I was having a very interesting discussion on a swedish music forum about filters and how they work. Someone claimed a filter could never boost frequencies and that filters and EQ's are different things. Alot of people stood by his side and a swedish book used by schools teaching sound engineering stated that a filter can only cut frequencies.
However this is not what I've learnt. When looking at a low/highpass, bandpass and bandreject filters.. well sure. But once the filters are resonant they suddenly can boost frequencies, there's also peaking and shelving filters. I don't see an EQ and a filter as different things. Filters are used in EQ's for the various bands.
I've looked around some though and the interesting thing is that a majority of sources seem to state that a filter is used to adjust the frequency content of sound by cutting and/or boosting. However there is a small number of sources, both international and swedish state that EQ's and filters are different things and that filters are only used to cut frequencies.
It would be great to hear some thoughts from developers and people skilled at electronics. Some digital filters, based on FIR seem to only be able to cut frequencies, while others based on IIR using a feedback connection seem to be able to boost. But this wouldn't explain the confusion, would it? In general filtering seem to be discussed with analog gear in mind and you still find alot of analog gear using resonant filters.
Any ideas and speculations on this subject would be very welcome.
/Majken
Is a filter able to boost frequencies?
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- KVRian
- 1023 posts since 8 Apr, 2003 from Östersund
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- KVRAF
- 13444 posts since 14 Nov, 2000 from Hannover / Germany
I allways thought an EQ was some sort of a filter as well. Or vice versa.
But then, maybe in the end a filter is indeed just some passive device that you route parts of the signal back in in case you want that resonance effect.
I'm not sure...
But then, maybe in the end a filter is indeed just some passive device that you route parts of the signal back in in case you want that resonance effect.
I'm not sure...
There are 3 kinds of people:
Those who can do maths and those who can't.
Those who can do maths and those who can't.
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- KVRist
- 265 posts since 26 May, 2004 from NYC
An eq is a set of filters which mix the source back (50/50) into the signal path i.e. the filters are in parallel with added to the source material. This is generally an internal not an external routing issue.
An eq may have gain compensation per band or before/after the run of filters to compensate for volume differences. Some hardware eq's only cut although the controls seem to say otherwise, they may do this for noise reasons as cutting will not add to the noise floor while boosting will.
You are also completely correct in that a resonant filter is by definition 'boosting' the signal buy reintroducing more of itself to the filter, a feedback path. Interesting to note that filtering in the digital domain (iir or fir) is accomplished via delay ! So ask your buddies why these are called filters not delays ?
An eq may have gain compensation per band or before/after the run of filters to compensate for volume differences. Some hardware eq's only cut although the controls seem to say otherwise, they may do this for noise reasons as cutting will not add to the noise floor while boosting will.
You are also completely correct in that a resonant filter is by definition 'boosting' the signal buy reintroducing more of itself to the filter, a feedback path. Interesting to note that filtering in the digital domain (iir or fir) is accomplished via delay ! So ask your buddies why these are called filters not delays ?
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- KVRAF
- 12235 posts since 18 Aug, 2003
A filter is simply anything through which a signal is passed and modified/processed. Technically speaking, anything we call an effect is a filter.
An EQ's operation is twofold: it isolates a band of frequencies and then applies gain control to that band.
If you want to know more about filters, follow all the links from the Wikipedia page.
Cheers,
Steve
An EQ's operation is twofold: it isolates a band of frequencies and then applies gain control to that band.
If you want to know more about filters, follow all the links from the Wikipedia page.
Cheers,
Steve
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- KVRian
- 775 posts since 13 Jul, 2004 from Hull, United Kingdom
Okay, here is the 101 on filters:-Majken wrote:I was having a very interesting discussion on a swedish music forum about filters and how they work. Someone claimed a filter could never boost frequencies and that filters and EQ's are different things. Alot of people stood by his side and a swedish book used by schools teaching sound engineering stated that a filter can only cut frequencies.
However this is not what I've learnt. When looking at a low/highpass, bandpass and bandreject filters.. well sure. But once the filters are resonant they suddenly can boost frequencies, there's also peaking and shelving filters. I don't see an EQ and a filter as different things. Filters are used in EQ's for the various bands.
I've looked around some though and the interesting thing is that a majority of sources seem to state that a filter is used to adjust the frequency content of sound by cutting and/or boosting. However there is a small number of sources, both international and swedish state that EQ's and filters are different things and that filters are only used to cut frequencies.
It would be great to hear some thoughts from developers and people skilled at electronics. Some digital filters, based on FIR seem to only be able to cut frequencies, while others based on IIR using a feedback connection seem to be able to boost. But this wouldn't explain the confusion, would it? In general filtering seem to be discussed with analog gear in mind and you still find alot of analog gear using resonant filters.
Any ideas and speculations on this subject would be very welcome.
/Majken
1. striktly speaking a filter, as the name implies removes something from something else to produce a new result - in the case of audio filters it does this by attenuating the incoming signal and distorting the phase of the various harmonics in that signal - this may lead you to believe that a filter cannot in that case boost frequencies but that is not so as...
2. most filters used in synthesis are known as active filters which is to say that they amplify the outgoing signal on some way (for example the signal modulating a carrier at the cutoff frequency), in addition to this, the carrier of the amplifier can itself create a pure sine at the cutoff frequency if driven hard enough by the filters resonance (and it should be noted that the sharper the fall off of the filter, the greater the resonance will be relative to the rest of the signal). So what about eq...
3. a simple parametric eq could consist of a number of sets of a low pass and high pass filter that are overlapping with the highpass filter having a lower cutoff frequency than the low pass filter - these are active filters so the output is subject to gain from each - if this gain is set higher than the basic threshold any harmoniics falling within the bandwidth of the filter should be boosted - it is more complex than this but at its most basic.
4. passive filters cannot boost a signals harmonics, they always remove something from the signal.
If you want to know more about this you should check out the early synth secrets articles from Sound on Sound (they are on-line although I cannot remember the address off the top of my head).
Have a better one - Saul Cross