The problem with high pass filters

  • I hope you will find this little video interesting. It shows that High Pass filtering can increase peak levels and reduce sound quality, even if the filter is set below all fundamental freqs of the sound itself.

    It comes from Paul Frindle, a guy with 40 years of experience in the pro audio and music industries. He has been part of the development of the SSL E and G-series analog consoles, the Sony Oxford digital mixing console, and many other things.

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  • the thing is: if for example you you make a high pass at 80hz, you are also boosting around 80hz +3db

    Well, it depends on the slope of the filter. Might be more, or it might be less.

    I wouldn't say this is a "problem" of high pass filtering, just the way they work. It can be advantageous or disadvantageous. This is why we mix with our ears, not our eyes :)

  • Filters have been used in recording almost since it’s inception. Just because someone shows you how they actually work, should not mean you should suddenly be scared of using them. They are a very necesary part of production and sound sculpting.

    And you are certainly not “degrading” a guitar sound by applying hi or low pass filters. Guitar amps are dirty and degraded by their very nature. All you are doing is helping the FOH engineer to avoid clash in the bottom and “distortion bees” in the highs. The advantages FAR outweigh any “mathematical danger”

    Btw, any EQ is also filtering ;)

    And in the end, the love you take is equal to the love you make.

  • Filters have been used in recording almost since it’s inception. Just because someone shows you how they actually work, should not mean you should suddenly be scared of using them. They are a very necesary part of production and sound sculpting.

    And you are certainly not “degrading” a guitar sound by applying hi or low pass filters. Guitar amps are dirty and degraded by their very nature. All you are doing is helping the FOH engineer to avoid clash in the bottom and “distortion bees” in the highs. The advantages FAR outweigh any “mathematical danger”

    Btw, any EQ is also filtering ;)

    Spot on. Use your ears and "watch" out for phase issues when using filters parallel to other paths. What is shown on the scope is phase shift of lower frequencies.

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  • What I thought the presenter was really saying is that we should be wary of just automatically using a HPF at a set frequency on every track (because youtube “experts” say so). This would have the danger of creating a significant buildup of unwanted frequencies at the crossover point. Whereas using your ears and applying HPF where required at the appropriate frequency for that specific signal would avoid this as you would have little boosts in many diff places.

  • What I thought the presenter was really saying is that we should be wary of just automatically using a HPF at a set frequency on every track (because youtube “experts” say so). This would have the danger of creating a significant buildup of unwanted frequencies at the crossover point. Whereas using your ears and applying HPF where required at the appropriate frequency for that specific signal would avoid this as you would have little boosts in many diff places.

    That's also what I got from it :)

  • Honestly, I'm not sure of the relevance for using a HP on the Kemper.

    It's interesting that a filter way below the fundamental of a sound should significantly change the waveform; that would be an unintended effect. Will probably depend on the specific implementation of the highpass filter algorithm; there may be others that will shape the sound differently. But it seems that a lot of highpass filter implementations do create harmonic distortion, even when set to low frequencies, which also explains the increase in level. As far as I understand, this may have to do with the phase response of a highpass filter - it changes the phase of a signal differently, depending on frequency.

    Overall, the increased level can be easily countered by just reducing the level after the EQ, so no real damage done. And if you don't want a highpass filter to change your signal, then don't use it. If OTOH you need to remove some lower frequencies in your signal, a HPF is your friend, and you don't care about any side effects.

    What the video does show is that using a HPF when you don't need it isn't good for your signal - fair enough...

  • The video is interesting at some level, but it doesn't tell the full story regarding linear system theory and filtering. There is no ideal filter in the real world, whether it is implemented in hardware or software. There is also potential to be misled if one does not understand the difference between the frequency and time domain. There are many concepts at play here and this video omits almost all of them. If you really want understand this then you need to understand linear system theory, Fourier transforms, the relationship between the frequency and time domain, magnitude, phase, group delay, sinusoidal superposition, Gibbs phenomenon, etc... Point being this is not a simple subject and this video does not begin to explain what is really happening. In fact, it creates more questions than answers and does not equip the user with any information to address the 'problem'. Probably best to file it in the circular file.

    Oh and the these types of effects are not limited to high pass filters. Similar issues exist with all filters.