In this article, we've turned once again to Tim Vear, Shure Senior Applications Engineer and author of numerous articles and literature on all things audio. He literally wrote the book on "Audio Systems for Houses of Worship," now in its 14th printing.

Here, we've excerpted a what you need to know primer on post-DTV wireless.


In the US, and many other countries, broadcast television is undergoing a transition from analog to digital. We've all seen the television spots and read about the shortage of converter boxes. But to many in the pro audio community, the transition represented a dual threat — unreliable audio performance and obsolete gear.

Specifically, the transition affects wireless audio systems in several ways:
• more "occupied" TV channels
• no "open" space in DTV channels, and
• future "re-allocation" of existing TV channels.

Understanding what has happened really requires a basic understanding of frequency bands — what they are, their former 'occupants' and current 'residents'. The good news? The wireless world did not end on June 12, 2009. The even better news: With a couple of exceptions, the wireless systems you had before still perform reliably when you follow some basic guidelines.




All full power television stations in the US are now broadcasting only digital signals (DTV). A few stations (mainly along international borders) and some low-power stations still broadcast an analog signal. Though both types of signal occupy similar channel "blocks", the nature of the signal within the channel is different.

An analog TV transmission consists of three separate signals, each at a specified carrier frequency within a 6 MHz block in the US. (See Figure 1.)

The picture or "video" information is an AM signal at 1.25 MHz above the bottom (low frequency end) of the block.

The sound or "audio" information is an FM signal located at 0.25 MHz below the top (high frequency end) of the block.

The color or "chroma" information is an AM signal at 3.58 MHz above the video signal.

The energy distribution and occupied bandwidth of these three signals is not equal: the video signal has the highest power and widest bandwidth, followed by the audio signal and finally the chroma signal with the lowest power and smallest bandwidth.




A digital TV transmission consists of a continuous signal that occupies the entire 6 MHz block. (See Figure 2.)

All of the video, audio, and color information is digitally encoded into this signal along with a variety of other data, control, and secondary audio information.

It is possible for the DTV transmission to carry one high definition television signal (HDTV) and/or multiple standard definition television signals. The energy distribution within a DTV channel is essentially uniform. However, the average signal level of a DTV transmission is somewhat less than the levels of the video and audio signals in an analog TV transmission.


The primary users of both high band VHF and low-band UHF frequencies are broadcast television stations.

In the US, these are VHF TV channels 7 through 13 and now UHF TV channels 14 through 51.

Each TV channel is allotted a 6 MHz block for its transmission. VHF channel 7 begins at 174 MHz and extends to 180 MHz. Channel 8 occupies 180-186 MHz and so on up to channel 13 at 210-216 MHz. UHF channel 14 occupies 470-476 MHz with successive channels up to channel 51 at 692-698 MHz.

The 6 MHz/TV channel block is found in the US, the rest of North America, South America and Japan. Other countries, most of Europe and India for example, use a 7 MHz/TV channel block, while France and China, among others, use an 8 MHz/TV channel block.

For analog transmission in these other systems, the video and audio signals are located at the same frequencies relative to the channel boundaries as in the 6 MHz systems, but the frequency of the chroma signal differs slightly in each to accommodate the various color systems: NTSC (6 MHz), PAL (7 MHz), and SECAM (8 MHz).



Traditionally, to avoid potential interference between broadcast television stations, regulatory agencies have not allowed adjacent analog TV channel operation in a given geographic area.

Then: In the US, for example, a local analog TV channel 9 would have channels 8 and 10 vacant. These vacant channels could then be used by wireless microphone systems with little concern for television interference. Historically, the prohibition of adjacent channels guaranteed the existence of certain vacant TV channels in a given area.

Now: The advent of DTV has removed this guarantee: DTV channels are allowed to exist adjacent to each other and also adjacent to existing analog TV channels.



This has resulted not only in more densely spaced TV channels but also in difficulty using certain "pre-selected" frequency compatibility schemes that relied on non-adjacent TV channel spacing.

The effects of interference from television broadcast are dependent on the type of television signal (analog or DTV) and the strength of the television signal, as well as the location and the operating frequency of the wireless microphone system.

Direct conflicts with any of the three signals that make up an analog TV transmission can produce noise, distortion, and short range or dropout. Pickup of the video or chroma signals (which are AM) may cause distinct "buzz" in the wireless receiver, while pickup of the audio signal (FM) may result in the TV sound being heard.

