Telex 38109-977 manual Ntsc Channel Configuration

Special Considerations

Wireless communications are here to stay. They have become an integral part of the total professional communications package. There are, however, many factors associated with wireless that need to be understood and addressed that do not come into play with hardwired communications systems. In this section, we look at the special considerations that must be considered when deciding whether or not to implement a wireless system.

The first area of study is the RF spectrum and how it can be used to implement a wireless intercom system. Traditionally, wireless intercoms have been a function of broadcast television productions, and as such have used, at least in part, a spectrum that falls under FCC Code 47 CFR, Part 74 in addition to itinerant frequencies. The spectrum most commonly used falls into two areas: VHF systems from approximately 154 MHz to 216 MHz, and UHF systems from 460 MHz to 608 MHz and 614 - 806 MHz. As mentioned in an earlier section, large chunks of this spectrum have either been reallocated, or will soon be reallocated. The FCC has found that auctioning spectrum is a good way for the commission to move from an expense center to a profit center, and they are pursuing it with a passion.

Wireless intercoms, like any other wireless system, require at least one transmitter to function. Under FCC rules, all transmitters must be licensed prior to operation (there are some very low power transmitters that can operate under Part 15 and do not need to be licensed, but that doesn’t apply to any modern RF intercom systems). There are different forms to obtain various types of licenses depending on what area of the spectrum your system will operate in, who will be operating the system, and what the system will be used for. The law is very clear in that no one is permitted to operate a transmitter typically used for wireless intercom systems without first obtaining an FCC license.

Wireless equipment often operates in areas of the RF spectrum that are designated for TV channels, but are unused in a given area. In all cases low power transmitters used by wireless intercoms and wireless mics must operate on a secondary, non-interfering basis. This means that wireless users must not cause harmful interference to television or other receivers, and must accept all interference sources. In keeping with this, the FCC rules state that VHF systems must not be operated within 50 miles of a television transmitter occupying a similar spectrum. The rules further state that UHF systems must not be operated within 75 miles of a television transmitter occupying a similar spectrum. See Figure 6.4, for a depiction of what a television station’s assigned spectrum looks like. Refer to Table 6.1 for the standard frequency allocations of television transmitters.

Figure 6.4 NTSC channel configuration.

Video Carrier

	NTSC Channel Configuration
		Audio Sub
Guard Band
	Chroma Sub
	Slot Area
TV Channel		TV Channel
Start Frequency		End Freguency

0 MHz	1	2	3	4	5	61MHz
		1.25 MHz
					4.8295 MHz
						5.75 MHz

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