The Difference Between Dynamic EQs and Multiband Compressors
Dynamic EQs and multiband compressors look similar, have similar controls, and behave similarly. With so many similarities, it can be difficult to identify how they are dissimilar. The difference between dynamic EQs and multiband compressors lays buried in their design. Dynamic EQs use their input audio signal as a sidechain source to trigger a parametric EQ that affects signal amplitude. Multiband compressors split their input audio signal using multiple filters, and the signal from each filter is run through its own compressor; these signals are then summed together to create the output signal.
Seems like a bit of a long and confusing explanation, right? The issue is that if you try to simplify the terminology any more than this, you begin to make statements that aren’t technically accurate. Ironically, trying to simplify the differences between dynamic EQs and multiband compressors can cause more confusion than clarification. I’ll be walking you through the design concept that powers each type of device, and I'll then provide you with some situations in which you may choose to use one type of device over the other.
Before getting into the nitty gritty of it all, there's one key difference I'd like to point out between these two devices. A dynamic EQ is capable of reducing and expanding the dynamic range of an audio signal, while a compressor that's acting as a compressor, and not as an expander, is only capable of reducing dynamic range. Great, so that's the difference? Well, unfortunately it's not that simple. Many multiband compressors allow you toggle each band between a compression mode and expansion mode, making the difference between dynamic EQs and multiband compressors just as convoluted as before. Hang in there, we'll get to the bottom of this!
If you're interested in learning how to use a compressor, check out "The Ultimate Guide to Compression."
How Multiband Compressors Work
Multiband compressors split the incoming audio signal into different frequency ranges, or bands, using bandpass or crossover filters. The audio from each filter is run through its own compressor which allows you to control settings like threshold, ratio, attack, and release. All the signals are then summed together to create the output signal. For example, a three-band multiband compressor would require three separate filters and three different compressors. These additional electronic components make analog multiband compressors quite expensive in comparison to their full-bandwidth counterparts (normal compressors).
When the incoming audio signal is split via the use of filters, a phase shift occurs at the crossovers; the points where the filters overlap. Whether or not this phase shift sounds “good” or “bad” is irrelevant. The point is that the audio signal has changed in some way. I’d like to note that some plugins like the Waves Linear Phase Multiband Compressor and the FabFilter Pro-MB do not cause a phase shift (if set to linear phase mode), but this feature is unique to the digital world.
If you decide to use a multiband compressor on an aux track to apply parallel compression, some frequencies found at the crossover points may be out of phase with the original signal. This doesn’t necessarily mean “don’t use a multiband compressor for parallel compression,” but it is something to keep in mind. Being aware of variables like this can help you listen for less apparent issues, and make more well-informed mixing decisions.
How Dynamic EQs Work
A dynamic EQ runs the input signal through a parametric EQ. This parametric EQ is manipulated by a control signal coming from its sidechain circuitry. The dynamic EQ’s input signal is used as the sidechain input and split using filters. Each signal is run through a level detection component that reacts based on the threshold, attack, and release that you've set. If the input signal has triggered the level detection component, a control signal is produced that tells the parametric EQ how to behave. In some cases this might mean applying a cut, and in other cases it might mean applying a boost.
You’ll notice in the diagram that there are no filters apart of the green signal path running through the parametric EQ. There are only filters present in the sidechain circuitry, which means that your input signal will remain free of phase shift when there is no control signal triggering the parametric EQ. This allows dynamic EQs to be more transparent than multiband compressors.
Narrow Bandwidths vs. Wide Bandwidths
Dynamic EQs tend to allow for extremely narrow bandwidth settings, while multiband compressors do not. Dynamic EQs are great for surgical forms of processing. Dealing with sharp resonant frequencies in your audio signal is something that a dynamic EQ is well suited for.
Transparency vs. Color
It seems as though dynamic EQs are often made to be transparent. After all, their design allows them to be this way. It would seem almost counter intuitive to create a colorful dynamic EQ. With the rise of plugins, manufacturers aren’t bound by the physical limitations of hardware anymore, so there are very transparent multiband compressors out there as well.
Lots of plugin compressors are modeled after analog units, so they aim to capture every detail found in the hardware. These details can include saturation, noise, hum, a feed-forward design, a feedback design, etc. People often reach for a compressor to color their audio signal on top of reducing its dynamic range.
Both analog dynamic EQs and analog multiband compressors exist. I've heard people say many times that dynamic EQs only come in the form of plugins, but this is untrue. The BSS DPR-901 II is a perfect example of an analog dynamic EQ. Analog dynamic EQs aren't something you'll find in most studios, and for good reason: Not that many of them exist, they're expensive, and the transparency they aim for can be achieved just as easily with a plugin.
These aren’t hard and fast rules stating that dynamic EQs have to be transparent and multiband compressors have to have a lot of character. This is merely a common difference between both types of devices and may allow you to find the right tool for the task at hand more easily.
It’s difficult to make any universal statements about the differences between dynamic EQs and multiband compressors, other than that they differ in their fundamental design. The results they produce can be quite similar to one another, or quite different depending on two main factors; the device being used, and the way in which you're using the device. If you’re trying to apply surgical processing, you’re probably better off reaching for a dynamic EQ, and if you’re trying to add some color to your signal, your best bet is to start by reaching for a multiband compressor.
Don’t lose sleep over how each device works. The right tool for the job is whichever one gets the job done. If you don’t like how a dynamic EQ sounds on your material, try a multiband compressor or vice versa; it’s as simple as that.
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