The B&H Introductory Guide to Portable Field Mixers
By Sam Mallery
The heart and soul of a sound kit for field production is a portable field mixer, and no sound bag is truly complete without one. The role of a portable field mixer in a production can be a little confusing at first. We'd like to illustrate the main functions of a portable mixer, and to explain why some models are more expensive than others.
In basic terms, a portable field mixer is a battery-powered device that allows you to combine the signals from multiple microphones (or other audio sources) and mix them down to a single mono or stereo output. In most situations, the output of the portable mixer will be plugged into the audio inputs of a video camera, a portable audio recorder, or both simultaneously. What's interesting is that a mixer's most important function is arguably not what its name implies— to mix audio signals. A professional sound person will rely upon the portable field mixer constantly, even when just a single microphone is in use, and there are no other sound sources to mix.
A portable field mixer allows a sound person to quickly access various controls that may need a speedy adjustment during a production. With the mixer close at hand, the sound person can adjust the live microphone levels, adjust their headphone volume, check what the mix sounds like inside and outside of the mixer, etc. (these functions are discussed in detail below). It's a common practice for the sound person to strap the mixer to their body in an audio bag when production is underway. The audio bag makes it possible for the sound person to adjust the controls on the mixer quickly with one hand, while the other hand is free to do other tasks.
Common controls found on portable field mixers
There are a few common knobs and switches on portable field mixers that you should familiarize yourself with. Some of these controls, such as channel level control knobs, are placed on the front panel for easy access. Other features, such as a limiter threshold control, may be placed in a less obvious location.
Gain and Level Controls
These are the most fundamental controls on a portable mixer. These components really define the overall quality of the mixer. The quality of the circuitry of a preamp in a mixer truly defines the overall quality of your sound. The finer the preamp is, the more expensive the mixer will be. Inferior portable mixers only have a single level control for a channel with no separate knob to control the gain.
It's important to understand the distinction between gain and level controls. The gain knob controls the preamp. The preamp is a small amplifier that boosts the audio signal coming into the mixer's input. The level control determines how much of that boosted preamp signal gets to the main outputs of the mixer. You use the gain control to set your signal in the sweet spot, so it's loud enough to be heard in full, but not so loud that it clips and distorts. You use the level control to determine how much of that channel needs to be sent to the output and into the video camera or portable recorder.
For example, say you have three microphones plugged into all three inputs of a three channel portable mixer. Shortly after you gave microphones to your subjects, you had them test their mics and you set the gain levels to get their mics in the sweet spot. Once you began working, there would be long stretches in which you would only need to hear one of the microphones in use. During those stretches, you would ride the levels of the unused microphones down. This way if the other talent accidentally bumps the microphones, or if they happen to cough and clear their throat, that noise won't get picked up and sent to the outputs of the mixer. But, if they were to suddenly start talking again, you could easily ride their level back up because you have the ability to set the gain on their microphones.
Headphone Monitoring (tape return/fold back)
Some portable field mixers allow you to change what you're listening to through the headphone output. There are various names for this such as "tape return" or "fold back." Basically, there's a special input on most mixers that allows you to connect a cable from the headphone output of the video camera you're plugged into. The cable that allows you to do this is a breakaway cable (more on this below). The tape return enables you to listen to what the mix sounds like in the video camera you're sending audio into. It's a very reassuring thing to be able to hit a switch on your mixer and know for certain that your work is being recorded into the camera, and that there is no problem in the connection between the two.
Panning is the ability to control the balance of an audio signal from left to right in the stereo spectrum. For example, if you were wearing headphones and you panned a signal all the way to the left, you would only hear the signal on the left side of the headphones. In music production, panning is used mainly in creative ways, to give music a more natural ambience and to help separate different instruments and voices in a mix.
In electronic field production (EFP) and electronic news gathering (ENG), panning is primarily used as a utility to prioritize audio signals. For example, say you are operating a 3-channel mixer for a television show. You are shooting outdoors away from the studio and you have the host of the show's wireless lavalier mic in channel 1, and the guest star's wireless lavalier mic in channel 2. In the third channel you have a handheld microphone that the host is holding. There is a crowd of people behind the two hosts, and they plan on speaking to individuals in the crowd. They need the handheld mic to ask the crowd questions. Since the host of the show is the most important audio, you pan that microphone all the way to the right. You pan the guest host's microphone to the left, and you pan the handheld microphone to the left as well. Remember, there are only two outputs on the mixer, left and right. This way the host of the show has a dedicated output on the right, while the guest host and the crowd members' audio is mixed together on the left output.
Unlike traditional audio mixers, the panning controls on a portable field mixer often come in the form of a switch with three positions: L, C, and R (Left, Center, and Right). Pan controls may also come as knobs.
Low Cut Filters
Low cut filters (also known as high-pass filters) remove low frequencies that can be problematic for a production. The microphones used on a production are often very sensitive and capable of picking up low rumbling noises that can be potentially distracting for the viewer. These low-frequency sounds can be created by vehicles, HVAC systems, even by the sound person themselves if they accidentally mishandle their equipment.
Low cut filters are usually switches or buttons. Some people always leave them engaged. Others may choose to turn them off on occasions where they're confident they won't have any rumble issues. Disengaging the low cut filters provides a more full-frequency sound.
