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Audio mixing for live production and streaming.

Live audio is challenging. Unlike many of the issues that arise on the video side of the equation, none of the problems that affect audio quality--RF interference, feedback, ground loop hum, hiss, over-modulation--can be seen. All of these challenges take a knowledge and understanding of the problem to assess and fix them properly. On live productions, more often than not, the time to fix audio issues is as soon as you hear them starting to happen.

You might find yourself dealing with over-modulation on the input to the mixer, or bad equalization in your mixer. Your potentiometer or fader might be too high or too low, and you'll find yourself trying to compensate for it later in the mixer. Or your master audio may be distorting, which means you need to try to tamp it down.

Finally, there's the dreaded feedback. Most of these are problems you can learn to address by mastering the controls and features of a good audio mixer (Figure 1, opposite page), or--better yet--instructing presenters, talent, and other members of your crew on how to make proper use of the technology in their hands so that they send you a clean and usable signal.

Microphone Placement

One of the most important things you can do to ensure good audio is proper microphone placement. For example, you can use a good lavaliere microphone on the body of the speaker, as opposed to a podium microphone he or she can walk away from, or a handheld microphone that he or she can wave around the air. You might even try a headset microphone if the venue is particularly noisy. These have become much more discreet and more socially accepted when the situation warrants it.

Also, be sure your source audio won't overwhelm your microphone. Usually, this happens with music, for instance, when your performer is using a vocal microphone for drums or a harmonica and the audio distorts at the microphone. But it can also happen if you set up a microphone for someone who speaks softly, then hand it to a football coach who shouts everything to the back of the room. You can adjust the input level of your wireless microphone so that it picks up a good signal, but does not overmodulate.

With handheld mics, the as-near-the-mouth-as-possible rule applies. If you ever do weddings or other events, watch the DJ. DJs know how to work a microphone. A DJ might stand in front of the speaker--which will often cause feedback--but hold the mic extremely close to his or her mouth to avoid talking very loudly. The system will also be set so that unless the microphone goes into the speaker, it's not going to create feedback.

When it comes to an instrument mic, naturally, the best approach is to put the mic on the instrument. On some productions you could encounter, say, an acoustic guitarist who wants to play to a mic on a stand. That's a lot farther from the instrument than an on-instrument guitar mic, which will pick up the strings and the resonance of the chamber, and give you much better leeway in the rest of the chain to avoid feedback and other issues.

These are all areas in which effective mic placement--and aggressive mic guidance from you--can make a big difference in the quality of the audio you capture and deliver.

Overwhelming Source Audio

In situations where audio recording issues arise from source audio overwhelming the microphone in use, the first thing you want to check is the sound pressure level (SPL) of your microphone. Microphones have sound pressure level ratings, which vary depending on the type of microphone, the manufacturer of the microphone, and the size of the microphone.

One microphone that a lot of people use is the Shure SM-58. It's a classic vocal mic that's great for certain tasks and not so great for others. For instance, the SM-58 has an SPL rating of 94dB. To put that in perspective, some common approximate sound pressure levels include 110dB for an airplane taking off, or 100dB for a rock concert. If you're working with the SM-58 on a live event and recording a loud instrument such as a kick drum or trumpet blaring right into it, that sound can easily overwhelm this mic and distort the audio.

With wireless mics, you'll also find potential for distortion in the transmitter. You'll need to adjust the input trim. This is one strategy many people either don't know about or simply don't like to fiddle with. But in theater productions, we do this a lot because we match a microphone to a performer. Whether handheld, lavaliere, or instrument transmitter, it will have an input trim. For instance, on the Sennheiser wireless lav system, you go into the menu to access the sensitivity of the microphone (Figure 2), and you can bring it down or bring it up depending on what your situation requires.

If you're working with a performer or presenter who's going to sing or speak on stage, you'll need to adjust the gain of the transmitter to his or her voice. If you're going to clip that microphone to the horn of a trumpet, you're going to need to set it differently.

If you're adjusting input trim for a guitar, the setting will vary depending on how the microphone is used, whether you're attaching an acoustic microphone or jacking into the guitar directly, as with an electric guitar. You need to adjust the sensitivity of the transmitter appropriately; otherwise, it can distort right away and there's absolutely nothing you can do to fix it down the line.

To adjust input trim, have the presenters/performers speak or sing in a normal tone. If the level starts to bounce a little bit while the person is talking or singing, that's good. If it sits, or if it just goes up and hits the top of the audio level indicator, it's too loud. If they're talking and it doesn't light up, you'll have to add so much gain later that you'll raise the noise floor and hiss. The same principle applies to instrument mics. Have the performers play they way they will during the performance and set the transmitter's input levels accordingly.

