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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 part of a track, which helps to manage instruments with wide dynamic ranges, like vocals and bass, and helps the quietest bits of a track to become easier to hear. Let's take a look at the standard compressor limiter dynamics 3 plug-in in Pro Tools. I've got one loaded up here on the bass track. If a signal comes into the compressor beneath the threshold, nothing is done to the signal.
Lower volume signals are not touched except by the Output Gain Adjuster, which is right here. As we decrease the threshold, signals can be touched by the compressor. Right now, the threshold is set at -30 dB. So if a signal is above -30 dB, it will be compressed. Now let's try this with the bass actually playing. I am going to bring the threshold all the way back to 0 and press Play.
As I bring the threshold down, watch the levels over here, the input, the output, and the gain reduction. (Music playing.) If the signal crosses the threshold, the compressor reacts according to the attack speed parameter, measured in milliseconds.
The ATTACK right now is set at 10 milliseconds. The compressor then begins to reduce the volume of the signal according to the ratio and the knee. The ratio down here dictates how much the signal is compressed. For example, a compression ratio of 3 to 1, as we have here, means that an input of 6 dB over the threshold will come out of the compressor at 2 dB. Compressors with ratios of 10 to 1 or higher are considered limiters.
Now, we can boost this ratio all the way up to 100 to 1. That's pretty steep. Let's talk about the knee value, which is right up here. A low knee number like this indicates a hard knee setting, and means compression will take effect very quickly, applying the maximum amount of compression. If we twist this and create a soft knee that means the compressor will ease into the maximum amount of compression.
You can see this represented in the graph up here. This is a soft curve right here, a soft knee. However, if we twist this back down, it becomes much more angular, and becomes the hard knee. A signal will stay compressed until it falls below the volume threshold. Once the signal is below the threshold, it's still compressed until being let go at the release time, and it's then allowed to return to its regular uncompressed volume.
So once this signal is below the threshold, it will still be compressed for 80 milliseconds, as shown here, even though it's below the threshold. 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 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 limiter.
So how do you apply compression to a track? First, you should ask yourself whether you think the track actually needs compression. If you think that it does because the dynamic range is too wide, start by choosing the threshold. A high threshold, like this, only lowers the peaks of the signal; a lower threshold will constantly compress the track. Obviously, you can choose anywhere in between, but honestly those are the two common ways to compress: either compressing the track constantly or just touching the peaks.
We'll bring the threshold down. I am going to press Play now, and I am going to adjust the threshold so that I get a certain amount of gain reduction. (Music playing.) I am going to try to get it to average around -3dB in the gain reduction. So that means that I am going to set the threshold right around here, around -18 dB. After setting the threshold we can set the ratio.
You can use a light ratio of 2 to 1 or 3 to 1 for some light compression, boost this up to 4 to 1 or 6 to 1 for more volume leveling, and anything over 10 to 1 is considered limiting, and that squashes the track level. So let's give it some heavier compression. We'll give it 6 to 1 here, or thereabouts. After you set the ratio move on to the 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 consider the type of instrument and part that you're compressing, and whether or not you want to compress the initial transient of the instrument. For example, the initial transient on a drum track is always very fast. So if you want to compress the initial transient on a drum, the compressor's attack time has to be extremely short, just a fraction of a millisecond, and you can do that here. You can go down to 10 microseconds.
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. The release parameter on a compressor is just as important as the attack because it determines how long the compressor stays active once the 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 to 20 milliseconds or less.
For a smoother sound, use values of over 100 milliseconds, or even longer releases on bass tracks. So I am going to increase this. 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 has been compressed out of the loudest parts of the signal. If a signal comes into the compressor and it's reduced by 8 dB, you can increase the output gain and add 8 dB back to the signal without the loudest parts clipping.
So when I set the threshold, I set it so that we got about 3 dB of gain reduction. So I could lift this back up to about 3 dB. Let's take a listen to this track. (Music playing.) Now, you'll see that the gain reduction was actually more, so I can boost this higher, and now we'll listen and compare using the bypass button.
(Music playing.) Aside from just controlling the dynamics of a 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. Let's go over here and choose the Steamroller preset. Now, there are a lot of other useful presets up here; in fact, the bass guitar one actually sounds pretty nice.
But we are going to try the Steamroller and check out how that sounds. (Music playing.) So obviously we are using this preset to create a distorted tone with the compressor. What about trying the bass guitar setting? (Music playing.) Yeah, that sounds nice, and look at these parameters, what we are set at.
Got a low ratio, a soft knee, pretty long attack time, and a very long release, a medium threshold, and a lot of gain makeup. Another popular technique is to add a compressed copy of a track back to the original to increase the punch of the overall sound. This is called parallel compression, and it's a common technique used on vocals, guitars, and drums. Let's take a look at what I've got set up here. Let me close this. Now, I have got these two acoustic guitar tracks, and I've bussed them out to Bus 1-2, which is picked up by this aux track right here, Bus 1-2, and I've got a compressor on the aux track.
So, what we are going to hear is the mix between the dry unaffected tracks that are going out the main outputs and a compressed copy of those tracks mixed together in the overall mix. So I am going to mute the parallel compression track first, so we can hear just the guitars, and I'll mute the bass as well. And then I'll unmute the parallel compression, and we'll hear what it sounds like. (Music playing.) 00:10:15.09] So you can hear that there is obviously a volume difference here, but there's also a sonic difference too.
It gives us a little bit more power with the parallel compression in there, and some clarity, but also it still has some dynamics left over from the dry tracks. Now, let's talk about limiters. Limiters are essentially compressors with ratios of 10 to 1, or higher. The 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. As you saw before, you can select a ratio of up to 100 to 1 in the dynamics three 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 often used on the same types of dynamic tracks as 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-peak signal. I am going to open up the maxim plug-in here.
Limiters like maxim here are often used on submixes and almost always in mastering applications to make sure that the tracks don't peak or cause unwanted distortion. Let's check out how adding this maximum limiter to the master fader track in the session can boost the output signal by reducing the dynamic range of the song, but without creating distortion. We don't want to push it too hard, or else you will get distortion. But let's hear how this sounds as I change the threshold and the ceiling values. (Music playing.) So you can hear in this example that you can bring the threshold down, keep the ceiling high, and get the track to be a little louder without adding any distortion.
However, if you bring the threshold down too low and keep the ceiling high, it's definitely going to create distortion on the track because you are trying to push it too hard. Adding compression and limiting to your mix correctly takes some knowledge of the parameters. As you are 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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