Practical EQ — From Plugin Knob to DSP

And now for something you might recognise… We built filters from samples, delays, coefficients and feedback. Put a friendly graph on the front and we arrive at the EQ interface engineers use every day.

Plugin view → response curve → coefficients: three views of the same filter.

1. A familiar draggable EQ

Click a node to select it; drag to move it. Left/right = frequency; up/down = gain for bell/shelf bands.

Frequency
Gain
Q
Stages

2. Hear it

Volume 10%

3. What does the computer see?

Same machine — different coefficients.

Low-pass, high-pass, band-pass, notch and bell filters can look like fundamentally different kinds of processor. But they do not necessarily require fundamentally different DSP structures.

The same basic biquad can become different filter types simply by calculating different values for its coefficients:

b0   b1   b2   |   a1   a2

Change how those coefficients combine the current input, previous inputs and previous outputs, and the same structure can produce a low-pass, high-pass, band-pass, notch or bell response.

So these are not fundamentally different species of filter. They are different behaviours produced by changing the numbers inside a familiar DSP structure.

The button on the plugin says what we want. The coefficients tell the computer how to do it.

4. More nodes = more little filters

Adding another parametric-EQ node commonly adds another filter section with its own coefficients. These sections are often cascaded in series.

BAND 1
→
BAND 2
→
BAND 3
→ OUTPUT
More FIR taps make one FIR longer. More EQ nodes commonly mean more complete filter sections.

5. Cascading

Select Low-pass or High-pass and increase Stages. Repeating the section in series makes the overall response steeper.

We'll meet this again: complex processors are often many simple stages connected together. More stages also mean different phase/time behaviour and more computation — steeper is not automatically better.

6. Why do EQs have “character”?

Curve & Q behaviour
Nominally similar controls can produce different shapes.
Phase response
Topology changes phase as well as magnitude.
Band interaction
Multiple sections need not combine identically.
Nonlinearity / implementation
Saturation, oversampling and analogue modelling can add further behaviour.
Character needn't be mystical: different designs make different engineering — and sometimes aesthetic — compromises.

7. Series isn't the only topology

SERIES
IN → FILTER A → FILTER B → OUT
PARALLEL
IN branches → A / B → Σ → OUT
Same building blocks, different topology → different behaviour. Keep this idea: it returns in phasers and reverb.

8. The engineering lesson

There is rarely a perfect solution — only the right compromise for the job at hand.
Filter because you have a reason, not because the channel happens to have an HPF button.

Next: all-pass — magnitude stays flat while phase changes. Mix it with dry and we get a phaser.