Dealing With Hum

Another very annoying problem with audio is hum. Usually it's a 60Hz (in the U.S.) hum from AC power. It comes when the ground used for the audio is coupled to different things in different places. This happens, for instance, when the venue is mixing sound behind the stage and run a couple mic lines to you at the back of the house, and you're also plugged into a circuit that's not grounded in the same way the house audio is. Suddenly, the 60Hz hum crops up and invades everything. If you hear it when you plug in, or even touch, the long audio lines into your mixer, and it goes away as soon as you remove them, you've pinpointed the problem.

The first thing to try is a ground lift adapter. You should always have at least one of these in your kit, because they can help you solve so many different problems. That said, it's a temporary fix, and not one you should rely on in a permanent setup.

If a ground lift adapter doesn't solve the problem, try to work on the audio cables themselves. There are multiple ways of doing this. Simple accessories such as an XLR adapter can be put inline on the XLR cables to try and find which pin is carrying the offending voltage. Alternatively, there are more substantial transformer devices that completely isolate the in and out, while still passing audio. These devices can usually resolve the most stubborn hum problems.

Lastly, make sure power cables aren't taped down along with the audio cables. That's also a great way to pick up extra noise. They should be separated by at least a few inches if not a foot.

RF Noise

If you're still working with old 700mHz wireless microphones on frequencies that have been allocated for other uses--including public service--you should replace these systems as soon as possible.

If you've moved on to systems that use approved frequency ranges, you're less likely to encounter interference, but it can still happen under certain circumstances. For example, if you're filming a lecturer who has his lav belt pack just a few inches from his cellphone, you're going to get RF interference from his phone. He needs to leave it on the podium, or on a table. The farther the phone is from the microphone, the less it will affect your signal.

The same goes for your own crew. Don't put your phone on the audio mixer, even if you're using it to play music into your system. The RF noise can get into any part of the signal path.

Overmodulated Mixer Input

Another problem that arises frequently when you're mixing audio for live events is overmodulation (or overmod) on the mixer input. Several factors can cause overmodulation, some easy to avoid, others a bit more subtle. To begin, if you're feeding line-level input, don't plug it into the mic jack. It will distort. Some mixers have a universal gain that you can dial down to avoid clipping. If yours doesn't, you'll need an external PAD to knock the line level XLR down to mic level. These should be in your kit and always stay with the audio mixer.

On the other hand, if you feed a mic level source into the line-level jack, you'll find yourself cranking everything all the way up and wondering, "Why the heck am I not getting any sound out of this?" The more you crank things up, the more hiss you'll get in your signal path.

Bad Equalization

Next we'll look at equalization (EQ) adjustments (Figure 3, next page). Let's say you want someone to have a strong, booming presence in the room. You'll need to increase the gain of the low frequencies, noting that the input level you had previously set is now considerably higher. The EQ does affect the input level, even though it technically shouldn't since it's further down in the audio path. But it does, and you have to go back to the input level and readjust it after setting your EQ.

The same goes for high frequencies, although you likely won't notice them as much. But they can get shrill if your high-frequency EQ is too high and you haven't adjusted your input trim to compensate.

Pot Too High, Pan Wrong

Down at the bottom of the mixer you'll find the pan control and potentiometers (Figure 4), both of which are critical to getting a consistently professional mix. On smaller mixers, the potentiometers, or pots, are boring little knobs. On the mixer shown in Figure 4, they're much more appealing sliders. Whenever anyone walks up to a mixer, the first thing they go to touch is always the sliders.

On audio mixers, you can see that there's headroom above zero. That's gain, or amplification, of the signal. When you need to make the signal louder than it was when it came in, that's where the slider goes. If you find your slider/ pots are always above zero, it's time to recheck your input trim. It's probably too low.

When people who are not familiar with the hundreds of knobs found on an audio mixer--especially bigger mixers, with their multiple sweepable EQs, and with multiple aux sends and returns on each channel--they see the "pan" knob down at the bottom of the mixer and decide it's just another knob to ignore. If someone else has adjusted the pan on the mixer and the pan knobs are all adjusted in some way, you'll wonder, "Why am I getting sound out of only part of the system?"

Using pan controls effectively can be especially important if you're producing multiple feeds with only one mixer. If you're using one mixer for both streaming and house sound, you can take that main mix and split it. The left channel goes to your stream and your right channel main mix goes to the house. This makes it easy to push one slider up and raise the level equally on both your stream and in the room. But you have to make sure the pan is set correctly, or you might find that one of your microphones is audible only in the room, and not on the webcast stream. Mastering the pan control is critical to avoiding those situations.

