Korg Forums

I've been meaning to try this for a while ... this morning, with the help of template program 111, I figured out what settings would let you use the comb filter to generate correctly pitched plucked string type sounds and a whole lot more.

Here's the basic recipe for creating a starter program for experimenting with this really cool feature of the Radias.
_______________________________________________________________________________________________

Select an initialized Program (you'll find one in the P bank by default) and go into Program Edit mode using the EDIT/YES button and make the following changes to the Program :

1) In the Mixer section set the level of OSC1 to 0. The Radias is now silent if you play it.

2) Set Filter Routing to Serial

3) Go to the FILTER2 edit page and set the following :

FiltType = COMB
Cutoff = 48
Resonance = 127
Keytrack = +1.00

It's important that you choose those exact values, especially in the case of Cutoff and Keytrack, as they ensure correct pitching

4) Go to the EG3 edit page and set following :

Attack = 0
Decay = 10
Sustain = 0
Release = 0

5) For V.PATCH 1 set the following modulation routing :

Source = EG3
Dest = NoiseLvl
Intensity = 48
_______________________________________________________________________________________________

That's the basic setup ... play the keyboard and you'll hear something akin to plucked strings, almost
like the beginnings of a harpsichord or clavinet type sound.

More on this later ...

So you might be thinking ... how does that work ?

Have you ever been in a round tunnel or underpass where the walls were tiled and there was a lot of echo ? If you shout out a word you hear all the echoes bouncing off the walls and kind of interacting with one another for a short period of time.

When the comb filter resonance is set to a high value it behaves just like that, except the echoes happen much more quickly and each echo is also modified slightly. So in the case of the recipe I described a very short burst of noise triggered by EG3, is the initial "shout" that goes into the comb filter and bounces around in there for a while creating a whole bunch of different echoes. These echoes happen really quickly and it sounds like just a single burst of sound rather than loads of echos. It sounds like a string because when you pluck a string it doesn't just simply wobble about a bit and create just one tone, but rather the string vibrates in a whole bunch of different ways and creates a number of different overtones that are multiples of the basic pitch, resulting in a much richer sound.

Setting the Filter Cutoff to 48 and Keytrack to +1.00, effectively makes the size of the "echoey tunnel" change according to the key played ... so you get a different pitch. Just like you get a different pitch if you blow across the neck of a half full or totally empty beer bottle. This is important because we're just sending in a short burst of white noise and white noise by definition doesn't have a pitch element, so the Comb Filter has to do the work of making the sound pitched for us.

TI DSP Technote wrote: This model, while simple, is suprisingly realistic; the burst of noise
represents the plucking of the string and the comb filter, which acts
as a resonator, represents the resonating body of an string
instrument such as an acoustic guitar or bass.

More soon ...

Daz.

Next I want to talk about how you can refine this basic sound we've created and some of the kind of experiments you might want to try to expand on the basic technique.

Firstly, to make the basic sound a little more natural, increase the EG2/AMP Release to about 70, so that the sound doesn't just die when we release a key.

This is now our sonic base camp from which we can explore the various different places that we can take this. You want to save this Program as a useful starting point.

Let's try a bunch of different things and see (hear!) where they take us.

1) On the Pitch page set the AnalogTun parameter to 0. Currently it's 5 and this makes the Radias behave more like an analog synth by making the pitching of the oscillators and filter a little different each time you hit a kit. It's easier to control our Program if that isn't happening.

2) Currently the Program doesn't do much in response to velocity so lets address that and make the sound a little more expressive. We'll start with the idea of changing the length and volume of the burst of noise we send into the comb filter in response to velocity.

Go to the EG3 edit page and make the following changes :

LvlVelInt = 40
TimVelInt = -20 (note the minus sign there)

Now try playing with different velocities and notice how both volume and tone of the sound change. Setting TimVelInt to a negative value made the decay of EG3 get longer when you played harder, so a longer burst of noise is used to trigger the comb filter. Setting LvlVelInt just made EG3 vary the peak level it hits on attack and therefore makes the noise burst louder. The combined effect of these two settings gives a nice response to velocity.

