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Compressors and limiters reduce the dynamic range of signals that exceed a selected volume level or threshold. They turn down the loudest parts of a track, which helps to manage instruments with wide dynamic ranges like vocals and bass and helps the quietest bits of these tracks become easier to hear. Let's take a look at the standard Compressor/Limiter Dynamics plug-in here in Pro Tools. Within this plug-in, you can see we've got a lot of parameters over here. And first, what I want to do is put all of these down to their lowest values, so I'll set the Ratio 1:1 and the Threshold down to 0 dB.
We'll start with that. And the Threshold parameter is where I want to start our discussion. When a signal comes into a compressor beneath the Threshold, nothing is done to it. So if I have a Threshold of 0 dB, that's the highest it can possibly be, so no signal is going to cross that. However, if I bring this down and I press play on this bass track--I've got it soloed here--you're going to see the signal coming in, and it'll turn red when it is above this threshold here. (Music Playing) So it's red here; it's above the threshold.
If I raise this up here, now when it's white, it hasn't crossed this threshold level that we've set here. (Music Playing) So now the signal is crossing the threshold pretty regularly, at this setting here. However, no compression is occurring yet, because our ratio is set to 1:1. A 1:1 ratio means that what comes in is also what comes out of the compressor. The ratio dictates how much the signal is compressed.
So if I increase this ratio up to 3:1, we'll say, then a signal that comes in at 6 dB above the threshold will come out of the compressor at 2 dB. For another example, a signal that comes in at 12 dB over the threshold will be reduced to 4 dBs over, according to this Ratio setting. Compressors with ratios of 10:1 or higher are considered limiters, and we're going to talk about limiters a little bit later in this movie. Now with this ratio set here, I'm going to increase the threshold all the way to the top, and then I'll press play and bring the threshold down, and you'll hear the bass starting to be compressed at this ratio of 3:1.
(Music Playing) And you can see that the output level is lower than the input level. And we can look here at the gain reduction--that's the GR meter right here--at how much the signal is actually being reduced. So it's in between 6 and 12 here. (Music Playing) If I raise the threshold, then we have very little compression going on.
(Music Playing) If I increase the threshold and bring it way down, then the signal is very much compressed here. (Music Playing) Now let's talk about some of these other parameters. When a signal crosses the threshold, the compressor reacts and clamps down on the signal according to the attack speed that we have set here. And this is measured in milliseconds, or it can be even in microseconds, if I roll this all the way down here to the left in microseconds.
The Knee parameter also affects how quickly the compression kicks in. A low knee number indicates a hard knee setting, meaning that the compression will take effect very quickly, applying the maximum amount of compression, while a soft knee means the compressor will ease into the maximum amount of compression. So let's take a look on the graph here. With 0 setting, this is a hard knee, and it's a very angular look here, if I twist this, then it becomes more rounded, and that's a soft knee setting right there.
We'll talk a little bit more about these in just a minute. Now let's talk about the Release parameter. So a signal will stay compressed until it falls below the volume threshold. And once the signal is below the threshold, it's still compressed until being let go at the release time, and then it's allowed to return to its regular uncompressed volume. So once a signal is below the threshold, in this case, it'll stay compressed for 80 milliseconds, even though it's below the threshold. And our last parameter is Gain over here. The compressor's gain will be applied to the output level of the signal, regardless of whether the signal is compressed or not.
That means that the uncompressed softer parts of the track are increased in relation to the compressed louder parts, creating a track with a more uniform volume level and less of a dynamic range. And that's the whole point of using a compressor or a limiter. So how do we really apply compression to a track? First, ask yourself if you actually think that the track needs compression. If you think that it does because the dynamic range is too wide, first, we want to dial in a threshold, and you can set it high if you only want to affect the peaks, or we can set it lower if we want to affect the entire track, and that will keep the track constantly compressed.
So I'm going to press play and then continue talking about these parameters here. (Music Playing) So now we've got constant compression. You'll see that the signal is constantly compressed because this is red. If we raise this up and only wanted to give the peaks, we could do that. (Music Playing) I'm going to bring this down. (Music Playing) Now we can adjust our ratio, and a ratio of 2:3 is a good number for light compression.
If we raise this up to 4:6, then that's a little more volume leveling. And if we take it up to 10 or above, then that's considered limiting, and it squashes the track. So I'm actually going to bring this down to about 6, and I'm going to stop it here. And now we want to set our attack and release times, and this requires some thought. The attack time determines how quickly the compressor reacts to a signal that's over the threshold. So you actually need to consider the type of instrument and the part that you're compressing and whether or not you want to compress the initial transient of the instrument.
So for example, the initial transient on a drum track is always very fast. So if you want to compress the initial transient on the drum, the compressor's attack has to be extremely short, and that's when we bring it down into these microseconds here. However, if you want the drum's transient to come through the compressor before compression is applied, set the Attack time to allow enough time for the initial drum transient to pass through before the signal is compressed. So in this case, we would increase the attack time. The Release parameter on a compressor is just as important as the Attack because it determines how long the compressor stays active once a signal falls below the compressor's threshold.
