Lesson 6: Wireless Systems.


Wireless or Radio?

It's funny how things get their name. In the electronics section we came across "capacitors". The original name for these devices was "condensers", but even though the name of the electronic component has changed, devices using that principle are still referred to by the original name eg, "Condenser Microphones". Now we run across another such name anomaly. Originally, the box that we now refer to as the "radio", was called the "wireless". The name of the devices in that field have changed, but the stage system has retained the name "Wireless",mostly because the name specifically points to the fact that there are microphones etc. with wires still in use, Wireless Microphone and Guitar Connection and that with wireless stage devices there are no wires to clutter up the stage, or restrict the movement of the user.

Besides the hand held microphone with the built-in transmitter, there are also transmitters in boxes that can clip on to the user's belt or in a pocket, or they can often be seen taped to an instrument strap, usually behind the user. These boxes can take inputs from many sources, such as headset microphones and instrument outputs. Secondly there is a transmitter box with an XLR sockets as part of the casing which is designed to plug directly into the back of a standard microphone to give it the portability of a wireless mic. Unfortunately, the transmitter box is square whereas the mic handgrip is round, and the box is about twice the width of the mic's diameter. Combining the extra weight of the transmitter unit upsetting the balance of the mic, and the unsightly dimensions, its easy to see why it was not a big seller for Shure. The system has even been reversed so that preformers can have reciever boxes in their pocket or on their belt that can recieve transmissions from off-stage to earpieces used for foldback.

Originally, wireless microphones came in the frequency band of a standard AM/FM commercial radio, but that caused too many problems, so they were moved into a different band between 500mHz and 900mHz. These devices are usually locked into specific channels that are set far enough apart so as to not interfere with each other. It's important to set the same channel on both the transmitter and reciever.

AM, FM, or Digital?
AM and FM modulation
The common element of all 3 is the "carrier". The carrier is the thing we tune in to when we choose a radio or TV station (eg, 104.9Mhz). The difference is what each principle does to the carrier. We'll start with AM or Amplitude Modulation. The carrier is produced at a constant amplitude, and then the signal changes the amplitude of the carrier in direct proportion to the shape of the signal.

An obvious disadvantage of AM is that, the further away you get the weaker the signal, and the more susceptable the wave is to static and electromagnetic radiation, as in a lightning strike, or power tools on a construction site, etc.

The second type of modulation is Frequency Modulation. In this type the carrier's frequency is influenced by the wave shape of the audio signal, so it is continuously changing. This way, it doesn't matter how low the ampliture of the carrier is the clear audio signal can still be extracted. FM also has much more immunity to static and electromagnetic interferance.

Digital transmission over an audio media is a technology that has been around since the 1970's in the form of computer modems over telephone lines. Digital modulation It is a form of FM except the only information being transmitted is ones and zeros, therefore, the modulation uses only 2 frequencies. The carrier frequencies used in those early systems had to be within the telephone network frequency range of 300Hz to 3.4kHz, so it was common to listen to the sound coming out of the dialup modem of your computer talking with the server for about the first 30 seconds of your online time. The speed of these connections was in the vicinity of 2.4kb/sec.

This type of technology still exists but the frequency is 10,000 times faster which means the carrier frequency is up in the megahertz range, which also means not all telephone lines can handle it. These days we call that technology "Broadband" and it requires an "ADSL" modem.

The same process applies for digital signals being transmitted over a wireless system, except the modulated carrier is amplified and sent to an antenna. In all 3 cases the demodulation process is simply a case of filtering out the carrier and the signal is what is left.



We'll soon be ready to move on to Recorders and Editors. When its ready you will be able to click here to go to the lesson about Analogue and Digital Recorders and Editors, or click here to return to the Stage & Studio Equipment index page.