_____

Sidebar : whilst you're on the EG3 page try increasing the Sustain from 0 to 20. Play and hold a chord and listen to the sound. As well as hearing a plucked kind of sound, you hear a kind of sustained fizz in there too, which is reminiscent of the sound of the sympathic strings on a sitar. Set Sustain back to 0.
_____

3) Because we selected Serial Filter routing, the noise burst is going through Filter1 before going into Filter2 which is our comb filter. By default the Cutoff Frequency of Filter1 is at it's max value, so at the moment it's not doing anything because it's low pass filter. Lets use it to modify the noise burst before it goes into the Comb filter. After having a bit of a play with the Filter1 Cutoff and Resonance to hear the effect it has, set it like so :

Filter 1 Type : LPF24
Filter 1 Cutoff : 73
Filter 1 Resonance : 32

Setting the Cutoff lower than the default 127 takes away some of the very high frequencies and setting the Resonance to a lowish value adds a little body to the sound. The sound is now more clavinet than harpsichord.

_____

Sidebar : it might be fun to flip the hood on our strange harpsichord/clavinet thing that we're creating and try strumming/scraping the strings instead of just plucking them with the keyboard. Set V.Patch #2 like so : Source is MWheel, Destination is NoiseLvl and Intensity is +24. Press and hold a chord and then after the sound has decayed try moving the modulation wheel back and forth to create the scraping/strumming effect. If you want to automate that effect, set LFO2 as the Source for V.Patch #2 instead of M.Wheel and then set the LFO2 Frequency to about 5Hz (or tempo sync it!). With some reverb or delay added to this, you could create some really cool ethereal sounds.
_____

4) Try moving the pitch bend wheel whilst holding a note. Nothing happens ! Normally, the pitch bend wheel modifies the pitch of a sound by changing the pitch of the oscillators, but we aren't using the oscillators so nothing happens. If we want to modulate pitch, we instead need to modulate the Filter Cutoff Frequency of the Comb filter instead. Try setting V.Patch #2 like so :

Source = P.Bend
Destination = Filt2Cut
Intensity = +4

Now the pitch bend wheel bends the pitch as expected.

5) Now we know how to modulate pitch we can add another little detail that makes the string pluck effect a little more realistic. Normally when a string is plucked the pitch is slight different at the beginning and then settles. If you've ever tuned a guitar you'll be familiar with this. We're just going to do something easy/lazy. EG3 is already setup to generate a short modulation blip at the beginning of the sound, so we'll use that to modulate pitch. Set V.Patch #3 like so :

Source = EG3
Destination = Filt2Cut
Intensity = +2

That just adds a nice little twang to it.

6) Lets warm the sound up a little by using the Waveshaper. Set the Waveshaper up like so :

Depth = 40
WS Type = Pickup

That's enough pseudo-clavinet action for the moment ... time for a break.

Picking up where I left off. Lets try triggering the sound we've created so far with something other than a burst of noise. How about a burst of Sawtooth ? No, too boring How about triggering it with the attack transient of a snare drum ... that's sounds like more fun ! I think you'll be surprised how this sounds too.

At the moment V.Patch #1 is set to let through a small burst of noise using EG3. Lets change that so instead of white noise we get a burst of OSC1 in PCM drum mode.

Set V.Patch #1 like so :

Source = EG3
Destination = OSC1Level
Intensity = +48

Now set OSC1 like so :

Wave = Drum PCM
WaveSel = 28 (SD-Brk1)

Before you play a chord on the keyboard, stop for a second and just try to imagine what you might be about to hear. We're sending a breakbeat snare sample through a couple of filters, what is that likely to sound like ? Now play a chord ... does that sound like anything you were imagining ?

It sounds to me rather like the classic M1 piano sound . We've entered the sonic twilight zone.

Take a few moments to try some of the other drum samples that OSC1 can spit out. I rather like the sound you get when you use 71:Tom Dry or also the Lynn Drum rimshot.

