Tips: Intermod & Interference
Intermodulation is a common problem in wireless audio systems. It can cause unwanted audio artifacts, interference, and distortion in the audio signal. When many wireless microphones are being used at the same time, this can be especially hard to deal with in RF environments that are crowded. In this article, we'll show you how to deal with the problems caused by intermodulation and make sure your wireless audio system is working at its best. We'll look at the different ways you can avoid or lessen intermodulation to make high-quality audio recordings, such as by coordinating frequencies, using advanced error correction techniques, and "hopping" from one frequency to another. This article will help you get the best results possible with wireless audio systems, whether you are a sound engineer, a film or TV producer, or someone else who works with them.
Frequency intermodulation (IM) is what happens when two or more signals with different frequencies are combined in a nonlinear device, like an amplifier or a wireless microphone transmitter. Intermodulation products are the new frequencies that are made when two signals are mixed together. These frequencies were not in the original signals. These products of intermodulation can cause noise and distortion in the audio signal, which can lower the quality of the sound recording. In its simplest form, intermodulation is like when you mix different colored paints together, and you end up with a new color that you didn't mean to make. A practical example you've likely heard on set would be with walkie steppage; when you're talking on a walkie-talkie and someone else is talking on a different walkie-talkie at the same time. The waves from the two walkie-talkies can mix together and make a new sound that you didn't want to hear.
Wireless microphones are often used to record sound on location in TV and film production. The sound signal from these microphones is sent to a receiver via radio frequency (RF) waves. When more than one wireless microphone is used, frequency intermodulation can happen. This is because the wireless microphones work on different frequencies and can interfere with each other, making intermodulation products that aren't what you want.
One of the hardest things to do in this field is to stop frequency intermodulation, which can happen when there are a lot of wireless devices running at the same time, like in big cities. This can be done with frequency coordination, which is the process of choosing and giving wireless microphones different frequencies so that they don't interfere with each other. This can be done by hand or with software that picks frequencies for wireless microphones based on where they are and how many other RF devices are nearby.
Another challenge is dealing with the intermodulation products that can be caused by cell phone towers, wifi routers, and other wireless devices that are out of the production team's control and can mess up the audio signal. Using frequency filters, directional antennas, and other ways to stop interference can help with this.
Also, it's important to know that in recent years, government regulations have limited the RF spectrum that can be used for wireless microphones. This makes it harder to find clear RF channels to use and puts more pressure on using frequency coordination and other methods to avoid intermodulation. Be that as it may, there are many tools at a sound mixer's disposal that can help reduce the chances of causing intermodulation, or at least mitigate its effect on the quality of the dialog being captured.
Before we dive into some solutions and prevention techniques, it's important to know that intermodulation is handled differently for digital and analog wireless audio systems.
Analog wireless audio systems send audio information using analog signals. In an analog system, the audio signal is always changing, and other analog signals in the same frequency range can cause interference and distortion. This can cause intermodulation, which is when the sound signal from one wireless microphone mixes with the sound signal from another wireless microphone, making noises that you don't want to hear.
On the other hand, digital wireless audio systems use digital signals to send audio information. In a digital system, the audio signal is turned into a series of ones and zeros (binary digits) before being sent. Digital systems are usually less affected by interference and distortion from other signals because the digital signal is more stable and easy to recover after transmission.
Digital wireless audio systems have a lot of benefits, but one of the best is that they use advanced error correction techniques that can find and fix mistakes that may happen during transmission. This means that even if interference or other things mess up the digital signal, the system can still get back to the original audio signal. Having said that, let's briefly touch on the tips you came here for.
Coordinate with other departments
Intermodulation can be caused by other RF devices on set, such as wireless microphones, cell phones, and Wi-Fi routers. Coordinating with other departments to ensure that these devices are not operating on the same frequencies as the audio system can help reduce the risk of intermodulation.
Use frequency hopping
Wireless audio systems that use frequency hopping can rapidly switch between different frequencies to avoid interference from other RF signals. This can help reduce the risk of intermodulation. Sennheiser, Shure, Wisycom and AKG are some manufacturers that make wireless audio systems that use frequency hopping to avoid interference. The Wisycom MRK16 and MCR54 wireless systems, as well as the Sound Devices Astral lineup all use a proprietary algorithm to rapidly switch between different frequencies in order to avoid interference.
Use frequency diversity
Multiple antennas can be used to transmit or receive the same audio signal on different frequencies, reducing the risk of interference. Sennheiser, Shure, and AKG also offer wireless audio systems that use frequency diversity to reduce the risk of interference.
Use directional antennas
Directional antennas can be used to focus the RF signal in a specific direction, reducing the risk of interference from other RF signals that are coming from other directions. Some manufacturers offer directional antennas for their wireless audio systems to focus the RF signal in a specific direction and reduce the risk of interference.
Use shielded cables
Using shielded cables can help reduce the risk of RF interference, as the shield can help block out external RF signals. Many manufacturers, such as Canare, Belden, and Mogami, offer shielded cables for use in audio and video production to reduce the risk of RF interference.
Lectrosonics, for example, has a wildly popular technology called Digital Hybrid Wireless. The design overcomes radio channel noise by digitally encoding the audio in the transmitter and decoding it in the receiver, yet still sending the encoded information via an analog FM wireless link. This proprietary algorithm is not a digital implementation of an analog compandor. Instead, it is a technique that can be accomplished only in the digital domain. The process eliminates a compandor and its artifacts from the audio chain, expanding the applications to include test and measurement of acoustic spaces and musical instruments.
Digital technology has rapidly advanced the features and capabilities of innovative approaches to mitigate both intermodulation and interference. It is especially important to take these steps when filming in crowded RF environments, such as large cities or venues with many RF devices present, as the risk of intermodulation is higher in these situations. Hopefully you can use some of these tips to help you on your next shoot.