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