early part of the last century, wireless communication has been a reality. But the possibility of wireless microphone systems only came about when the technology had moved beyond AM (amplitude modulation) and into FM (Frequency Modulation) systems. By the 1950s, the production demands of Television brought forth some of the first practical wireless mics, reducing the need for actors to shout their lines. This technology, along with the development of the shotgun microphone, revolutionized TV production.
FM has two advantages over AM for audio signals: first, it allows for wider audio bandwidth (greater frequency response) this allowing for a more natural sound. Secondly, FM has what is called “capture effect” meaning that with only 2-3dB of good signal over the background noise, an FM system can produce a useable output.
By the late 1960s, wireless microphones had made their way onto live concert productions, giving wealthy performers the chance to move about the stage without the tether of a cable. At that time, the systems were using low-band VHF FM platforms in the 27-75MHz range. The radio spectrum was relatively un-crowded and very few wireless microphones were in use. Systems at that time were also fixed-frequency, using crystal oscillators. This means that if by chance your system was on the same frequency as a local TV channel, you would not be able to have interference-free operation. Thus, traveling shows relied on backup systems for such eventualities.
Wireless systems advanced quite a bit during the 1970s, ending up with high-band VHF (174-216MHz) and giving fairly reliable performance and very good audio quality. However, these systems were still single-frequency and thus were susceptible to being “stepped on” if a local TV station or other powerful transmitter were present. It was not until the mid-1980s that manufacturers began developing frequency-agile systems using Phase Locked Loop (PLL) circuits, allowing the user to tune the operating frequency across the given band of that particular unit. This development coincided with the move into the UHF band (470-806MHz) although the two things are separate.
Although frequency agile systems give the huge advantage of flexibility and avoidance of interference, the trade off involves the fact that the receiver is no much more “open” to signals across the band, thus generating noise in the audio. The older, single-frequency system often used highly-selective crystal filters thus blocking out all signals except that from the intended transmitter. Audio quality and reliability were very good with the late-model VHF, single-frequency systems from Vega, Lectrosonics, Sennheiser and others. The new, frequency-agile systems were often considered somewhat noisy, but were still used because of the incredible flexibility.
Starting in the mid-1990s, new developments were incorporated into these systems to further reduce noise, resulting in improved performance. “Tracking Filters” are used in some systems that now rival the performance of the older, fixed frequency types. Today, Lectrosonics and Shure incorporate such filters in their top-end wireless systems. These systems work by tuning a highly-selective filter at the front end of the radio along with the selected operating frequency (see figure 1).
Since the late 1990s, several manufacturers have developed methods for transmitting digitally encoded signals in a purely digital fashion. This is similar to DTV (Digital Television) transmission and thus looks very different if viewed on a scope (See figure 2). The advantage of digital transmission is that typical channel noise is not picked up at all, and, if the signal is recovered well enough, the output is an exact copy of the input. The disadvantage is that digital transmission requires 10-12dB of good signal above the background noise to produce a useable signal.
This is in contrast to analog FM systems needing only 2-3dB! In addition, the energy of a digital transmission is spread across a wider spectral area rather than the single, focused carrier of the FM systems. One last side effect is that digital systems tend to produce much more out-of-band noise than their analog counterparts. These things combined make pure digital systems much more difficult to implement than good FM systems, and capable of far fewer simultaneous channels in operation. Good FM systems have been used for over 100 simultaneous channels, while pure digital systems generally have not been used to exceed 20 channels at once.
An issue that comes up when using multiple wireless systems is interference due to Intermodulation (IM) products. This occurs when two or more radio signals (from wireless microphones, TV stations, etc.) are combined in a non-linear device such as an RF amplifier, transmitter or receiver, producing new signals as sums, differences, and multiples of the original carriers (see fig. 3) These new signals act as harmonics (just like on musical instruments) at various frequencies mathematically related to the originals. With just a few channels of wireless microphones in operation, IM interference is relatively easy to avoid. However, the more channels that are added, the number of potential IM products increases exponentially.
Thus, coordination of large systems involved quite a bit of experience from the operator, along with the use of specialized computer programs allowing these IM products to be calculated. Some recent wireless systems, including top-end units from Shure and Audio Technica, offer “auto-coordination”, thus saving users time and effort in setup. However, there is no substitute for experience when coordinating large systems in high-profile installations. At huge events like the Grammys, national tours and the Super Bowl, among others, dedicated staff is on hand just for this purpose. Usually, some kind of RF spectrum scanner is employed to see what kinds of RF signals are already present on the site. Then, as more and more channels are activated and the spectrum gets more and more crowded, the whole picture can be viewed in real time.
One of the terms most often heard in discussions of wireless systems is “diversity”. This term refers to the fact that some receivers have two antennas, each receiving a slightly different mixture of direct and reflected signals from the transmitter. Just like with acoustic signals, radio signals bounce off of nearby surfaces, only to re-combine at the antenna. If the two (or more) versions of the signal are out of phase at the antenna, there can be what is known as a “multi-path null” thus cancelling the signal at that moment (see fig. 4). These relationships change based on movement of the transmitter, such as when a performer moves around a stage. So to remove or reduce the possibility of multi-path nulls from occurring, a receiver can be designed with two antennas, each in a slightly different place. If one antenna receives a null, the other antenna can be used to feed the signal to the receiver.
