Beginner's Mixing & Mastering

Part 10 -- Filtering & Gating

A typical bell filter boosts by 18 dB at 1 KHz, Q set from .5 to 6.0.

Band-pass Filter
Allows a specific band of frequencies to pass while blocking the others, by passing chosen frequency range surrounding the centre frequency. Has centre frequency instead of cutoff.
It's obsolete by even free multipass compressors, though.

A typical band-pass has centre frequency of 1 KHz, slope at 18 dB per octave. 

Band-stop
Passes frequencies except those ranged around the centre frequency. Like band-pass, band-stop has centre frequency instead of cutoff frequency.
eg. a band-stop can cut frequency down from ~15k+ to ~9k - ~6k to reduce harshness of a cymbal.

A typical band-stop has centre frequency of 1 KHz, slope at 6 dB per octave.

Sometimes, the band-stop is simply another name for Notch Filter, as they have the same response shape. Sometimes tricky to work the steepness of middle slope, where output is nearly zero. Be sure to adjust while the song plays for real-time determination.

All-pass Filter
Passes frequencies equally without actually boosting or cutting. Creates a unique phase shift similar to a time delay where affected frequencies exit later than arrival to input.
Although the main source of phase shift, it's also another means to create stereo from a mono signal. This pseudo-stereo signal allows the mixer to be a bit more unique, if desired.

An all-pass can also induce vastness and spaciousness by applying different values to the left and right channels, which won't combine the separate signals. In extreme parameters, they make audio seemingly come from outside the speakers.
Useful for Dolby Surround and cinematic music, especially when modulated to continuously change overtime. This creates an effect similar to a Leslie rotating speaker but without the Doppler effect (when pitch changes while the rotating horn driver turns).

Phase Shift / Phasing
Not to be confused with polarity!
Phasing refers to frequency and timing, delaying certain frequencies by defined amounts, whereas polarity is whether a frequency is positive or negative, there's no adjustment to time. Reversing polarity is not the same as flipping the phase shift 180 degrees!

Phasing is a filter producing an effect similar to time delay, where certain frequencies exit an all-pass filter later than arrival to input. It's sometimes blamed for causing audio issues, though in small amounts, phasing is often inaudible.
Also refers to a certain modulation effect, as described in the previous chapter as a mod, but here, its explained from a filtering perspective.

Can alter ratio of peak and volume without affecting level, tone or clarity. The reduced peak also helps increase volume without needing a limiter (eg. as done for broadcasters). When used alongside a limiter, additional volume can be gained without lowering quality.

A stereo synthesizer is like a phaser unit, but uses the phase shifted output twice while reversing the polarity of one. This can be done in your DAW to make things sound synthy. In fact, phasing is the basis for filtering, EQ and comb filtering.

Comb Filtering
An unusual filter, not logarithmic and instead having many narrow peaks and nulls, all equally set at linear intervals with the same Hertz. If time delay used instead of phasing, results in repeating peaks and nulls. Comb filtering is the most unusual phenomena in audio engineering, as it affects us from music to radio to your microwave!

Flanging was described in the previous chapter as a mod. Here, I describe it in the context of filtering, where you won't actually hear the phase shifting but instead the swooshy sensation. You're hearing response changes, regardless of boosting treble in EQ (which simply moves comb filtering into high-frequency).

The flanging effect is in fact a result of Comb Filtering, characteristic for its hollow, robotic sound.

Comb filtering also occurs whenever audio is combined with a delayed version of itself, the delay time having some frequencies flipped 180 degrees. So, if the original audio is positive, the delayed version is negative, and vice-versa. If original and delayed audio are at same volume, the nulls deepen while peaks boosted by 6 dB.

Comb filtering helps induce motion, giving life to static sounds, such as to pads, leads, ambience, rhythms, or repetitive guitar strums. Faster speeds may induce warbling or vibrato.

Note: If the original audio is combined with the delayed when shifted 180 degrees, the audiowaves cancel each other and you may get complete silence; other frequencies of same multiples may also cancel each other out.

eg. a delay time shifting 100 Hz by 180 degrees also shifts 300 Hz per cycle plus its own 180 degrees, creating deepest nulls at 100 Hz, 300 Hz, 500 Hz, and so forth.

A comb filter can be somewhat replicated in real-life when sound bounces off a hard, smooth surface then combines with air pressure along with original audio waves.

eg. a microphone placed down the lid of a grand piano receives delay from the direct to bouncing sounds, creating an acoustic comb filter. If you boost treble in EQ here, the EQ won't create the effect, but expand it.

Comb filtering may also occur when recordings had multiple microphones. eg. when a microphone by a snare drum also received the kick drum, changing the low end.

Note: Sometimes, flanging is hard to hear when using microphones or speakers while standing around a dense boundary, as each ear gets different audiowaves, caused by diffusion.

Comb filtering sometimes heard in AM radio. The flanging occurs when your receiver gets a direct signal from a transmitting antenna and a delayed version bouncing from the ionosphere, usually during evenings.

For FM radio, you may have heard of picket fencing, where the signal rapidly fades in and out, where the receiving antenna experiences deep nulls caused by waves bouncing off hard, smooth surfaces especially when slowing down, such as a school bus driving by and stopping at traffic lights. This is another case of comb filtering.

Ever wonder why reception dropouts happen with wireless microphones? Sometimes called multi-path fading, occurs when a path comes from two signals but one delayed, aka comb filtering. The solution is a diversity system, where multiple receivers and antennas are spaced apart, so when the transmitter (on the performer) moves on stage, all peaks and nulls change continuously, preventing dropouts.

Why do microwave ovens have a rotating base? Or why is the rotor hidden while attached to a microwave antenna? Why are there uneven hot and cold spots on the food?

Did you ever guess it was--you guessed it--comb filtering? Where the food is hot, there were peaks. Where food is cold, there were nulls. Waves and frequencies absolutely do affect us in our everyday lives, not just in music!

#9
Gating
A Noise Gate (aka downward expander) is a bit like a limiter, but blocks low sounds until reaching a certain threshold, then the low sounds are released.

If your music has unwanted sounds like coughs, lip smack, buzzes, burping, amplifier hums, or what have you, this will remove them. Some 3rd party VSTs have deep amplifier hums so a noise gate can remove that, too.

As a beginner, you are unlikely to come across these issues, but this is nevertheless worth knowing! Can also be used on recorded samples, excerpts from movies or games, or what have you.

Noise gates are useful for real drums and live performances to avoid unwanted sounds.

Some good, free gates are Renegate by Auburn Sounds or Bob Perry Gate 2.