How do filters work?

[fellis]

I've been using filters ever since I started Djing and I know what it sounds like to twist each knob. From twiddling around I've worked out that a high pass filter only allows high frequencies to pass through and for a low pass filter only low frequencies. As for the resonance I assume that it increases the volume of the frequency you have the filter set to?

If anyone with any technical knowledge out there could explain what exactly resonance is that would be great! Also, how does a filter work on a sound being played at only one frequency? (ie how can a high pass filter cut out the "low frequencies" of a note being played a 440 Hz, for example, if there is only one frequency being played? is it something to do with harmonics?)

[FascistDonut]

I know you are probably just looking for a simple answer from some knowledgeable peeps (and unfortunately I'm not qualified for that) but in case you'd be interested in reading a book about all that stuff, I would recommend the Dance Music Manual. It's got pretty technical and in depth (but very comprehensible) explanations about all that frequency and wave form jazz.

[ame]http://www.amazon.com/Dance-Music-Manual-Second-techniques/dp/0240521072[/ame]

[djhipnotikk]

first, take a look at this:

http://www.bhphotovideo.com/images/EQ-05z.jpg

it does the opposite of what you're thinking it does.

HPF: lets high freq's pass, while CUTTING others.
LPF: opposite
BPF (band-pass filter): let's a certain band of freq's through.

so as you turn the knob, more and more freq's are being cut, whether it be on the low end or the high end.

while writing this i'm finding it quite difficult to put into words as it's much easier to explain it visually.

so as you increase the filter amount (the more you twist the knob), the sloped section of that line moves along the frequency spectrum, cutting more frequencies. the actual slope of that line is usually predetermined, and referenced in db/octave. a 12 db/octave slope will be more gradual, while 60 db/octave will be steeper.

HPF's and LPF's are usually "shelving", like in the picture linked above (notice how it kind of forms a "shelf"). BPF's on the other hand are boosted/cut in something that looks like a bell, and how wide/narrow the "bell" will be is determined by what's called the Q. some EQ's have an adjustable Q (parametric)...anyway.

that's basically how filters work.

edit: forgot your question about single frequencies.

single frequencies, like a 440hz sine wave, has no other harmonics present in its purest form. so if you take a HPF on 440hz, once you pass 440hz, it should slowly be cut (because of that gradual slope down). it probably won't *completely* go away, maybe just be cut by 20db or whatever (depending on the filter).

not sure what you need clarified on resonance. everything has a resonant frequency, whether it be a speaker, a door, or you. if you've ever listened to a subwoofer, you may notice how some frequencies seem a lot louder coming out than others. this can be because the speaker itself is more resonant at those frequencies. this just means that the material of the speaker feels more natural pushing those freq's. you can do a self-demonstration of this by grabbing all kinds of different materials and sitting next to a sub that is playing various freq's, while touching the different objects. if the sound is loud enough, those different materials will vibrate at different times, because they all have different resonant freq's.

hope this helps

[fellis]

Wow thankyou so much, it's all coming together now. What I meant about resonance is that most filters have a resonance knob, and the more you crank it up the more you get that "sweeping" sound when you turn the filter knob. I was wonderig what is actually going on to actually create that sound?

[sine143]

If i'm not mistaken, Resonance is a product of the Q, which is the severity of the slope. when you crank the Q UP, the filter decreases more rapidly at the cuttoff frequency, however, RIGHT below the cut off freq (say its a lowpass) it actually increases by a very steep amount, before it drops off. think about the filter point being like a sine wave. you can only make it decrease so quickly before you actually have to increase it to decrease at such a great rate once you crank the Q up.

[patjuh90]

Correct me if i'm wrong but if you select the normal filters in traktor, they have a high ressonance whilst the xone filters have a lower ressonance?

[sine143]

That is correct. I always use the xone filter... just a choice, cause i love cheating through mixes with clever filter use.

[fellis]

ahh yes i think i understand. so changing the resonance changes the slope of the cutoff (ie increases the dB/octave units). But if you increase it past a point, you start to introduce a spike at the cutoff frequency which creates a "resonance" at the pitch of cutoff frequency which gives you that sweeping sound....yes?

[djhipnotikk]

found this that explains it pretty nicely:

Resonance effect originates from an artifact in the original cutoff filter used in old hardware synthesizers. It is narrow band of frequencies, near the cutoff level, where the sound is amplified. Today this artifact can be easily avoided, but it is still available though, because it can be used as a special effect. Changes in the cutoff level together with high resonance produces interesting phaser-like effect, which is one of the reasons for the popularity of the TB-303 synthesizer.

[lola]

interesting post! +111111