There are many ways to implement a diversity design into a receiver. The most basic type is to have two separate receivers, each with their own antenna. Then an audio switching device, triggered from noise or low RF levels in the receiver, decides which receiver feeds the audio output. This approach is known as “dual diversity” (see fig. 5). An enhanced way to implement this design is to use an audio panning circuit instead of a switch to choose the ratio of receiver A to receiver B feeding the output. This type is known as “Ratio Diversity” and provides 3dB better signal to noise than the standard “dual diversity” type.
Another common method is “antenna phase” diversity. Here, there are two antennas, with one of them connected to the receiver through a phase switch. Normally, the RF signal from both antennas is combined in phase. However, if the RF signal drops due to a multi-path null, the second antenna’s phase is reversed to strengthen the signal into the receiver. This method is very effective and also efficient in terms of space and power consumption, and thus is used quite often with small, portable receivers such as for camera-mount applications (see fig. 6).
One last bit of information about the radio side. FCC (Federal Communications Commission) specifies the amount of RF power allowed for wireless microphone transmitters. In the VHF band, the transmitting power is limited to 50mW (50 milli-Watts). And for UHF transmitters, it is limited to 250mW.
In general, wireless system performance is greatly enhanced when the system has a well-designed receiver front-end, including good RF filters, and a well-designed transmitter with filtering on the transmission side. These are things that are more difficult to describe on a product spec sheet, because they are very technical. The rule of thumb, however, is that you get what you pay for. Systems designed for entry-level musicians will not perform to your expectations in a more demanding environment.
The Audio Side
Due to FCC Restrictions on RF output power and bandwidth occupied by wireless microphone systems, manufacturers must ensure that their products comply with such rules. And once a product is designed, it must be submitted to the FCC for testing. If the product passes the various tests (power, output “shape”, spurious emissions, etc.) then an FCC designator is assigned to that specific product. To be within the law, each and every product producing RF emissions must be tested by the FCC for these specific areas, depending on the type of product and its intended use. Wireless manufacturers design their transmitters to output a signal that fits into a specific “spectral mask” as specified by the FCC (see fig. 7). This means that the signal must not produce anything outside this mask.
Due to these restrictions, it is very difficult to get a wide bandwidth audio signal with wide dynamic range through a radio link. One problem to overcome is “channel noise” or basically the background RF noise getting into the receiver and causing noise at the audio output. Because of this, an untreated audio signal can only have a dynamic range of about 50dB – clearly not enough for good quality music transmission. Thus, designers have employed a variety of means to overcome this problem.
Compandors: a Brief History
Although wireless microphone systems have been in use for commercial purposes since the 1950s, it wasn’t until the early 1970s that the first companded systems were employed. John 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. In fact, the reasons for companding along with the audio compromises when comparing analog tape, particularly cassette, and wireless microphones are remarkably similar.
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 finally 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 can causes 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 noticeable if trying to play old cassette tapes.
One solution employed by several manufacturers is to carefully tune the compandor to the type of source signal expected to be passed by the system. Most often, the tuning scheme chosen will work well with voice but the drawback become apparent when the vocalist picks up a tambourine and bad audio is the result. Similarly, stage leakage from cymbals can cause the same effect. In other words, the typical compandor can’t cope with the abundant high-frequency transient information.
More recently, some manufacturers have gone as far as 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 source material.
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 five operating frequencies (four in the Sennheiser version), had reliability problems, and did not stay on the market very long. Nevertheless, development continues in the area of digital wireless, including systems that offer data encryption, stereo operation and other handy features. However, thus far, no digital system has been offered that competes with analog wireless for the sheer number of channels that can be used at once, nor the long range available with good FM systems.
One new system employing analog FM UHF transmission but with a digital audio path is the Digital Hybrid Wireless. In this type of system, 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).
One additional challenge is that each manufacturer uses their own proprietary method for treating the signal. Thus, no two manufacturers’ systems are compatible with each other or even in some cases with their own, older equipment. So far, only the Digital Hybrid system allows for such cross-compatibility by emulating the various companding schemes in the DSP realm. 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.
One of the main advantages of a digital platform is the ability to transmit encrypted data. This is especially important for government agencies, but may also be used for private meetings or to avoid eavesdropping of any kind. Encrypted wireless systems first digitize the audio signal then apply some kind of encryption algorithm to the data stream before transmitting. Then at the receiving end, a pre-arranged key is used to decode the signal. Modern encrypted systems are very robust and provide a high degree of security. For example, one system uses a 128-bit encryption scheme, resulting in several Trillion Trillion possible combinations. To break such an encoded data stream using brute-force computing techniques would take millions of years using today’s high-speed supercomputers!
An issue affecting wireless microphone use in our digital age is the shrinking of available RF spectrum. DTV, wireless computer networking, cellular phone services and upcoming use of unlicensed wireless computing devices (like Blackberry, iPhone, etc.) has brought about legislation aimed at auctioning existing spectrum to the highest bidders. Indeed, some of this has already happened. DTV signals already occupy a much larger chunk of the RF spectrum than did their analog predecessors. Also, DTV signals are allowed to be adjacent to each other (every 6MHz) whereas analog TV signals must have at least 6MHz of clearance between them (see fig. 8). The current plan is to shut off all analog TV broadcasts by February 19, 2009.
While the technology of wireless microphones has improved dramatically in the past two decades, it has actually become more difficult to coordinate and use larger, multi-channel wireless systems. And it will get even more difficult by the end of this decade. Fortunately, today’s systems are more flexible than ever, and new developments are always taking place in manufacturers’ R&D departments. The advice I would give to anyone considering purchasing a wireless system is to evaluate the current offerings in terms of flexibility, ease of use, sound quality and customer support. And don’t forget to pay for what you expect to get. With these things in mind, your system should provide many years of reliable operation.