Direct conflict with a DTV signal also causes short range or dropouts. Though an analog receiver can't generally "hear" anything from a digital transmission, the DTV signal acts as a very strong broadband radio noise source. In some systems this may result in increased noise or distortion in the audio output. Since there are no "unoccupied" frequencies in a DTV signal, wireless users will find it nearly impossible to operate anywhere within a very strong DTV channel.



The most effective solution for broadcast television interference is to avoid using frequencies of local active TV channels. Television transmitters may operate at power levels up to several million watts while wireless microphone systems typically have only less than 50 mW (fifty one thousandths of one watt!) of output power.

How local is local? "Local" is generally considered to be up to 50 or 60 miles, depending on the coverage area of the particular TV transmitter and on the location of the wireless microphone system. The good news is that indoor setups are at less risk than outdoor setups because building structures will usually strongly attenuate TV signals. Inside buildings of substantial construction it may be possible to ignore TV stations as close as 30-40 miles. Still, since the locations and assignments of television stations are well known, it's pretty easy to choose relatively safe wireless microphone system frequencies in a particular area.

One frequency does not fit all. One consequence of the newly dense TV channel distribution in the US is that it is not generally possible to use a given set of wireless microphone frequencies everywhere in the country.

As the distribution of vacant channels changes from city to city, it's inevitable that a fixed-frequency system will experience interference from a television station in some location. Today, nearly all current wireless systems are "frequency-agile" (tunable) — their operating frequencies may be changed as needed.

Many manufacturers pre-select groups of wireless microphone system frequencies based on the availability of locally vacant TV channels. If the TV channels are known, it's relatively straightforward to choose appropriate wireless frequencies for that location. Information on TV channel distribution throughout the US is widely available from manufacturers and other sources. It is usually sufficient to indicate the destination of the wireless equipment at the time that it is specified in order to avoid broadcast television interference.



In the United States, the Federal Communications Commission (FCC) has been supervising the transition of broadcast television from its traditional analog format to an all-digital format (DTV). In the process, the commission has been mandated to increase efficient use of TV spectrum and the amount of spectrum available for public safety and other wireless services. This transition has provided both consolidation of the broadcast spectrum and the reallocation of the resulting open spectrum for other uses.

The FCC has consigned all broadcast television into a "core" band, TV Channels 2-51. All existing TV stations above Channel 51 have migrated into the core band. The upper frequency limit of the broadcast band is now 698 MHz and (nearly) all full-power TV stations are now digital. All former television spectrum above Channel 51 (698-806 MHz) has now been reallocated for new services. This spectrum is generally referred to as the "700 MHz" band.

Within the 700 MHz band, most of this spectrum has been auctioned by competitive bidding to telecommunications companies while the rest has been set aside for public safety services. The telecommunications spectrum, 698-763 MHz (roughly TV 52-62) and 775-793 MHz (roughly TV 65-67), is being developed for various services such as broadband internet access, streaming video, and other digital data transmission. The public safety bands total 24 MHz in two "paired" bands: 763-775 MHz (roughly TV 63-64) for fixed transmitters and 793-805 MHz (roughly TV 68-69) for mobile transmitters.

The 700 MHz band is no longer available for use by wireless microphones or other wireless systems. Not only will they no longer be authorized in this spectrum, but the interference from new services in this band will make operation of such systems unreliable at best.

For this reason, manufacturers and distributors of wireless systems in the US market are no longer supplying wireless systems in the 700 MHz band. New wireless systems continue to be available mainly in the core television band (TV 2-51) and in a few other narrow frequency bands.



There are two items that should be on the "to-do" list of every wireless system user:

1. Take an inventory of your present wireless systems to identify any remaining equipment that operates in the "700 MHz" band (698-806 MHz). As a practical matter, these systems will eventually become problematic as the new users of this spectrum come on the air. As a legal matter, it is also likely that the FCC will eventually prohibit their use.

Presently, Shure and other manufacturers are offering rebate programs to encourage the replacement of 700 MHz wireless systems. This opportunity allows you to recover some value from these systems regardless of age. However, most of these programs will expire at the end of this year (2009).

2. Take a close look at your present frequency coordination plan. Although most TV stations in the US moved to their final assignment on June 12, there have been significant re-assignments since then. In several cities it has been found that some VHF assignments have not yielded the expected coverage area and so these TV stations have relocated to the UHF band.

This means that even though you re-coordinated your frequencies on June 12 it may be necessary to make changes if these TV channel re-assignments have occurred in your area. Shure regularly updates our Wireless Frequency Finder to reflect the latest information from the FCC.

As always, the Applications Engineering Group at Shure is standing by to assist in your frequency coordination efforts.