As well changing drum samples to achieve different tones, you can also use the Pitch parameter on the OSC1 edit page to change the sound and spectral mix. You'll notice that changing that Pitch setting does not change the pitch you hear when you play the keyboard ... it only changes the tone of the sound.

Just to recap. With this setup we're using the normal synthesis rules have been reversed. The Cutoff Frequency of the filter controls the pitch and the pitch of the oscillator(s) controls the tone.

Keeping this fact in mind lets try using the other source of samples that is available to us, OSC1's SYNTH PCM mode, to trigger the comb filter instead of using DRUM PCM mode. So far we've used noise and drum hits to trigger the comb filter, which worked well because both of those sound sources are effectively 'pitchless', they generate the same sound regardless of the note you played on the keyboard. The Synth PCM mode of OSC1 is pitched, so when we use that to trigger the comb filter, notes of different pitches will have radically different tones. That's not necessarily what we want, so we need a trick to defeat the normal pitching of OSC1, so that the pitch doesn't change too radically from end of the keyboard to the other. The trick is to modulate Pitch with some inverse keytracking.

Set things up like so :

1) Set OSC1 to Synth PCM mode and set WaveSel to 20 (SlapBass)
2) Set Virtual Patch # 4 like so :

Source = Keytrack
Destination = Pitch (sadly there is no modulation destination for OSC1Pitch)
Intensity = -24 (note the minus sign in there!)

Play up and down the keyboard and you'll that the tone changes subtly between high and low notes and sounds fairly natural. If you set the Intensity of V.Patch #4 to 0, then you'll hear more dramatic tonal changes, because we're not defeating the pitching of OSC1 at all. If you set the Intensity of V.Patch #4 to -48, OSC1 will play the exact same pitch regardless of which note you play on the keyboard, so the tone of our will not change at all.

When the intensity of V.Patch #4 is set to -48 you're creating the following situation; The keyboard is telling the oscillators to increase pitch by a semitone as you play successively higher notes and the inverse keytrack modulation of Pitch is doing the opposite, telling the oscillators to decrease the pitch by a semitone as you play higher notes. The net result of this little battle is that the pitch doesn't change at all. This is a useful thing to know and emulates the behaviour of the button on the Nord Lead that stops osc2 from tracking pitch.

Anyway, back to Comb Filter fun ... try out all the various samples that OSC1 Synth PCM has to offer and experiment with Filter1 cutoff, type, and resonance to modify the trigger before it goes into the comb. I also tried putting the Waveshaper before Filter1 and setting it to decimator mode, that offered up even more sound design opportunities. Also try plugging in the headset Mic and setting OSC1 to Audio In and use your god given noise generator to create different trigger sounds

If you want to get really funky add an extra timbre to the program and set it's output to the Internal Bus and then bring it into this timbre through OSC1 audio in. Or use the mod sequencer to change the drum sample used to trigger the comb continually. There are so many different avenues to explore. Changing the EG3 attack to a slower value creates effects more akin to bowing than plucking. Also try the parallel and individual filter routings which will allow you to mix the pluck sound with a more traditional kind of sound. To enhance the pluck effect, also check out the Punch Level parameter on the page where you edit the Drive/WS. Finally, for FX sounds rather than instrument sounds, try radically modulating the Filter Cutoff of the Comb Filter.

Daz.

looks very interesting. now i go buy some weed and try this tonight!

Lets go back to the beginning and try something different, something less stringy sounding.

Reload the settings described in the first post and set the Amp Release to 70.

Now do the following :

1) Increase EG3 decay from 20 to 73

2) Change V.Patch #1's Destination from NoiseLvl to OSC2Level

3) Set OSC2 to use a Triangle wave.

If you play the keyboard at this it doesn't sound terrifically interesting. However try sweeping through the various Semitone values for OSC2 as you're playing and you'll lots of different tones, some very metallic and others that seem to make the Comb filter resonate. The (fine) Tune knob also adds an extra element to the sound.

