Filters and EQ

Frequency Response Curve

Plots amplitude vs. frequency

Most accurate = Linear/flat amplitude across frequency spectrum

Practical devices ALTER this and in doing so, act as a kind of what? A filter for frequency content!

http://www.headphone.com/buildAGraph.php

Filters and EQ

The most common form of signal processing

Controls amplitude of various frequencies

Controls harmonic or timbral content of a sound

Filter types

4 basic types of filters High pass Low pass Band pass Band reject

Aka notch

High Pass Filter

Pass HighsAttenuate Lows

-cutoff frequency-slope

Stop band Pass band

Low Pass Filter

Pass LowsAttenuate Highs

-cutoff frequency-slope

Stop bandPass band

Band Pass Filter

Center Frequency

Band Pass Filter components

Passes a band of frequencies

2 Cutoff Frequencies

Center Frequency

Quality Factor (Q) Width of bandwidth Narrower = higher Q Q=fc/BW

Fc = center frequency; BW=bandwidth

Band Reject Filter (notch)

Shelving Filters

Discussed briefly in tech 1, but had little use of them with synthesis

Shelving filters are used for attenuation near the extremes of the frequency band High shelf and low shelf; names are inverse of what

the do High shelves remove low frequencies Low shelves remove high frequencies

Shelving filters to not remove frequencies entirely, but instead boost or attenuate after the use of high and low pass filters

Shelving Filters

Equalization

Group of filters sent the same signal in parallel

Each filter is usually a narrow bandpass

Filters are combined to create output sound

Spectrum shaper = each filter has its own output control

Equalization (contd.)

2 types of EQs Graphic EQ

Fixed center frequency, fixed bandwidth, fixed !

Usually features linear faders

Equalization (contd.)

Parametric EQ Fewer filters than graphic Can adjust cf, Q, cut/boost

Importance of EQ

Good equalization is possibly the most important factor in digital audio signal processing

Your processed sounds are only as good as your source recordings

Some things are unavoidable, such as quantization error, but other factors can be limited and removed through EQ

Importance of EQ (contd.)

Low-pass filter – removes high frequency noise, hiss Recordings with high amounts of white noise can

benefit from a high-pass filter. White noise contains random distribution of frequencies across the frequency band; applying a highpass filter removes higher frequency content and softens the added noise to sound more like pink noise

High-pass filter – removes low rumbles, “thud” sounds, woofiness

Importance of EQ

Human ears perceive mid-range frequencies at higher amplitudes than high and low-range frequencies (more on that later)

Recordings that contain high-level mid-range frequencies can sound blurry and unclear Using notch filters to reduce, but not remove, mid-

range frequencies can add clarity to recorded sounds (let’s look at that graphic equalizer on slide 13 again)