External Filter vs. Native Filter

[Libertine Lush]

Hi everyone,

I'm curious to know if an external filter affects the sound of a synthesizer as if it was the synthesizer's internal/native filter.

So if you set a software/hardware synth's lowpass filter to fully open and resonance to zero, then filter the signal with a plugin filter/hardware synthesizer/filter module, is it being filtered as if the external filter were the native filter? Or are there some minor discrepancies?

My curiosity is born out of my love for the new Cyotomic filters in Ableton 9.5, sounding as gorgeous as you'd want any filter to sound and my desire to use them on so many of my software/hardware synths.

Thank you.

[stillshaded]

if it's a monophonic synth: yes.

If it's polyphonic: no, because there is one filter per voice.

[Libertine Lush]

If it's polyphonic: no, because there is one filter per voice.

Even though there isn't a filter for each individual voice, when the signal from a polyphonic synth is sent through an external filter, the totality of it is being filtered.

So I can better appreciate the distinction, could you explain how the sound is different when used on a polyphonic synth?

[ghettosynth]

If it's polyphonic: no, because there is one filter per voice.

Even though there isn't a filter for each individual voice, when the signal from a polyphonic synth is sent through an external filter, the totality of it is being filtered.

So I can better appreciate the distinction, could you explain how the sound is different when used on a polyphonic synth?

It's not always the same with a monophonic synth either, it depends on how the filter in the synth is being modulated With a polysynth, each filter envelope affects each voice independently. If you are not modulating the filter, then the result is similar. There may be some difference owing to how individual voices drive the filter and interact with resonance, however.

[stillshaded]

certainly.

You kind of answered the question yourself. When you send a polyphonic synth through a single filter, all of the voices are being summed into one stereo (usually) signal that will be filtered as a whole. Think of it as putting eq on a channel. this is essentially what you are doing.

Now, within a polyphonic synth, each oscillator is sent through it's own filter, and the output of these filters will be summed (usually). think of this as if you had several monosynths and an eq on each one. Essentially the same thing.

Now, you can run a polyphonic synth through a single filter, of course. It will just yeild different results. But they can be cool.


I used to have a korg poly-800. cheapest synth with an analog filter around. Miss that thing. It only had one filter for all the voices. But it was triggered by an the envelope of the most recent voice played. It yeilded some cool results!

In conclusion.. don't listen to me.. listen to your ears.

[Libertine Lush]

stillshaded and ghettosynth, thank you! I think I understand the difference better now.

[RichieWitch]

Consider this too.

The filters inside a synthesizer have access to the envelopes and LFOs of the synth. These modulation sources are what breathe life and character into the filters. Your external filters will not (usually) have access to these modulation sources, although your external filter might have its own modulation or you can add external envelope generators and LFOs.

I have five different external filters that I regularly use with my hardware synths. When I use them, I generally completely open the filter in the synth, and neutralize any modulation being used on the internal filter. Having extra filters can really expand your sonic palette.

[Libertine Lush]

I have five different external filters that I regularly use with my hardware synths. When I use them, I generally completely open the filter in the synth, and neutralize any modulation being used on the internal filter. Having extra filters can really expand your sonic palette.

Good to know. That's how I've been using an external filter, but wasn't sure if I was using it correctly.

[ghettosynth]

I have five different external filters that I regularly use with my hardware synths. When I use them, I generally completely open the filter in the synth, and neutralize any modulation being used on the internal filter. Having extra filters can really expand your sonic palette.

Good to know. That's how I've been using an external filter, but wasn't sure if I was using it correctly.

It's not necessarily right or wrong. The question you should ask is why do you want to use an external filter? You could be trying to get something of a paraphonic sweep, which is what the parent is alluding to. Often, you want to just add the character of the external filter's response. In that case, you may not want to open up the synth's filter, it completely depends on the sound. I often use external filters in subtle ways, for example turning up the resonance and then playing with the cutoff on sounds that have no or few harmonics on their own, e.g., a kick drum.

Or, perhaps you want the modulation on the internal filter, for example, you have a filter sound with a lot of motion owing to an LFO, but, you want to bring the high end in slowly. This is common in dance music. Again, you just leave the sound alone and sweep the external filter manually.

There's no right or wrong way to use it, every situation is different. As a DJ, I always have filters on the DJ busses these days, always! They are probably used more than any other knob on the mixer, including the levels and when I'm playing records I have no control at all over the modulation that's in the records.

It's really just elementary synthesis, there's nothing special about an external filter. It's just a monophonic filter "effect."