Figure 5 shows a mixer that has separate left and right controls for the main mix. But if you have a smaller mixer, it will most likely have only one knob. You won't be able to adjust one channel or the other independently. The pan control will be where you adjust each channel's contribution to the main stereo output of the mixer. But even with separate Main Out sliders, you still have to pay attention to the pan setting of every signal you're working with.

Overmodulation After the Mixer

If the final mix that you export is wrong, you can actually clip the audio on your amplifier or streaming appliance/laptop input. One of the key reasons is that not all line levels are created equal. Professional line level is +4dB, and consumer line level is -10dB. If you're using a professional mixer, on the back you'll find a switch that allows you to adjust the output level; if you don't have such a switch, you'll need to adjust your mix to compensate.

If you're going to feed your mixer into a piece of consumer gear like the computer that's going to handle your web stream, make sure you're not feeding it professional line-level out. Even if you have the computer set for line input, it will still be too loud. And if you happen to plug the mixer into the computer's microphone input, you'll have to take the mixer's output way down.

As for the house mix, the power amplifiers that feed the room are set to professional line level so they expect a hotter signal. Your mixer's output for the web stream won't sound very loud in the room because the amp is expecting to get a hotter signal. You'll have to make sure that you're outputting the right level for each feed that you're delivering.

Feedback

How many times have you heard feedback during a presentation? We all know what feedback is: The room audio gets into the mic, which gets into the room, which gets into the mic. Figure 6 shows what feedback looks like on a scope. This has a spike around the 1k tone, but there's the main feedback peak and then you can actually see that there are two overtones: one above 2k and one above 5k.

If you have a 30-band EQ, you can use it to correct room audio. Operators will often notch the feedback without notching the overtone. Those frequencies are hanging around, ready to give you some feedback. These days, thankfully, feedback is easy enough to manage that we can use hardware to kill it. The Behringer Eurorack mixer that has appeared in several figures in this article has a feedback exterminator (Figure 7). All you do is push the button, and the digital circuitry puts a notch in as soon as it hears the feedback coming.

Generally, the Behringer box offers enough capabilities to handle most easy room problems. Then, of course, there's an EQ for the room as well. If you're finding that the room mix is a little bassy, you can correct for that as well.

If you don't have a mixer that handles feedback effectively, you can buy outboard devices, such as the Shark shown in Figure 8, next page. A lot of companies make similar boxes now. The Shark goes in between the mixer and the amp. It takes the output of your mixer, listens for the rising tone, and adds a digital notch that kills it. It can also be used to delay the audio so it matches your video if there's any video delay, such as through a video mixer. This can tighten up the audio sync on your stream and is a handy tool to have in your kit.

It's important, though, to note that you need to set these feedback exterminators properly to use them effectively. You'll find producers who go into a room before a big show, take out the mics, put them on a table, and slowly raise the levels. As the "silence" gets louder, they'll hear those frequencies that are first to produce feedback, and eliminate them. Then they'll raise the house a little more, hear the second note, and give it a notch. Technically, you can try and keep going, but if you do, you'll get more and more feedback. What these devices do is let you get a few dB louder in the room than you could without them. In many rooms, it will help, but in a difficult room--say, a room with marble on the walls--that's going to be tough, because it's just so reflective. You might have to keep your overall audio level much lower.

I worked in a facility where the main hall had a glass ceiling and tile floor. It was quite reverberant. They had big speakers set up on top that shot outward over the whole hall. The sound bounced around a bit before it came down, and without a feedback exterminator, I couldn't create a usable mix in the room. I could never get it loud enough on the floor, especially if there was a big crowd, because there was always a percentage of the crowd not paying attention, carrying on their own conversations, chatting, phones ringing, and so on. In those situations, feedback exterminators are essential.

Mastering the Mix

The key to getting good audio on live event productions and streams is mastering the entire signal path from the source of the audio to all of your intended destinations. There's much more to it than plugging in a mic and pushing up a slider. You need to be certain of proper audio levels all the way through the chain, and that includes several areas before and after the mixer.

You also need to know how to avoid many of the pitfalls and potential problems that inevitably arise along the way. By mastering the entire audio path, you can deliver professional-quality audio on almost any production.

Anthony Burokas (video@ieba.com) has provided corporate communication services and consulting through IEBA Communications for 20-plus years. His award-winning video has been seen on PBS for more than a decade, he helped build in-mall advertising, and he is currently transitioning to 4K. Comments? Email us at letters@streamingmedia.com, or check the masthead for other ways to contact us.
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Title Annotation:HOW to MASTER
Author:Burokas, Anthony
Publication:Streaming Media
Date:May 1, 2015
Words:3149
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