Whenever I find that something really interesting happens when sweeping a knob value through its range, that's a sure fire sign that I've found something worth modulating with an EG or LFO. So lets try modulating OSC2 pitch with LFO2. Do this :

1) Set V.Patch # 2 like so :

Source = LFO2
Destination = OSC2Pitch
Intensity = 46

2) Set OSC2 like so :

Semitone = 16
Tune = 39

3) Set LFO2 like so :

Wave = S & H
Freq = 8.25Hz
KeySync = Voice

Finally tame the comb filter a little by reducing the Resonance from 127 to 120 or so.

Play this in a low octave and what you should hear is a spacey chime sound, with a random glissando effect from the S&H LFO.

Thanks Daz,

I'll check this out soon.

Here is a little flavour of what a comb filter proggy sounds like in action.

I took the program I was discussing above that was using a snare drum to trigger the comb filter and polished up the programming by doing the following :

* Lengthened the EG3 decay a little
* Set up the amp EG properly with velocity sensitivity
* Added a little OSC2 and some noise into the burst, as well as the snare hit from OSC1
* tweaked Filter 1 to get the right tone
* Added 2 voice Unison
* Added some effects - Chorus, Reverb & Delay

short mp3 snippet here -> http://www.korgforums.com/daz/misc/radcomb.mp3

The drums and bass are from the Oasys, which I sequenced this with ... I just recorded a single pass and then recorded the result in a wave editor on my Mac. Quick and dirty, but hopefully gives an idea of what I am talking about.

Personally I think it sounds better than some of the sample based sounds I've used of this type.

Daz.

p.s. this technique is not only useful for EP, Clav, Harpsi types or other stringed stuff, but a whole bunch of cool abstract type sounds too. I just seemed to drift in this direction whilst writing up this experiment.

A useful trick to know when using the comb filter for this kind of physical modeling, is how to quickly disable the comb filter so that you hear just the burst of sound you're using to trigger it. Just set the Resonance of Filter 2 to 0 and you'll hear what your triggers burst sounds like so. I found that very useful when creating the program you hear in the mp3 snippet.

Daz.

Reading up on physical modeling I kept noticing references to the "ideal triangular pluck", which I didn't get a first because I was thinking they meant a triangle wave - which makes no sense. Then the penny dropped

If you picture the shape of EG3, using the settings I originally described (Attack = 0, Decay = 20, Sustain = 0 and Release = 0) it's something more like a single cycle of sawtooth. If you change EG3 so the Attack is 10 and the Decay = 10 (or any other pair of equally small numbers) the shape of the pluck is now triangular.

Daz.

Hey Daz - superb tutorial mate, absolutely awesome for the less experienced like me, thanks.

Anyway - I got stuck at:

6) Lets warm the sound up a little by using the Waveshaper. Set the Waveshaper up like so :

Depth = 40
WS Type = Pickup

Where do I find that...?

Sorry!!

Thanks mate

6) Lets warm the sound up a little by using the Waveshaper. Set the Waveshaper up like so :

Depth = 40
WS Type = Pickup

Where do I find that...?

There are two ways of getting to that spot, assuming you are in Edit mode.

1) Press the DRIVE/WS button in the AMP section until the WS LED lights. The LCD screen should now display the P08 AMP page.
2) On the left hand end of the Radias panel look for the "16 Keys" section and ensure that the "Page" LED is lit (using the Select button if necessary). Then Press button 8 (labelled AMP) to jump to the AMP page.

So now we're on the AMP page use the up/down cursor keys to scroll through the parameters on the LCD display, down to the parameter named WS Type. That's the chappy you want to change to Pickup using the alpha dial (or whatever they call that big white knob). The Depth you can set on the same page or use the dedicated Depth knob in the AMP section.

Daz.

Any give this a go yet ... come up with anything interesting ? Let us know

I will be continuing through this sometime over the next few days Daz. Will try to put a few chords together if anything interesting transpires.

Superb tut though mate... (looks for thumbs up smilie)

Thanks Daz! This is brilliant stuff! Advanced programming made easy.

I never really realized the potential of the comb filter until today.



I've got some hours of sonic exploration in front of me now!

Thank you.