Portable field mixers often include a tone generator (also referred to as oscillators or slates). Their main function is to allow you to set the input level of the video camera or recording device with the output level of the mixer. The basic idea is to go through the menus on the video camera and find the camera's audio meters. With the mixer plugged into the camera's audio inputs, turn on the mixer's tone generator and adjust the audio levels on the camera so the signal is strong but not going into the red.
The tone generator often comes in the form of a switch. The switch will often have three positions: Off, Locked On, and Momentary On. You lock the tone on when you are calibrating the output level of the mixer with the input level on the camera or recording device. The slate tone is also used as a marker between takes, so you would use the momentary setting to create short pulses of tone. You can often hear these tone pulses on blooper reels of onscreen talent flubbing their lines. These pulses are used as markers in post production. The pulse creates a square block in the digital audio waveform of the audio you're recording. In post production, these blocks are easy to see, and help editors manage the footage. Some mixers even feature a built-in microphone with the slate. This way you can trigger a pulse to then audibly say into the microphone "Scene one, take two… "
A limiter creates a ceiling for the volume of an audio signal in a channel. Limiters help to keep the audio from clipping and distorting. High-quality portable field mixers have limiters on both the inputs and the outputs. For a deeper understanding of this subject, check out this related B&H educational article about compression.
Many portable audio mixers feature multiple outputs. Mixers always have a main output (usually two XLR jacks) to send the mixer's signal to the most important source (video camera, portable recorder, etc.). There are often more outputs as well. It's common to have a headphone jack with a volume control. It's also common to have an auxiliary output to feed the mixer's signal to a second source. This will enable you to send the mixer's audio to both a video camera and a recorder at the same time. IFB systems are also used in shoots, and multiple outputs are needed to feed them audio. IFB systems are used to send the mixer's audio wirelessly to a set of headphones for monitoring. The director of a shoot will often wear an IFB receiver to hear the audio as the shoot is taking place.
Multiple outputs are also useful if you're doing a two-camera shoot without a portable recorder. You will be able to feed your mixer's audio to both cameras. This way, if something goes wrong mechanically with the audio in one of the cameras, you have a second back-up copy in the other camera. This practice also makes things easier in post production. Since both cameras are fed the same audio, there is no need to sync sound back and forth between the footage.
What am I paying for?
The prices for these portable field mixers can range from under two hundred dollars to several thousand dollars. There are great differences between the entry-level mixers and the more professional ones: the ruggedness of their construction, the quality of their switches and knobs, etc. The biggest difference between good, better, and best, though, is sound quality.
Earlier in the article we explained how the preamp of the mixer is controlled by the gain knob. The preamps of a mixer are an excellent example of what makes one portable field mixer more expensive than another. A basic preamp can be manufactured very inexpensively. Video cameras all have inexpensive preamps built into them. The problem with inexpensive preamps is that they don't sound nearly as clean as a properly built preamp.
If you've ever turned up the volume of a stereo system all the way with no music or sound playing, chances are you could hear a hum when the volume was maxed out. That sound is called the "noise floor." The noise floor of an inexpensive piece of equipment is much higher than a properly built one. As a professional sound person, one of the many things you are offering your clients is a much higher caliber of sound that simply cannot be obtained with inexpensive equipment. Your skills as an audio engineer are crucial for getting good sound as well, but without quality equipment it's impossible.
Other features that go into higher quality mixers:
Essential equipment used with portable mixers
In a production situation involving video cameras, you often have to move around. When one shot is finished and the camera or audio has to be relocated, breakaway cables will make the process of connecting and disconnecting your audio a lot easier and faster. These cables are essentially three cables bundled into one. They have two XLR cables for the master output of your portable field mixer to plug into the inputs on the camera. Some breakaway cables also have a stereo mini-plug to plug into the headphone output of the camera, so you can monitor audio in the camera, from your mixer. They're referred to as "breakaway cables" because they have a mechanism near the camera's end of the cable that detaches the hose of the cable — in a snap you can disconnect the audio from the video camera, without out having to unplug the XLR's and headphone cable from the camera's inputs. Breakaway cables help keep the pace of a production fast and they give the camera operator peace of mind because the audio people don't have to touch the camera more than necessary.
You can't underestimate the value and importance of having good headphones to complement your portable field mixer. Good fidelity headphones are as important to a sound person as a camera's viewfinder is to a cinematographer.
Portable field mixers and portable wireless systems are as deeply espoused to one another in electronic field production as burgers and fries are in the traditional dietary habits of many North Americans.
Accessories for advanced audio field production professionals:
In situations where you need to operate a boompole a short distance away from your mixer, the Duplex Boom Cable allows you to keep monitoring your audio with a high-quality belt clip snake system.
No audio bag should be without an ample supply of audio adapters. Comprehensive has adapter kits that will cover you in all situations.
It's also really important to have barrel adapters to cover you in situations where you may need to transform a line signal to a mic signal, or to pad a signal down 30dB. B&H sells a wide variety of barrel adapters that just might save the day for you down the road.
All those tools in your audio bag can get a bit heavy during the long production day. Do yourself a favor by distributing the weight with an audio bag harness.
Easily keep your cables organized and ready on your audio bag or on your belt with the Rip-Tie Carabiner Cable Carrier.
Below we've created a chart with some of the popular portable audio mixers sold at B&H:
If you'd like to learn more about electronic field production, be sure to check out these other B&H educational articles:
Thanks for reading this Pro Audio Insight article! Naturally, if you have any further questions about portable field mixers, electronic field production, or any questions about professional audio in general, don't hesitate to contact us at 1-800-416-5090.
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