Tech Tips:
Wireless Collision
By Andy DunnEverywhere in the world, people are becoming more and more reliant on wireless technology of some kind. Even right now you may have a cell phone on your hip and a wireless network connection on your lap top computer.
What happens when you are planning a shoot with wireless communication, wireless video, wireless microphones, wireless IFB, and all that jazz. Add these to cell phones, pagers, and wireless ethernet on the set and who knows what will happen.
First let’s get a little background. All these devices transmit their signal as radio frequencies. In fact, radio frequencies are used by television stations, radio broadcasts, fire stations, and security systems as well as many other industries. There are different groupings of radio frequencies. ELF or Extremely Low Frequencies from 0 to 3Khz, VLF or Very Low frequencies range from 3Khz to 30Khz and LF or Low frequencies are transmit at 30Khz to 300Khz. HF or High Frequency consists of frequencies ranging from 3Mhz to 30Mhz; VHF or Very High Frequency consists of frequencies ranging from 30Mhz to 300MHz while UHF or Ultra High Frequencies range from 300Mhz to 3Ghz. Groupings within each range are allocated for different purposes. For example, short wave radios operate in the HF range. FM radio stations, general aviation and television stations channels 2 through 13 operate in VHF ranges. Military aircraft, television channels 14 through 70 and cellular telephones transmit in the UHF range.
Unfortunately, there are a limited number of radio frequencies. Those that are allocated for each purpose are strictly monitored and enforced. This is because 2 different transmissions of similar frequencies in close proximity to each other will cause interference. Each one fights to be the dominant transmission source and both end up being affected by the other.
Most devices used in production tend to fall within the VHF or UHF range. These are then broken down into blocks, which are dedicated to certain companies, groups or types of devices. For example, the Lectrosonic wireless microphone systems we have at Rule range from transmitting at 563.2MHz to 742.3MHZ. Other companies may rent wireless microphones using the same frequencies as ours. As I said, frequencies are limited. However, there is little chance that you will be working in an area close to another microphone transmitting at the same frequency as your own. This has been known to happen though, especially in New York City’s theater districts or during high profile news events where there may be news stations from around the country in one area.
In these situations of high RF traffic, knowledge and versatility are key. You need to be able to recognize interference versus a problem with your own unit. Also, using a piece of equipment where your frequency is selectable can be a great advantage. Interference occurs when a receiver attempting to pick up the frequency of a specific transmitter, begins to pick up another transmitter sending out RF similar or near the frequency it is scanning for. Depending on the type and strength of the signal, you may hear (or see) exactly what the interference transmission is, or you may hear (or see) static. A receiver may pick up two microphone transmitters on the same frequency. The stronger signal will be dominant and be prominent in your headphones, however, the weaker signal will still occasionally break in. By shutting off the dominant transmitter, you should be able to hear the secondary signal more clearly. This may not always be true since, in some cases, the weaker transmission will need the stronger transmission to help boost it along. By shutting off the dominant transmitter, the weaker or more distant transmitter may not have the range to make it to the receiver.
Transmitting frequencies do not have to be on top of each other to cause interference. While wearing a belt pack for a wireless communication system on one hip and a cell phone on the other, be ready to hear when a call is about to come in through the wireless system’s headset. The transmission to the cell phone is very strong and the cell phone is always in receiving and transmitting mode so the cell towers can find it. When a call is coming in (even before the phone rings) you may pick up interference from it.
A quick thanks to Pat Kelly who discovered on a recent job how interference could be a great testing tool. One of his video transmitters did not appear to be working, however, the problem could have been in the transmitter, the receiver, the cable to the transmitter, the cable from the receiver or even the output of the video source. By placing his belt pack from his HME wireless headsets near the video transmitters, he noticed two important things. Of the two transmitters, the first produced interference audible in his headphones even if his belt pack was not transmitting. The second transmitter created no interference at all. If the cables, receiver or video source were the problem, the transmitter would still be sending out RF and would still create interference. Lack of any interference quickly pointed to a bad transmitter.
So the bottom line is that knowledge is key. First, make sure that all wireless devices are transmitting on their own frequencies. Set systems with variable frequencies to ones that don’t conflict with other devices on the set. Keep transmitter antennas as far apart from the transmitters or receivers of other systems, a minimum of 8 to 12 inches. Keep transmitters in non-transmit mode whenever possible. For example use PTT (or push to talk) with the HME wireless communication system rather than locking the transmit button in the always on position. Finally, turn off cell phones on a set if possible.
If some of these are not possible on a set, try to recognize the pattern in the interference so that after you hear it once, you know what to look for the next time. The cell phone interference has a very specific sound. If you hear this pattern on the set, you will know what it is and not panic. If you hear voices from one system on another system, the frequencies may be on top of each other and the frequency for one of the systems will need to be changed. And most importantly, if a single transmitter seems to be cutting in and out, the problem may be with that transmitter only. The antenna may be broken, the batteries may be getting low or the user may be getting out of range. Since a base station transmitter is more powerful than a belt pack transmitter, it is very possible to be in range to receive broadcasts but not in range to transmit.
Wireless use doesn’t always mean wireless collision but a little knowledge can prevent a lot of frustrations. Talk about planning for wireless BEFORE production begins and work with your rental house or wireless device provider to identify possible collisions before they become major accidents.
