The Big Picture – What’s in a Wireless System?

Wireless systems consist of a number of components. Transmitters and receivers are the most obvious of these componenets, but coaxial cable (sometimes referred to as transmission line), antenna splitters, amplifiers, and antennas also contribute to wireless systems. In this section, we will discuss these components in depth.

Transmitters

In simple terms, a transmitter is a device that takes a signal (such as the output of a microphone), and broadcasts it using a set of rules, such as format (modulation type) and channel (frequency). Transmitters can take many different forms, but in the theatre and broadcast industry they are most commonly in the form of a small bodypack, the handle of a handheld microphone, or a rackmount box.

Transmitter Types

Wireless audio transmitters may take any of a number of forms. Most common in the theatre is the so-called beltpack transmitter. This device is about the size of a large-sized pack of cards, and is designed to clip to the talent’s belt or be hidden under a costume. The beltpack transmitter requires an external microphone (often referred to as a microphone element, or simply element) to be mounted on the talent’s head or body, and this element must be connected to the transmitter with a small, thin wire.

Another common type of transmitter is the hand-held transmitter. These units often resemble typical stage microphones, except that they are typically longer and fatter than their wired brethren. Handheld transmitters typically have a microphone element built-in, although many transmitters allow the user to swap the microphone element as needed.

A third type of wireless transmitter is the plug-on transmitter. This device usually has a female XL-type connector on the top and is designed to literally “plug on” to a typical corded microphone so that microphone can be used wirelessly. These transmitters are commonly found in the news and ENG industry, and are very flexible as they can be used with any microphone that is desired. They also often have an option to provide phantom power.

Finally, IFB (Interruptible Fold-Back) transmitters are often table-top or rack-mount devices. These are typically powered from a power supply and not a battery, and often require an external antenna to operate. Additionally, these transmitters often use slightly higher power than battery-powered transmitters.

Audio Circuitry

Microphones generate an output signal that has a very wide dynamic range–that is, the amplitude of the signal may be very weak or very strong. In a wired sound system, we use components capable of handling signals of this sort, and we also have immediate control over the gain we apply to the input signal (in the form of a gain control on a sound console).

Wireless systems face two primary problems, though. First, we cannot represent a very wide dynamic range due to the limited bandwidth we have available to us (recall that in FM, the output RF signal’s frequency varies as a function of input signal amplitude, so high amplitude signals will cause us to swing way outside of our allocated frequency band). Second, we cannot gain access to a wireless transmitter’s gain when it is on stage and in use–meaning we have to make a prediction regarding the volume of our talent in advance to avoid having a signal that is too loud or too soft for our wireless link.

Modern wireless systems alleviate the dynamic range problem by using a compander circuit in the audio chain of both the transmitter and the receiver. In simple terms, when the input signal gets too loud, the compressor in the transmitter limits the volume by a specific ratio (sometimes, this is done over multiple frequency bands–lows, mids, highs). In the receiver, this process is reversed using an expander circuit. When this process works well, it is natural and hard to detect, but when it is pushed to its limits (with harsh transients), the transmitter and receiver can begin to work slightly out of sync and one can hear the signal “pumping” in and out.

Modulation (Signal Format)

There are many, many different methods that are used to modulate the RF signal (that is, superimpose the data onto the radio carrier signal). For instance, cellular telephones use very complex digital encoding techniques to maximize the number of users and minimize dropouts. However, wireless audio devices generally use a much simpler modulation technique that has been around for decades: frequency modulation (FM). In an FM system, the frequency of the RF wave is changed (modulated) by the input audio signal. As the input signal raises in magnitude, the frequency increases slightly, and as the input signal lowers in magnitude, the frequency decreases slightly. This is a very robust system (as the amplitude doesn’t change), and offers very high fidelity and nose immunity.

In addition to varying the frequency (really the phase) of a radio signal, it is also possible to vary the amplitude. AM radio modifies the amplitude exclusively, and many different digital modulaton standards vary both amplitude and phase together. More complex digital standards use parallel signals to increase the amount of information that can be transmitted (OFDM and MIMO are examples of this). Typically, wireless microphones are designed to be very simple devices, as this tends to reduce power consumption and signal processing delay, though af ew manufacturers do emply digital techniques (an example of this is Lectrosonics’ Digital Hybrid series and their 700-series encrypted devices.