Short release times let the compressor cut out more quickly on notes that fall below the threshold. To make the compressor really work, set the release time at about 20 milliseconds or less. For a smoother sound, use values over 100 milliseconds and even longer releases on bass tracks like this one. So I'm going to increase this even more. Finally, move over to the Gain, and this is often called makeup gain because it's the output gain on the compressor, and it's used to make up the gain that's been compressed out of the loudest parts of the signal.
If a signal comes into the compressor and is reduced by 6 dB, you can increase the output gain to add 6 dB back to the signal without the loudest parts clipping. Let's take a listen and compare, and I'll use the Bypass button to show you what it sounds like with the compression and without it. So I'm going to play this and adjust these parameters. (Music Playing) First I'm going to increase the Threshold to give us about 6 dB of gain reduction. (Music Playing) And I'll keep all these the same for now and adjust the Gain to give us our gain back.
(Music Playing) So I'll bring this down just a little bit, and you can tell that these levels are just about the same when we bypass and un-bypass. (Music Playing) Now I'm hearing a little bit of distortion here, so what I'm going to do is actually increase the release time, and that kind of smoothes out the sound.
(Music Playing) And obviously, if you have other parameters that you want to adjust while you're working on the sound, you can go on and do that, but I'm kind of happy with the way this sounds right now. Now aside from just controlling the dynamics of the track, you can actually use compression as a special effect. For instance, you can use the Squash technique to really mess with the sound of a track. I'm going to go up to the Presets here and choose Steamroller.
And look at this setting here. We've got a ratio of 100:1, we've got a really short release time, and a lot of gain, and a deep threshold. So let's take a listen to what this sounds like on the bass. (Music Playing) And I'm going to increase the gain. (Music Playing) It sounds very distorted, and we're using this compression to create that distortion.
It's by this deep threshold and this short release time. Now let's try one that actually sounds good. I'll switch over to the Bass Guitar preset, and let's take a listen. (Music Playing) That's pretty smooth. Getting away from compressing a single track, I want to talk about another popular technique that's called parallel compression.
And this adds a compressed copy of a track to the original to increase the punch of the overall sound. It's a common technique that we use on vocals, guitars, and drums. So let's take a look at what I've got set up here. I have these two acoustic guitar tracks, and I've bussed them out to Bus 1 and 2, which is picked up at aux track down here. I've got this compressor on the track with a setting called Fatten. So what we're going to hear in this mix is the dry unaffected tracks and the compressed copy of the tracks mixed together in the overall mix.
So first, I'm going to solo these, and we'll hear the acoustic guitar tracks by themselves, and then I'll add in the parallel compression track. (Music Playing) Now I'm going to add in parallel compression. (Music Playing) So you can hear that there's obviously a volume difference, but there's also a sonic difference too.
It gives the tracks a bit more power with the parallel compression in there, and also some clarity, but it still has the dynamics left over from the dry tracks. Now let's quickly talk about limiters. Limiters are essentially compressors with ratios of 10:1 or higher. These large ratios are used to prevent transient signal peaks from exceeding a chosen level. Because of this, limiters, like compressors, allow you to increase the overall track level while avoiding clipping, and you can select up to 100:1 on the Dynamics 3 Compressor/Limiter plug-in.
That means that a signal that's 100 dB over the threshold will come out of the limiter at 1 dB over the threshold. Limiters are useful on many types of tracks and are used most often on the same types of tracks that compressors are, such as vocals and electric bass. Limiters can be used in cooperation with compressors to take care of the peaks, while the compressor performs the main compression duties on the rest of the non-peaked signal. Now let's take a look at this Maxim plug-in. Limiters, like this Maxim plug-in, are often used on submixes and almost always in mastering applications to make sure that the tracks don't peak and cause unwanted distortion.
So check out how adding this Maxim limiter to the master fader track in the session can boost the output's signal by reducing the dynamic range of the song, but without creating distortion. (Music Playing) If I bring the ceiling way down, then the volume level goes down, and ultimately, what I want to set this at is -0.1.
(Music Playing) Now why do I want to set it at -0.1? That's just in case we have a potential over or a clip at 0.0, and we just want to make sure that we don't have that so that we don't clip our MP3 file or create an over for a CD that might potentially make it skip.
So that just gives us 0.1 dB of headroom, just in case. So as you can see here in this movie, adding compression and limiting into your mix correctly takes some knowledge of the parameters as well as some focused listening. As you're learning, be sure to try out some of the presets, tweak the knobs, and listen to the effects on a variety of instruments. Revisit this video to review the explanations of the compression parameters if needed. Used correctly, compression can make your mixes sound more powerful, more balanced, and more radio-ready.
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