What is Aud­yssey’s Dynamic EQ and Dynamic Volume?
Audyssey’s Chris Kyriakakis explains how these automatic sound control features work and what we can expect in the future.
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Audyssey has developed technology to correct sound distortion automatically.
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November 06, 2008 by Dennis Burger

If you have a passage of music that has loud and soft parts when you’re at high volumes, and then you lower the volume, the soft parts need more correction than the louder parts. So we had to invent a way of measuring perceived loudness in every channel, running in real time. And that’s what Dynamic EQ does. Every channel has its own loudness meter that looks at the content as its coming through and estimates—based on the signal and our human models—how a human will perceive it. So it makes minute adjustments, and of course it doesn’t just rapidly go up and down or you would notice it. We smooth it out. That’s part of the innovation here—that’s why we called it Dynamic.

So how did you develop the human model for this?
We work a lot with professional mixers at the university because of Tom [Holman]’s connections. So we did a number of experiments with them. In one of them we put them in front of the console and turned down the master fader, and the first reaction they had to their own mixes was, “what happened to my surrounds?” It turns out that loudness perception is spatial—it falls off faster behind us than it does in front of us. And we asked these mixers, “OK, you’re down 10dB, what would you do to the surrounds to maintain the surround impression?” And they would move it up, and at different levels they would move it up by different amounts. So if you do that with enough people you can come up with a set of rules that mimic what they’re doing.

So we integrated that into Dynamic EQ—as you turn the volume down, the surround levels go up a little or a lot, depending on how far down you are, to maintain the impression of surround. And the best way to demo that is to turn the volume down 20dB and turn off Dynamic EQ, and all of the sound collapses to the front.

How does Dynamic Volume fit into all of this?
So, continuing with our story of the mixers—we had probably 20 people who participated in this set of experiments, initiated by Tom Holman, who was puzzled for a long time by one particular problem: why is it that a mix that is done in the dubbing stage doesn’t work as well in our home theaters? Why do we need to keep adjusting the volume—up to hear the dialogue and down when the action gets heavy? Why do we need to do it at home, but not in the movie theater? We have high end equipment. What’s going on?

What we found was that the content is mixed for movie theater systems with extremely high dynamic range—very powerful amps, big speakers, in huge rooms. But when you bring that into the home the sound pressure levels that you perceive change, the perceived dynamic range changes, and it’s affected by the size of the room and the ability of your system. So the first thought was, We have to control the dynamic range, but we can’t do it by putting a compressor in the signal path, because that doesn’t work—it introduces artifacts.

By “compressor,” you mean things like Night-time Mode or Midnight Mode, and similar dynamic range controls on many A/V receivers, which make loud sounds quieter and quiet sounds louder…
Exactly. So to find a better solution, we brought mixers in to work with content that they knew and said, “Pretend your daughter is sleeping next door, and make adjustments to this mix in real time.” And we set up a system to electronically track their movements and in parallel record the content in 5.1. When we were done we had a seven- or eight-terabyte database that we gave to our research team and said, “We need to reverse engineer their decisions. Why did they turn the sound down when they did, and when they did turn it down how fast were they reacting? What were they reacting to? What was the content in every channel?” And we came up with an algorithm that mimicked those decisions. And so Dynamic Volume came out of these experiments. It uses a look-ahead method, with a small buffer, and looks at what’s about to play in each channel…

How far does it look ahead?
That’s adjustable. It depends on who we’re working with. A TV manufacturer will give us a lot because they’re already doing video processing and they don’t care. An A/V receiver manufacturer will not give us as much look-ahead time—it’s about half a frame. The more the better, obviously, but you don’t want to be introducing delay more than you have for video, so we’re limited by that. 

So, aside from the look-ahead buffer, what makes Dynamic Volume different from other dynamic range controls?
Well, the average person adjusts the volume level so dialogue sounds right. That’s what’s important to them. They just don’t want to have to run from the room when the explosions come, or at commercial breaks. So we keep dialogue as reference, and then monitor the signal above or below and decide how much to bring it up or down. The trick was to do that without using a compressor, as I mentioned before, because a compressor has a fixed time constant. If things are too soft it boosts, and if things are too loud it cuts. But it doesn’t have any knowledge of how quickly they’re becoming soft or loud. And so by the time it catches most things they’re already changing in the other direction.

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