Final Stages, Power Amplifier, and Transmit Power

Wireless microphone transmitters typically have RF power outputs anywhere from 10 mW to 250 mW. Generally speaking, it is desirable to have the lowest transmit power necessary to ensure a reliable link with the receiver. In most situations, 10-30 mW is sufficient transmit power. Not only does this help conserve battery life, but lower transmit power reduces the possibility of RF interference, such as intermodulation, RF signal clipping (different from audio clipping), and other problems.

The Receiver

The receiver is arguably the most important part of a wireless system, because the design of a receiver is the most critical factor in determining whether a signal will be heard or not. Good receiver designs are capable of pulling a signal out of a significant amount of noise while maintaining audio quality. Other good characteristics include very good intermodulation rejection, an easy-to-use user interface, and other things.

Almost all wireless microphone receivers have two antenna inputs. These two inputs offer the receiver a much better chance of receiving a strong enough signal, and reduce the odds of a dropout or other problems. It is essential that both antenna inputs be connected to an antenna, and it is preferable that both inputs be connected to similar-types of antennas.

Block Diagram

The reciever is in many ways the exact opposite of a wireless transmitter. The received RF signal is first filtered, and then it is amplified. The front-end filter is very important as we will see in a moment. The filtered and amplified signal is then fed into a mixer, which mixes the signal with a local oscillator to move the signal to a fixed frequency. The now-fixed frequency signal is then filtered again with a fixed-frequency filter, mixed down again, filtered again, and the fed to a demodulator. After the demodulator, the resulting audio signal is once again filtered, amplified, and fed to the XLR connector on the back of the receiver.

Front-End Filters

Front-end filters are extremely important for two reasons. First, the front end filteres remove any out-of-band signals that, when mixed with the LO, might produce a signal on the IF used in the receiver. Second, the filters reduce the amplitude of out-of-band signals that might cause the RF amplifier to distort, and this creating new mixing products.

Most wireless microphone systems have fixed front-end filters that cover the unit’s passband. However, some manufacturers have used tracking filters that follow the signal around. These filters are much narrower, and are particularly good at reducing the chances of intermoulaton.

Diversity Reception

ALmost all wireless audio receivers designed today employ some sort of diversity reception. Diversity reception techniques reduce the probability of a complete signal drop-out by using a combination of multiple antennas, multiple frequencies, and/or multiple receiver sections.

Spatial diversity is the most commonly employed type of diversity reception. With spatial diversity, two antennas are connected to the receiver. It is very important that these antennas be at least one wavelength apart from each other, and preferably a few wavelengths apart. As a rule of thumb, at 500 MHz one wavelength is approximately 24 inches, at 600 MHz one wavelength is approximately 20 inches, and at 700 MHz one wavelength is approximately 17 inches.

In more critical applications, two transmitters operating on different frequencies are often used. Not only does this reduce the chance of a transmitter failure affecting the show, but different frequencies have different standing wave patterns in a space, and therefore the odds of a complete dropout of the signal are greatly reduced. Some systems, such as the Lectrosonics Venue receiver system, automatically manage channel diversity and switch between receivers as necessary. Other systems must be manually managed.

2 Comments

  1. Hi Mike,
    I have been trying to figure out a way of doing 10 channels of mono audio at 16 bi 44.1 quality wirelessly to 10 modified antique gramophones without resorting to commercially available in ear monitor type devices from such companies as sennheiser which start at around a thousand dollars a channel. would you have any tips in finding a efficient and affordable solution?

    cheers
    jdf
    Melbourne
    Australia.

  2. Mike says:

    Hi David, I have a couple of ideas on this, though I will say that ten channels is pushing it. First, you may be able to find wideband FM headphone products in the 900 MHz or 2400 MHz band designed to let users watch TV quietly. If these have a channel select option, you may be able to hack the receiver into the gramophone and use the transmitter in the booth. I am concerned that you won’t be able to get ten channels of this, though. I have used FM iPod transmitters and pocket radios in the past, and these can work but are often less than reliable. I have also used a wireless microphone system in reverse, putting the receiver inside of a large old radio set piece (with a car stereo amp and speaker), and the bodypack in the booth. That system worked quite well. Sadly, I think that an IEM system would provide the best solution, even if it’s expensive. Does that help?

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