How does an EQ plugin work?

[MisterLevy]

I was looking at the EQ curve on my plugin and was wondering if each band is processed in series or parallel.

For example is my HPF processed and then that sound is processed by my next band which may be a bell filter. Then the sound of that is processed by the next EQ band I add. As if I am adding a new plugin filter for each band.

On my EQ plugin it will show a curve that is created by all the bands I have added. I have noticed that when I add a lot of bands to some extreme settings and then I add another band to a moderate setting it will not affect the main curve that the EQ displays. So is that band being processed at all and is it having any effect on the sound? (My ears aren’t trained well enough to hear a difference)

I guess what I am saying is does the plugin just process that one main EQ curve or each curve individually.

plus…

Should I even care?
Feel free to tell me I’m stupid to even ask a question like this or that I have not made any sense with my query.

[bozmillar]

95% of plugin EQs run in series, exactly the way you described here.

Some EQs run in parallel, where instead of having a bell filter that boost at 1k, you make a bandpass filter at 1k and add it to the original signal. When you do it this way, you get different results once you start adding more bands. Think of it like this (the fact that you are asking this question at all makes me assume the next stuff I say will make sense to you).

Series EQ: If you boost 6dB at 1kHz, and then boost 6dB with another band at 1kHz, you will end up with a 12 dB boost at 1kHz. You can test this with pretty much any EQ and that is the result you will get.

Parallel EQ: To boost 6dB at 1 kHz, you take your input and blend it with bandpass filter at 1kHz. The bandpass filter will have a gain of 1, so that when you add them together, you get a gain of 2 (or 6dB) at 1kHz. But if you take another bandpass filter with the same gain as the first and add it in parallel with the other filter, you end up with a gain of 3 (or 9.5dB).

Each of those parallel filters on their own would be exactly like the series filter. But when you combine multiple bands, they don’t act the same.

The filters in the +10db EQ plugin run in parallel, because that’s how the hardware did it. TDR nova is an EQ that works in parallel. http://www.tokyodawn.net/tdr-nova/

This page explains it in more detail: https://vladgsound.wordpress.com/2014/08/09/nova-67p-and-parallel-equalizers-explained/

From a plugin design perspective, parallel eq’s are a bit tricker to do, and take more CPU. The CPU hit comes from when the EQ has a cut instead of a boost, you actually have to run through the filter twice to do the feedback loop on the negative gain bands. But the upside is that it is better for dynamic EQs, because you don’t have to constantly recalculate the filter coefficients when changing the gain.

[MisterLevy]

Cool thanks for that.

I don’t fully understand the details of a parallel EQ but I understand the principle. You explained it well.

So with an EQ that runs in series does it make much of a difference which order you add each band/filter/shelf?

[bozmillar]

So with an EQ that runs in series does it make much of a difference which order you add each band/filter/shelf?

It makes no difference whatsoever. Unless, of course, there are distortion stages in between each filter. Then it does make a difference.

[holster]

@bozmillar

95% of plugin EQs run in series, exactly the way you described here.

Some EQs run in parallel, where instead of having a bell filter that boost at 1k, you make a bandpass filter at 1k and add it to the original signal. When you do it this way, you get different results once you start adding more bands. Think of it like this (the fact that you are asking this question at all makes me assume the next stuff I say will make sense to you).

Series EQ: If you boost 6dB at 1kHz, and then boost 6dB with another band at 1kHz, you will end up with a 12 dB boost at 1kHz. You can test this with pretty much any EQ and that is the result you will get.

Parallel EQ: To boost 6dB at 1 kHz, you take your input and blend it with bandpass filter at 1kHz. The bandpass filter will have a gain of 1, so that when you add them together, you get a gain of 2 (or 6dB) at 1kHz. But if you take another bandpass filter with the same gain as the first and add it in parallel with the other filter, you end up with a gain of 3 (or 9.5dB).

Each of those parallel filters on their own would be exactly like the series filter. But when you combine multiple bands, they don’t act the same.

The filters in the +10db EQ plugin run in parallel, because that’s how the hardware did it. TDR nova is an EQ that works in parallel. TDR Nova | Tokyo Dawn Records

This page explains it in more detail: https://vladgsound.wordpress.com/2014/08/09/nova-67p-and-parallel-equalizers-explained/

From a plugin design perspective, parallel eq’s are a bit tricker to do, and take more CPU. The CPU hit comes from when the EQ has a cut instead of a boost, you actually have to run through the filter twice to do the feedback loop on the negative gain bands. But the upside is that it is better for dynamic EQs, because you don’t have to constantly recalculate the filter coefficients when changing the gain.

How did I miss this thread before? I was JUST looking for info on this. Thanks @bozmillar! Hope you had a great Christmas!

[OMas]

bozmillar got right. MisterLevy, if you’d like to dig into this a bit, you might want to read the user manual for thEQblue https://www.maat.digital/support/#manuals, which goes into some detail on series versus parallel as it includes several implementations of both topologies.