Welcome back to the Auria for iPad Blog’s three-part series examining the ChannelStrip. The ChannelStrip is available on the FX panel of each channel in Auria and has three components: the Expander, EQualizer, and Compressor. In the previous post, we used the Expander to clean up the kick drum track from the Auria demo song, “The Approach.” This time, we’ll continue working on the kick drum using the Compressor.
Again, before we get into using the Compressor, we’ll brush up on the theory behind compression.
A compressor is used to decrease the dynamic range of a recording. Downward Compression accomplishes this by attenuating the louder parts of the signal, while Upward Compression works by boosting quieter parts. This can be helpful when dealing a recording where there are extreme or excessive fluctuations in volume and more consistency is desired. Common applications include adding sustain to a guitar part, softening the attack of a drum, or smoothing out the dynamics of a vocal track. Downward Compression is more common, as well as being the type of compression employed by the Compressor on the ChannelStrip, so it is this type of compressor we will be examining.
A compressor works by setting a volume threshold, above which any signal will be attenuated. The amount the signal will be reduced is dictated by the ratio. For example, a ratio of 2:1 implies that an incoming signal originally peaking at 8db above the threshold will peak at only 4db above instead. With a ratio of 4:1, the same signal would peak at just 2db above the threshold. As the ratio increases, a compressor will act more like a limiter. A ratio of ∞:1 will theoretically attenuate every signal to match the level at which the threshold has been set.
Other standard controls on a Compressor include attack and release, which are both generally measured in milliseconds. The attack parameter determines how quickly the volume reduction will kick in after the threshold has been surpassed. Release, conversely, determines how quickly the attenuation will be abandoned after the signal has fallen back below the threshold.
Threshold – Sets the threshold of the compressor.
SOFT – Use this button to engage the soft knee mode.
MkUp – Use this button to engage automatic make-up gain.
Gain Reduction Meter - This five LED meter shows the average compression level.
RATIO – Sets the compressor ratio. LIM puts the compressor in limiter mode.
High Pass Side Chain Filter – Sets the cut off frequency of the side chain (control path) filter. Use the IN button to engage this filter.
ATTACK – Sets the attack time of the compressor.
RELEASE – Sets the release time of the compressor
RMS – Engages the RMS processing mode, which offers a slightly more “polite” compressor response.
CMP – When lit, this button engages the compressor module.
OUTPUT – Sets the output gain of the compressor module
Now, let’s resume working with the kick drum track from “The Approach.”
Applying the Compressor will be an inherently more subjective process than using the Expander was. Ultimately, the only true consideration is how the kick drum sound fits as part of the overall mix. However, since we’ll be focusing on the track individually, we will need to make some assumptions about what we may ultimately want to achieve. There are a number of things that we could take into account, but let’s focus on two important ones: the overall sound of the kick drum and the drummer’s performance.
Before we begin, let’s take a listen to what our kick drum sounds like, after having applied the Expander in the last tutorial:
It needs some compression!
First, make sure the entire ChannelStrip is not in bypass mode (Bypass button not lit) and that the Compressor is activated (CMP button lit). For now, set the ‘THRESH’ control low and the Ratio high, so that you can easily hear the effect of the Compressor. We’ll set these more accurately later.
Note: Having automatic make-up gain (MkUp) turned on is a convenient way to hear what effect the Compressor has, independent of volume. Enabling it will automatically increase the volume of the post-compression signal comparable to the amount of volume reduction. This way, as you adjust the controls on the Compressor, the volume remains relatively constant, and the affect of the changing parameters is more readily apparent. Realistically, however, automatic makeup gain may enact more or less post-compression gain than is desired.
To set the attack and release parameters, let’s consider the sound of the drum itself. First, notice the prominent attack of each hit. Depending on the style of music, this may or may not be a good thing. Some styles may call for more of a dull thud than something with a clean and crisp attack. In our case, we’ll want the drum to cut through the mix and provide a clear center for the drum kit, as well as the rest of the band. So, let’s set the ‘Attack’ parameter to accordingly.
To accomplish this, we’ll need to set a high enough value that the transient of each hit passes through unaffected. On the Compressor, this is likely either the 10ms or 21.5ms setting. 4.6ms kicks in too early, while 46.4ms lets too much of the hit through. 10ms should be used if you’d like to tame the attack slightly, while 21.5ms will preserve it entirely. If the Compressor is working, you should see the Gain Reduction Meter engaging shortly after the attack of each hit.
Now, let’s set the ‘Release’ control. Our goal is to have the compression fully disengage before each new drum hit, since we want each attack to be preserved. The shortest time between two hits looks to be about 120ms—we’ll set the release to 95ms since it is the longest setting below 120ms.
With the attack and release parameters are set, we’ll now set the ratio. The ratio affects how drastically each hit is compressed. A higher ratio will squash each hit and really bring out the body of the sound. If you want a resonant, booming sound, set a higher ratio. If you just want to add a little punch, a lower ratio will do. We’ll set it to 3:1 to add a bit of thump, but not an overwhelming amount.
The last thing to adjust is the threshold—this will determine how much of the signal is compressed and will have a substantialeffect on its dynamic range. If we set the threshold higher, it will attenuate only the loudest hits, decreasing the overall dynamic range slightly and adding a little consistency. If we set it low, almost every hit will be compressed, bringing them all closer to the same volume. It’s a good idea to consider the drummer’s performance here—is the dynamic variation in the kick drum part something that is desired? Or are the dynamics inconsistent in a bad way? If we need to correct an overly dynamic part, a lower threshold is the way to go. If the drummer was precise and the variations in dynamics were produced as intended, then a higher threshold will tighten the performance subtly without destroying its desirable dynamics. Again, much of this comes down to personal preference, especially when dealing with how it plays into the rest of the mix. In this case, the drummer was spot on with his dynamic choices and performance, so we’ll set the threshold relatively high, around -2.0db.
That’s it! We should now have a tight, punchy kick drum that, while noticeably more consistent, still retains most of its original dynamics. Let’s take a listen:
Sweet! Next time we’ll tackle the EQualizer…