February 7, 2006
How to Choose a Lectrosonics Wireless Frequency

The purpose of wireless frequency coordination is to effectively get clean sound when using wireless systems such as radio mics. When transmitting sound, there are a number of things to be aware of so that extraneous unwanted sounds are not picked up as well.  The radio waves used in microphone transmitting are on the UHF and VHF bands (ultra-high and very high frequencies), the same ones at which TV stations broadcast their frequencies.  Finding clean airwaves is essential for recording sound with no noise or distortion

Go to www.lectrosonics.com and click on their “Television Station Lookup” link

Lectrosonics has set up a search that allows you to pull up the television stations in your area, their corresponding frequencies, and Lectrosonics’ corresponding block numbers for those frequencies.

Select the city or cities to which you will be closest.

The resulting chart tells you several things about the broadcasting in your area:

The frequencies at which the television stations broadcast
 
The location and distance from your area of TV stations broadcasting nearby (both digital and analog)
 
The block numbers and switch settings that will provide interference-free sound

THINGS TO KEEP IN MIND

1 – FREQUENCIES

  • The stations highlighted in red (channels 14-20 and 60-69) are frequencies also used by radio stations, so are consequently more prone to interference.  Channels 14 to 20 are also used by life safety operations in some cities, so usage may not be legal in some areas.
  • Federal law prohibits the use of Channel 37 (also highlighted in red) for radio mics at any time
  • Lectrosonics Blocks start with Block 20 in Channel 21. N.B.: Block 20 is not available in any current products, although it may be found in older equipment.

2 – TV STATION DISTANCES

  • It is best to select frequency ranges that do not have any broadcasting stations listed nearby, indicated by the absence of a distance and direction.
  • A = analog station, D = digital station
  • The number is the distance in miles in the given direction
  • For example, A4NE = Analog station broadcasting 4 miles to the North East of your city
  • If the need arises, it might be possible to share frequencies with a distant station.
     
    Bruce Jones of Lectrosonics says: "This can be a tricky one because it requires a judgment call after studying the distance and power of the station. A digital transmission is basically broadband noise, so it just raises the noise floor. An FM system naturally rejects the noise as long as its own signal is a few dB above the noise. Range will typically suffer, but how much depends on how strong the TV noise signal is. Digital signals also do not penetrate structures very well, so often an FM system will be usable in the middle of a digital TV signal if the receiver is in the RF shadow of a structure.

    An analog TV station transmits a video carrier that looks like noise to an FM receiver, a sync pulse which looks like noise but may shorten range a bit, and a wide deviation FM carrier for the sound. The symptoms of interference will vary depending upon where the FM wireless system is in the analog TV channel. If it's in the video part of the TV signal, range typically suffers. The sync pulse is very narrowband, so it is not often a problem. The audio carrier will come through loud and clear and the range of the wireless system will go right down the tube if the wireless frequency is within 100 kHz or so of the sound carrier."

In addition to the television station lookup, Lectrosonics’ Frequency chart pdf allows you to coordinate the frequencies when using multiple radio mics.

In addition to the frequency interference from TV and radio stations, use this to determine the block numbers you need in order to minimize intermodulation between the microphones themselves.

(INTERMODULATION = The result of 2 radio signals of different frequencies mixing together to form additional signals at frequencies that are not harmonic frequencies (integer multiples) of either.  Basically, it creates undesired output frequencies that were not present at the input)

The broadcast-free TV stations for New York City are 26, 32, 42, 46, 49, 55, 59. This will be the example for using the frequency chart to choose microphone blocks and switch settings


THINGS TO KEEP IN MIND
  • For blocks that are not adjacent to each other, such as 26 and 28, there will be minimal intermodulation trouble
  •  When dealing with adjacent blocks, there are things that can and cannot be done to avoid intermodulation
    • CAN cross groups within the same block and the same row

    • A & B  or  C & D  NOT   A & C, A & D, B & C, or B & D
    • CAN cross adjacent blocks if the color coding matches
    • CANNOT cross adjacent blocks within groups (not matching color coding)
    • CANNOT cross rows
  • On the frequency chart are diagrams for the aforementioned compatibility and incompatibility
  • See illustration on Frequency Chart for more information

Using the frequency chart, we know that channels 26, 32, 42, 46, 49, 55, 59 are the optimal blocks for New York and we can determine which will not interfere with each other.


Circled on the chart are the optimal channels for Blocks 21 through 24.

Once you’ve selected the blocks, the SW SET tells you which frequencies to set the rotary dials on the transmitters and receivers for the TV channels and blocks you want

For the New York City example on Block 21, Group A, the optimal channel is 26.
Its corresponding switch settings are:

  • 3,0
  • 3,5
  • 3,D

On the transmitter itself, the rotary dials (behind the sliding door on the side) can be set with a small screwdriver to match one of these optimal switch settings. The dial on the left should be set to the number on the left of the chart, and the right to the number on the right

Frequency Adjustment Switches

The first dial should be set with the arrow pointing at '3', and the second at either '0', '5', or 'D.'

Once this is done, the receiver frequency must be set to match in the menu on the LCD display. You can scroll through the options using the white buttons until the menu reads the same switch setting as the receiver.

Special thanks to Bruce Jones of Lectrosonics for his input.


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