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The TB-03 has a knob labelled 'CUT OFF FREQ'. I assume it´s a cut-off filter because Roland´s manual describes this knob as 'Filter cutoff (brightness of the sound)':

knob labelled "CUT OFF FREQ"

The Allen&Heath Xone 4D mixer has a filter called 'Low-pass filter':

Filter labelled "LPF" with "FREQ" knob below

When I turn both knobs it seems to me that they do the same thing: they filter out the high frequencies and pass the low frequencies.

Therefore my question: What is the difference between a low-pass filter and a cut-off filter?

Note: I think there is no difference. I think the term 'cutoff' refers to the point where Low-pass and High-pass filters begin to cut off the frequency.

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  • Are you absolutely sure that the "cut off frequency" means you have a "cut off filter"? Low pass filters require a cut off frequency.
    – user50691
    Commented Jul 3, 2018 at 11:09
  • it seems that in the audio industry "cut off" is used as a generic term for band pass filters, high pass as well as low pass, etc. I am not used to this use of terminology.
    – user50691
    Commented Jul 3, 2018 at 11:17
  • @ggcg: regarding your first question, here my answer: "I assume it´s an Cut-off filter".
    – Simon
    Commented Jul 3, 2018 at 12:10
  • See my second. It is typical for specialized industries to make their own jargon out of physics and engineering terms. I am not that adept at synth, or the use of effects on my guitar (I play clean). But from a purely signal processing point of view "cut off" is a term that applies to all filters so it's hard to tell. Which is why I'm asking in comments rather than answering. I find it interesting.
    – user50691
    Commented Jul 3, 2018 at 12:20
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    You may find it interesting that the "wah-wah" pedal, is effectively a variable band-pass filter - that is, it cuts off all frequencies both below and above certain points. These cut off points are changed more or less in parallel so that the 'slot' of frequencies that are passed shifts upwards or downwards as the control is used.
    – user33337
    Commented Jul 3, 2018 at 16:31

4 Answers 4

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"Cut-off" is ambiguous, since it could be cutting off either low or high-frequency material. Since the manual you quote refers to 'brightness' of sound, my guess is that it's a low-pass, high-cut filter.

For completeness' sake, a "bandpass" filter is just that: cuts everything above and below a specified frequency range.

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  • @David Z: Yes, his description of a bandpass filter sounds also accurate to me. But both filters, the one of the TB03 and the one of the Xone 4D are no bandpass filters.
    – Simon
    Commented Jul 3, 2018 at 13:45
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Every filter requires a cutoff frequency, which determines the point at which the filter begins to attenuate the frequencies in the appropriate direction (low pass filter attenuates high frequencies, hipass filter attenuates low frequencies).

This is almost certainly a low pass filter, and the knob controls the cutoff frequency, thus controls the brightness of the sound based upon setting the frequency beyond which we begin to attenuate.

Every filter has a cutoff frequency. I’ve never seen the term “cutoff filter” used in a professional context.

Note: you will almost never find a hi pass filter on a synth or drum machine that does NOT have a low pass filter. Hi pass filters are considered somewhat of a luxury on hardware and are quite rare to find. I cannot think of a single synth or drum machine that has a hi pass filter which does NOT have a low pass filter. Although, they probably exist (very rare) and perhaps someone can share one in the comments if they know of one. The rule of thumb is to always assume a low pass filter. Also, band pass filters are even more rare.

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  • “Every filter has a cutoff frequency” – you wouldn't call it cutoff though for many filters. That term doesn't make sense for notch filters, shelving filters, allpass filters, etc. etc.. For hi- and low pass filters I've heard it quite often though. As for hi pass filters... I know nothing about drum machines, however it's certainly not such an uncommon feature on general synths. But yeah, low pass filters are always much more common.
    – leftaroundabout
    Commented Jul 3, 2018 at 13:55
  • It is extremely uncommon on a synthesizer to have a hi pass filter and NOT have a low pass filter. In the grand scheme of hardware synths, the majority do not have hi pass filters, but yes they are not terribly uncommon. As for shelving filters, the term “cutoff” seems wholly appropriate, given that if you attenuate the frequencies beyond that point you effectively have a low / hi pass. If you boost the frequencies instead you have inverted the filter and effectively attenuate the opposite end. Notch filters use a cutoff to determine where the notch begins, and Q to determine the width.
    – user51507
    Commented Jul 3, 2018 at 13:59
  • Allpass filters are essentially comb filters. Their cutoff value simply adjusts the phase relationship based on a certain frequency. It does effectively provide a multi notch filter that is dependent on the frequency and the notes played. In a technical jargon sense, one would still expect to hear this lingo imo.
    – user51507
    Commented Jul 3, 2018 at 14:01
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    Um, allpass filters have little to do with comb filters; those rather act like a cascade of notch filters. And like with notches, their frequency parameter doesn't set “where the notch begins”, but instead the singular frequency that is completely removed. What allpass filters do is simply rotate the phase of the signal without affecting its amplitude – which is similar to what a delay does. And a simple comb filter is built by mixing a signal with a constant-delay copy, I reckon that's what you meant there.
    – leftaroundabout
    Commented Jul 3, 2018 at 14:08
  • @leftaroundabout you’re right, I was thinking of an allpass filter being constructed from a feed forward and feedback comb filter. But that’s true, it does have a different spectrum than a simple comb filter.
    – user51507
    Commented Jul 3, 2018 at 15:44
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In this case, both filters appear to do the same, according to the symbols and language. High and lowpass filters, however, can also be implemented as "shelving" filters which is pretty common for equalizing purposes. In that case, the frequency response becomes level ("a shelf") again when going significantly beyond the corner frequency of the filter.

The cutoff filter symbol for the Allen&Heath appears to suggest a resonant bump in the frequency response at the corner frequency before going down. That makes for a steeper filter curve at the cost of, well, a bump.

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  • "... bump..." See "GIbbs Overshoot Phenomenon"
    – Carl Witthoft
    Commented Jul 3, 2018 at 11:11
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    This is nothing to do with the Gibbs phenomenon. It's a by-product of one approach to designing second-order filters, and musically it is quite useful, since many acoustic instruments have a similar "bump" at the top end of their frequency response. Usually, you can control the width of the "bump" with a control labeled "Q factor" or similar, and the center frequency of the bump with the "cut off". If you set the Q to 1, the effect is the same as a simple low-pass filter without a bump in response at the corner frequency.
    – user19146
    Commented Jul 3, 2018 at 11:33
  • Really "Many acoustic instruments"? Can you provide some links to test data on orchestral instruments?
    – Carl Witthoft
    Commented Jul 3, 2018 at 15:30
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If you look at the devices these knobs are on - a bass synth/sequencer and a DJ mixer offering some weird 'effects' - not just zooming in on one knob, you'll see that they are both part of a resonant filter with both a Cut-off control and a Resonance one. It's more than a 'tone control' setup.

It's generally futile to analyse the labels on music-tech gear too pedantically. But in this case I think we can say they do just about exactly the same thing.

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  • It sounds like you experience with these devices is quite similar to mine and labeling the knobs is affected by the perspective of the designers.
    – skinny peacock
    Commented Jul 4, 2018 at 18:54

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