How to Master Audio Mixing for Live Production and Streaming
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.
Next we’ll look at equalization (EQ) adjustments (Figure 3). 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.
Figure 4. Pan controls and potentiometers
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.
Figure 5. Some mixers have separate left and right main mix sliders; others have just one knob.
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.
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.
Figure 6. Feedback as shown on a scope
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.
Figure 7. The Behringer Eurorack mixer has a built-in feedback detector.
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.
Figure 8. Behringer’s Shark FBQ100
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.
This article appears in the May/June 2015 issue of Streaming Media as "How to Master Audio Mixing for Live Production and Streaming.”
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