Telex 38109-977 manual An example of a modern day wireless intercom system

duplex operation. This enables all users on the system to speak and hear other users simultaneously without “covering” other users’ transmissions.

The demands of modern broadcast productions make the full-duplex operation of wireless intercom systems an absolute necessity for stage managers, lighting and audio technicians, or any professional who has to deal with the breakneck speed and complexity of television productions.

The spread of digital television (DTV) and the ever-increasing number of wireless users has made the available frequency spectrum a more difficult place in which to find available channels for wireless intercoms. The spectrum has also become a lot smaller, especially considering that four television channels (24 MHz of spectrum) have been reallocated for public safety use and the upcoming reallocation of UHF TV channels 60 through 69 (60 MHz of spectrum). Broadcast professionals now have to consider such factors as the compatibility of frequencies with each other, as well as, how to best avoid interference with local TV transmitters. We discuss these topics in more detail, later.

Unlike wireless microphones that operate only in one direction, wireless intercoms have more specific frequency spectrum requirements because of the relationship between the transmitter and receiver frequencies. Each intercom (if it is to be full-duplex) must have at least one system transmitter frequency that broadcasts to all beltpacks and one receiver frequency for each individual beltpack in the system. For a four beltpack system, also known as a four up, that means it must have a minimum of five total frequencies. See Figure 6.2.

Figure 6.2 An example of a modern day wireless intercom system.

RECEIVER &

TRANSMITTER

FOR

WALK

AROUND

1

2 3 4

TRANSMIT ANTENNA	RECEIVER
TO ALL	ANTENNA FROM
WALK AROUND RX’S	WALK AROUND TX’S

		ANTENNA SPLITTER
COMMON TX	RX	RX	RX	RX
FOR ALL
WALK AROUND	FOR	FOR	FOR	FOR
PACKS	WALK	WALK	WALK	WALK
	AROUND	AROUND	AROUND	AROUND
	1	2	3	4

Each beltpack must have a receiver set to the base transmit frequency and a transmitter set to its own unique receiver in the base. Due to a phenomenon mentioned earlier called desensing, these two frequencies must have a fairly large frequency separation, typically at least 12 MHz for VHF systems, and even more for UHF, or the transmitter will interfere with the receiver’s operation.

88 H a n d b o o k o f I n t e r c o m S y s t e m s E n g i n e e r i n g