[Libertine Lush]

Or, perhaps you want the modulation on the internal filter, for example, you have a filter sound with a lot of motion owing to an LFO, but, you want to bring the high end in slowly. This is common in dance music. Again, you just leave the sound alone and sweep the external filter manually.

Why would you use an external filter to bring in the high end instead of the internal filter? Would the effect not be identical, except for the difference in how the sweep will sound? Or perhaps that's what's implied.

Thank you for the examples. They're exactly what I needed.

[RichieWitch]

It's not necessarily right or wrong. The question you should ask is why do you want to use an external filter?

I agree with this completely.

For my taste, I find that using multiple filters in series begins to remove so much of the harmonic information that you are left with a rather dull, flat sound. Keep in mind that filter cutoff removes harmonics while resonance adds in a (usually) completely different set of harmonics. Filter slopes are additive, so two 4-pole filters become an 8-pole filter when used in series (assuming the cutoff is the same). Any signal eventually devolves to a simple sine wave given enough filtering.

With that in mind, I find running the left channel of the synth through one external filter, running the right channel through a different filter, then blending the two together leads to some very interesting, highly-customizable sounds. This signal path puts the filters in parallel rather than in series.

All that being said, here's something I did to really learn my filters.

Set up a test tone, maybe a saw wave on a long-running C3 note. Record this note in your DAW while doing filter sweeps, then resonance sweeps. If you do this for every filter you own, including your synths internal filter, you'll have a nice set of reference recordings that will teach you the "voice" of each of your filters. This can really help you when you're looking to design a particular sound. For multi-mode filters, you'll have to make multiple recordings, one for each output mode.

[chk071]

Not sure if i get the question right, but also a point to consider is that the synths internal filter, if there's no option to completely bypass, or turn it off, will always affect the sound, even if cutoff is fully opened. I don't know if that's valid for every filter, but most of the ones i came along in soft synths already take away some of the upper frequences spectrum fully opened. So, if you don't want the internal filter to alter the sound, turn it off if possible.

[RichieWitch]

Not sure if i get the question right, but also a point to consider is that the synths internal filter, if there's no option to completely bypass, or turn it off, will always affect the sound, even if cutoff is fully opened. I don't know if that's valid for every filter, but most of the ones i came along in soft synths already take away some of the upper frequences spectrum fully opened. So, if you don't want the internal filter to alter the sound, turn it off if possible.

Yep, I agree with this too. I've looked at my synth outputs on a spectrum analyzer, and even with the low pass filter fully open, you can see the frequency cutoff remains at the upper end of the frequency range, about 18 kHz on my Korg MS2000R, for example. I think that's what gives the MS2000R its slightly hollow, woody sound.

[ghettosynth]

Or, perhaps you want the modulation on the internal filter, for example, you have a filter sound with a lot of motion owing to an LFO, but, you want to bring the high end in slowly. This is common in dance music. Again, you just leave the sound alone and sweep the external filter manually.

Why would you use an external filter to bring in the high end instead of the internal filter? Would the effect not be identical, except for the difference in how the sweep will sound? Or perhaps that's what's implied.

Thank you for the examples. They're exactly what I needed.

No, not necessarily. The internal filter may be modulated by many things, the external filter will not. This effect can be particularly noticeable if the resonance is capable of being modulated. You may have some complex set of interactions going on with the internal filter such that just changing the cutoff materially changes the timbre of the sound in a way that is different from just adding an external unmodulated filter. You may want that timbre to remain intact over the course of the sweep. I do this quite a bit with my synthacon filter in my modular. It's really animated and everything interacts with everything else so it's not just as simple as sweeping its own cutoff to just get a darker version of the same sound.

One thing that you should probably also realize is that the best use of these filters is probably not on just a synth. I use them far more often on busses. Everything from drum busses to the entire mix. They're also great for getting different timbres out of sample based instruments.

Finally, don't overlook different modes. An external hipass is very useful for just taking a bit of the mud out of the low end in a way that has a bit more character in the high end than just using an EQ.

[ghettosynth]

It's not necessarily right or wrong. The question you should ask is why do you want to use an external filter?

I agree with this completely.

For my taste, I find that using multiple filters in series begins to remove so much of the harmonic information that you are left with a rather dull, flat sound. Keep in mind that filter cutoff removes harmonics while resonance adds in a (usually) completely different set of harmonics. Filter slopes are additive, so two 4-pole filters become an 8-pole filter when used in series (assuming the cutoff is the same). Any signal eventually devolves to a simple sine wave given enough filtering.

Exactly. I don't often use the 8 pole mode in my FR-777 because it's just too dark. In fact, the most common use for it is when I just want insta-dark on whatever the current sound is.