Question about using a lav mic.

I

ironpony

Carbonite Member
I don't really understand the concept of how it works. The lav mic is wired to a device that gives it gain and fader. This device then sends a radio signal to another device, that needs to be fed, phantom power. But if I plug this other device into a camera or field recorder, more preferably, I will also have to fed that device gain and fader, along with phantom power. So we have two gains and two faders now. Why? Why does the lav come with one if I plug into an external field recorder anyway. Why doesn't the lav itself just record with it's own fader and gain, and save it two it's own datacard? Or why doesn't it just come with no gain and fader, and just rely on the external recorder's?

It seems to me that that's like having to plug a camera, into another camera, just to be able to power it, and save data. It seems redundant, and I don't understand how to use it therefore, unless I am missing something of course.
 
This probably deserves a lengthier answer than I have time to write at the moment, but let me see if I can clarify.

You're really asking about wireless microphones, not lavaliers in particular. A wireless mic can be of any physical configuration or directional pattern, or conversely, a lavalier mic might be used with a conventional cable in many setups. But, yes, using a lavalier mic with a wireless connection is one of the most common applications, so I'll assume that's what you will be dealing with here.

It's probably easiest to think of the wireless system as a substitute for the cable running between the microphone and camera (or mixer, or recorder). It consists of two pieces, a transmitter and a receiver. Generally, the transmitter (worn by the subject) will be able to provide power to the microphone, and receives an audio signal from it through an ordinary wire. The transmitter pre-amplifies the microphone audio and uses it to modulate a radio frequency signal that is transmitted through a short antenna.

The receiver's function is to accept this radio signal and demodulate it to audio again, which is fed into the camera/recorder/mixer at a voltage (level) equivalent to a standard microphone output, or, depending on your gear and preferences, amplified to a line level signal.

Because the transmitter and receiver are active devices containing amplifying electronics, like all such circuits, they introduce some noise into the signal chain, and have a limit on the maximum signal amplitude they can handle without distortion. To make the most of the dynamic range, or more simply, the "space" between the noise floor and the maximum possible level, the transmitter will have an input sensitivity adjustment to match it properly to the specific microphone you are using, and the voice (or other source) you are recording. You want to transmit as high a signal as possible without creating distortion by overloading the device.

Similarly, the receiver will have an audio output level control that allows you to adjust it between microphone (low) and line (higher) signal voltages, and precisely tune it to the requirements of your camera, recorder or mixer. Some units may lack such an adjustment and require you to do all the settings on the recording device itself; the output level control is a useful feature, but not essential, strictly speaking.

So compared to using a simple wire between the microphone and the recording device, there are one or two audio adjustments that need to be set to use a wireless system successfully, one each at the transmitter and receiver. Fortunately, these are not usually extremely "touchy" settings that have to be changed for each shot; you can generally set up the system once at the beginning of the day or when moving to a new scene/location, and they will remain usable unless there are gross changes in the conditions.

"Phantom power" does not come into play too much when working with wireless equipment. At the microphone end, the transmitter will typically provide what is called "plug-in power" through a third contact on the microphone plug which is sufficient to operate small electret condenser microphones like most lavaliers, but is at a lower voltage (and with an entirely different wiring arrangement) than full-scale phantom power. The transmitter itself is powered by batteries, typically a couple of AA cells, either single-use or rechargeable.

The receiver, also, will be powered by internal batteries for the most part (or in some cases, plugged into AC, as in a fixed installation in a theatre or club), and does not need (nor should it be provided with) phantom power from the camera/mixer/recorder. You should turn this off when using a wireless receiver as an input device.

There is one aspect of using wireless systems that I have (deliberately) not touched on here, which is selecting an appropriate radio frequency for the link between the transmitter and receiver; it's a whole treatise in itself and is usually covered in some depth in the manufacturer's manuals and other literature. Rental houses are also often a good source of advice about what frequencies are legal and will work in a given area.

In short, adding a wireless microphone does involve using two active electronic devices to substitute for the passive cable used to hook up wired microphones, and these do require additional attention to power and setting audio levels appropriately. But it is not by any means comparable to "plugging a camera into another camera." I've probably over-simplified a number of aspects here, but hope this will give you a useful general overview.

As a suggestion, you might want to visit a video set or a theatre production where wireless microphones are in use, and see for yourself how they're set up and connected into the sound system. Or just visit a sound or music store and ask for a demonstration; maybe you could even rent a unit for a short time to experiment with it.

- Greg
 
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Yeah OK I think there are some misconceptions here:

1) It's puff-puff-PASS

2) A lav or lavaliere mic is a mic. It's a device that converts changes in sound pressure into changes in electrical current. No more and no less.

3) Gain is the ratio of input signal to output signal. Its a number.

4) A fader is a physical control on a mixer. Its a sliding variable resistor.

5) Wireless receivers are never operated on phantom power supplies. Phantom power is (as far as I know) only used to energize condenser microphones, and is of a very low amperage...like 1-2 milliamps. A single AA battery can put out up to 700 milliamps.

6) Read more books and stickies about audio.
 
The short answer is by the time you hear the output of a microphone it's been through multiple gain stages anyway. When you choose to capture everything yourself then you're the one providing all those gain stages in a collection of gear.
 
Let me take advantage of this thread being up top and ask here, as I have only seen discussion of wireless Lav use, and that does not apply to me.
Forgive the elementary nature of the question please.

If I bought a DPA 4071 lav (as an example) to use wired direct to a Zoom H4N recorder for voice narration, (all use would be in fixed locations/non-moving scenarios) are there any additional connections between the mic and ZoomH4N aside from an XLR adapter to plug the mic into the Zoom?

In other words, for wired operation, is it as simple as LAV>LAV POWER>XLR>RECORDER?

Many thanks for your help-- and again, I appreciate your patience in helping me figure this out.
I'm looking at lavaliers because I have less than ideal environments to record in, and hope that my deeper radio-like voice will lay better with a close lav without collecting the environmental noise a shotgun might pick up. But all anyone talks about is wireless, which seems a waste to me for my uses.
 
If you have a lav power supply... which typically looks like a tube that an XLR plugs into... then yes, that's all you need.

Some recorders have a mini-jack mic input... and then you don't even need the power supply. You could plug a Sennheiser (or whatever) lav with a mini-jack right into it.
 
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