Audio Myths-Busting series EP.4 "High-Pass Filter, Low Cut Frequency" (Q&A Myths Demystified)
*Beware wall of text* (as always)
"EQ are not easy as it look. Proper EQ can make or When it's not proper. Can break a tracks"
"The common misconception that you should high pass all of your tracks except the kick and the bass."
Q: Apply High Pass Filter on the track that doesn't need Low end, isn't that make sense?
A: EQ isn't so simple as it look, Because every filters introduce phase shifting into your signal, It's affected the phase coherence.
Example: i use tone generator to generate sine wave at 1kHz and I do a HPF at 100Hz. In theory it shouldn't affect the sound at 1kHz at all, Because 100Hz HPF and 1kHz is too far away from each other. The result. is sound the same, It's not affected in the sound wave of course. But it affected in a phase coherency. This is actually how filter work "There is always some phase shifting happening when using a filter" The more steep of the filter slope the stronger effect of phase shifting become.
Q: Normal bell EQ does not affect from phase shifting?
A: No. The Bell EQ actually is just a 2 filter combine, No matter what cut or boost.
Q: Ok, We got the phase shift and we got another phase shift on every channel, That shouldn't matter right?
A: Not completely. EQ doesn't linearly affected the same way. Which mean if i start to sweeping frequency of tone generator around, The spectrum of phase linearity start to drift around, and that is expected. Because of the way the filter works. This is something to keep in mind the next time you adding High-Pass Filter.
Q: So, I just using Linear Phase EQ to compensate for that and it should be fine?
A: It's doesn't work that easy, Because Linear Phase EQ can give you more problem than it solved the problem.
I will explain about Linear Phase EQ, But that's something for different Audio Myths-Busting episode.
Conclusions (If you read all of this, Please don't take everything too serious)
- Don't do HPF by default, is like a blindly fix something that's not gonna work.
- If you high pass your tracks too much, you lose a lot of the low end energy in your mix and the highs make it sound harsh and brittle. You're basically taking away the balls of the track. Try attenuating the lows (perhaps with a shelf) instead of cutting them.
- Because filters introduce phase shifts into your signal, you're creating phase problems which build up in the mix between certain elements - especially if your're high passing multi-tracks instruments separately. Therefore if you must high pass such instruments, high pass them together at the bus stage.
- Because of the resonant bump (regardless of visual of EQ show to you) at the cutoff point of a pass filter, don't high pass all of your tracks at the same frequency. For instance, if you're cutting all of your tracks at 80Hz, you're going to get a build up around 80Hz.
