Receiver & Transmitter Types

Beltpack Transmitters

Beltpack transmitters are designed to be placed on the body of the user. This type of transmitter is generally used with an external microphone, either a lapel, headworn, or instrument pick-up. These transmitters will provide phantom power for most electret devices directly from the internal batteries.

Plug-in Transmitters

Plug-in transmitters are designed to be inserted into the connector of a standard dynamic microphone. This type of transmitter is generally popular with television ENG crews working in the field.

Handheld Transmitters

Handheld transmitters are a combination microphone element, and transmitter built into the same unitized assembly. These devices are popular for vocalists, and speakers that prefer to hold on to the microphone. Handhelds generally are available with a variety of different microphone capsules in both electret condenser, and dynamic elements. Sound performance on most good wireless handhelds will duplicate the performance of a wired counterpart without coloration or distortion.

There are also three basic receiver types.

Stand Alone Receivers

Stand alone or rack mountable receivers are the most common for use in applications where the receiver is usually in a fixed location. These are also popular for rental applications. These receivers vary in complexity and performance, and generally run the full gambit on pricing from very inexpensive to very expensive depending on sophistication, and features. Some less expensive units will have permanently attached telescoping antennas which makes them less desirable for permanent installation.

ENG/EFP Receivers

ENG/EFP (Electronic News Gathering, Electronic Field Production) portable type receivers. These receivers are typically used in remote work for broadcast applications where a small lightweight receiver is either attached directly to the camera, or placed on the body of the camera person like a beltpack. These receivers are also very popular with videographers for use in weddings, and other special events. Due to their compact size, some ENG type receivers do not have the filtering or advanced RF circuitry of their larger stand alone cousins. When they are equipped with advanced circuitry, they are generally very costly. These devices will generally run on internal batteries as well as external power sources.

Card Frame Receivers

Card frame style receivers are typically used in large multi-system venues where numerous systems are operating simultaneously. There are two types of receiver construction in these systems. One type uses a downconverter scheme that actually converts the operating frequency to a lower band, either UHF to VHF, or VHF to HF. The second type is actually a series of self contained plug-in receivers that simply share a common built-in antenna combiner and audio output section.

There are two subcategories of receivers:

• Non-Diversity which includes single antenna recievers.
• Diversity which includes twin antenna receivers.

Definition of Diversity Reception

There has been a great amount of marketing hype publicized in recent years on various names for diversity reception in wireless microphones. Diversity reception is a method of minimizing the effects of multipath during the reception of the radio signal. This is done by combing or selecting two or more sources of received-signal energy which carry the same intelligence, but differ in strength or signal-to-noise ratio in order to produce a usable signal. This always requires more than one antenna, but not necessarily multiple receivers. The term "true" diversity is merely a marketing term, as there are several ways in which diversity can be employed in a wireless microphone system. One particular design is no more "true" than any of the other methods, as this would imply that the numerous other methods are "false." Each type of design has its advantages and disadvantages. However, all of the methods described can be effective in combating the ill effects of phase cancellation.

Phase Cancellation or Multipath

Phase cancellation or multipath is a phenomenon where a direct radio signal and a reflected radio signal combine in the receiver. The two signals are out of phase from each other due to the reflection of the second signal off a nearby object. This reflection adds a slight time imbalance to the second signal which causes it to partially or even totally cancel the primary transmission.

The result is a noise-up or sometimes a complete loss of signal often referred to as a drop-out.

A very common example of phase cancellation or multipath has occurred to most people at one time or another. If you have ever driven your car listening to your favorite FM station and pulled-up to a stoplight and noticed that your FM radio station became fuzzy and full of whooshing sounds, you have experienced phase cancellation or multipath. Notice that when you pulled your car up just a few feet the station came back to perfect reception.

Technically, the direct radio signal, and a reflected signal bounced off a nearby object such as the stoplight are combining in your car stereo’s receiver at the same instant. Since the reflected signal or multipath is out of phase anywhere from 0 to 180° from the original signal, the two signals mix in the receiver and either partially or totally cancel each other out. The result manifests itself as noise and either partial or complete loss of the signal commonly called a “drop-out.”

The phase cancellation will only occur in those physical locations where the direct and indirect reflected waves interact. Moving away just a few feet away will often cause the multipath to disappear. A diversity system overcomes the multipath problem by the virtue of having two antennas in separate locations. The possibility of having a multipath cancellation at both locations simultaneously is very slim.

When direct radio signals and signals reflected off of a metal object, such as a steel stud mix, they produce a condition known as multipath. This condition will cause a null in the signal (drop-out) in the physical location that the mixing occurs. Diversity reception helps to overcome this phenomenon in wireless microphone systems.

Diversity reception is a common way to eliminate noise and reception problems that can occur in a radio link. Practical applications typically apply only to wireless microphones because noise-ups and complete loss of signal can be very undesirable when used in a sound reinforcement application. Similar problems in wireless personal hearing assistance systems and intercom systems are not as great of a concern because they will typically affect only a few users and not an entire auditorium of listeners.

CONTINUE: WIRELESS MICROPHONES

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