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Compressors reduce the dynamic range of audio signals that exceed a selected volume level. They turn down the loudest parts of a track, which helps to manage instruments with wide dynamic ranges such as vocals and bass. It also helps the quietest bits of the track to become easier to hear. Let's take a look at Compressor plug-in in Pro Tools. If a signal comes in beneath the Threshold of a Compressor, nothing is done to that signal. Lower volume signals are not touched unless there is some gain added to them. Now I have set this Compressor plug-in right now to have a Threshold of 0 which means that everything will come in below that, but as I spin this you will see the Threshold dropping and then this value becomes the new Threshold for when the Compressor will actually compress the signal. I'm about to playback a bass track here and I want you to watch as I lower the Threshold the signal will get lower in volume and that's what shown over here on this side where we have the Input level, the Output level, and the Gain Reduction.
So watch these as I twist the Threshold and listen to how the signal is affected. (Music playing.) If the signal crosses the Threshold the Compressor reacts according to the Attack speed parameter measured in milliseconds and we can see that right here.
The Compressor then begins to reduce the volume of the signal according the Ratio and the Knee parameters, the Ratio dictates how much the signal is compressed, for example we've got a Compression ration of 3:1 here and that means if the input signal is 6 dB over the Threshold, it will come out of the Compressor at 2 dB over the Threshold. A low Knee number indicates what's call the hard Knee setting and that means that compression will take effect very quickly applying the maximum amount of compression, while a soft Knee, if we twist this all the way to the other side will ease into the maximum amount of compression.
Now let's take a look at these as I bring the Threshold down and you can see this soft curve right here; that's a soft Knee. If I twist this back down, you will see this is a hard Knee and the angle is much more dramatic. If I change the Ratio, you will see this line get lower making the output value from the Compressor to be a lot less than what it would be with a higher Ratio. Now as long as the signal is above the Threshold, this line right here, it will be compressed. But once it falls below this Threshold, it will stay compressed until it's let go by the Release time, which is down here. It will then be allowed to return to the regular uncompressed volume. So once a signal below the Threshold, it's got 80 milliseconds here; it will still be compressed even though it's below the Threshold.
The Compressor's Gain over here 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 that are below the Threshold are still affected by the gain, and that means that the louder parts are compressed, the softer parts still have this gain, and thus you will create a track with a more uniform volume level or less of a dynamic range. And that's the whole point of using a Compressor.
So watch and listen as I tweak some of these parameters, the Attack and Release won't be that easy to hear, but you will definitely hear the effects of the Threshold, Ratio, Knee, and the Gain. (Music playing.) So how should you apply compression to a track? First, you should ask yourself whether you think the track actually needs compression. And if you think that it does because the dynamic range is too wide, start by choosing the Threshold.
A high Threshold like this will only lower the peaks. So anything that is above this Threshold will be compressed while everything down here will not be compressed. However, if you lower this way down, then almost the entire track will be compressed. So it will be constantly compressed with a low Threshold. Obviously, you can choose anywhere in between, but those are two common ways to do it, to compress the entire track or to compress just the peaks.
Next, you want to choose the Ratio. Choose 2:1 or 3:1 for light compression, 4:1 or 6:1 for more volume leveling and then as you get up to 10:1 or above, this is considered limiting and it squashes the track level. After setting the Threshold and the Ratio you can go on to the Attack and Release times. And the Attack and Release times require some thought. Now let's talk about the Attack time. The Attack time determines how quickly the Compressor reacts to a signal that's over the Threshold. So you have to consider the type of instrument and the part you are compressing and whether or not you want to compress the initial transient of the transient. 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. So we bring that way down to just a fraction of a millisecond.
However, if you want the drums transient come through the Compressor before compression is applied, then you need to set the Attack time to allow enough time for the initial drum transient to pass through before the signal is actually 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 and this makes the Compressor really work, if you set the Release time to 20 milliseconds or below. For a smoother sound use values over 100 milliseconds and maybe even longer for bass notes, because they sound better with long Releases.
Finally, we'll move over to the Gain, and this is often called Make up Gain, because it's the output gain on the Compressor and it's use 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 is reduced by 8 dB, you can actually increase the output gain up to 8 dB and not have the loudest part of the signal clip. Now 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. Now I'm going to take a listen to this track and use a preset called Steamroller and we'll hear the difference.
I'm going to bypass it first and then I'm going to drop it in and you can hear the difference. (Music playing.) So you can use that as a special effect, if you want. There are a lot of other useful presets up here, in fact the bass guitar one actually sounds pretty nice. (Music playing.) You will see the parameters, how they are set here. It's small Ratio, kind of a softer Knee, kind of a longer Attack and definitely a long Release, a lot of Make up Gain and a fairly low Threshold.
Another popular compression technique is to add a compressed copy of a track back in with the original to increase the punch of the overall sound. This is called Parallel Compression and it's common technique used on vocals guitars and drums. So let's take a look at what I have got set up here. First, I have got this two acoustic guitar tracks and I have bussed them out to Bus 1 and 2, which is picked up by this Aux track here and I have got a Compressor on it. So what we are going to hear in the Mix is the dry unaffected tracks and a compressed copy of those tracks mixed together in the overall mix. So I'm going to mute the Parallel Compression first and we'll hear the acoustic guitar tracks by themselves. Let me hit the bass as well and then I'll add in the Parallel Compressed tracks and you can hear the difference. (Music playing.) So you can hear that there is obviously a volume difference, but there is also a Sonic difference too. It gives it a little bit more power with the Parallel Compression in there and some clarity, but also has still some dynamics left over the dry tracks. Adding compression and limiting to your mix correctly take 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 in a variety of instruments. Revisit this video to review the explanations of the compression parameter, if needed. Use correctly, compression can make your mixes sound more powerful, more balanced, and more radio ready.
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