The TwinPeak Filter

The best filter ever?

In essence there are two independent 18 dB lowpass filters in parallel, only sharing the resonance knob and their outputs mixed in 'anti-phase', or in other words their output signals are subtracted from each other, instead of added. In practice this means that if both filters get the same input signal the LP band set to the lowest cutoff is subtracted from the LP band with the highest cutoff and this creates a bandpass response that only passes what is 'between the two cutoff frequency settings'. The filters can 'pass' each other, meaning it doesn't matter which filter is tuned higher or lower it is always the band between the two cutoff settings.

The trick in the Twinpeak is how the input to both filters is mixed before they go into the two parallel filters. So, not only the outputs are mixed but there is mixing on the inputs going on as well. The A and B input connectors both have their separate input mixers. Basically the subtraction is not done at the outputs, those mix positive, but the input to one of the filters is inverted in phase. This has exactly the same effect as subtracting the outputs. For input A its signal always goes into filter one and the curve A knob sets how much of the input signal goes into filter two. So, if the curve knob is closed the IN A signal only goes into filter one and in the final output you only hear the effect of filter one on the IN A input signal. And thus it has a lowpass response as filter two does not get the input signal. That means that there is also not the subtraction at work that create the bandpass, as filter two gives nothing to subtract. Then, by opening the IN A curve knob, the signal level of IN A into filter two is increased and now there is something coming out of filter two and the final output curve changes towards a bandpass curve. For IN B it works the same, but this input always goes into filter two and through the curve B knob to filter one.

Basically it means the following options:

1) When both curve knobs are at 'lowpass' IN A goes into filter one only and IN B goes into filter two only. Now the Twinpeak works as two independent lowpass filters with their outputs mixed into one output connector. Because they are mixed in anti-phase the IN B signal part will be in antiphase on the output, while the IN A signal part is still in phase. Modulate each filter independently by the PEAK 1 MOD and the PEAK 2 MOD knobs only, as the CUTOFF MOD knob will modulate both filters in a linked way.

2) When curve A is at lowpass and curve B is at bandpass you hear the lowpass part of IN A, set only by the cutoff peak 1 knob. And from the signal on IN B you will hear the band between the cutoff peak 1 and the cutoff peak 2 knobs .

3) When curve A is at bandpass and curve B is at lowpass you hear the lowpass part of IN B, set only by the cutoff peak 1 knob. And from the signal on IN A you will hear the band between the cutoff peak 1 and the cutoff peak 2 knobs .

4) When both curve knobs are at bandpass both IN A and IN B will be bandfiltered with the same band between the cutoff peak 1 and cutoff peak 2 knobs.

5) And there are many settings in between, also when there are two totally different signals on IN A and IN B. It is best to judge the effects by ear.

If one only want to use one input on an external signal one can either close the level knob on the oscillator or one can plug the signal into IN A and plug a matrix jackplug into IN B  to completely break the normalization with the oscillator outputs.

The 1V-OCT input will affect both paralllel filters equally, like the CUTOFF MOD knob does.

The filter has a very good 'ping' characteristic at its maximum resonance. It lets itself also be modulated very well by audio rate signals. E.g. feeding a slow B PULSE from the TriLFO into e.g. IN A on the Twinpeak and feeding the A SINE from the TriLFO into the CUTOFF MOD on the Twinpeak will create a wide range of 'metallic' percussive sounds when the LF-VCO A RATE is tuned to audio rates.

The PEAK 1 MOD and PEAK 2 MOD have there zero level at twelve o'clock and turning them to the left will 'reverse' the direction of e.g. an envelope sweep.

The VCA can be totally independent if VCA IN is used for the audio input and the VCA CV with any unipolar, positive control voltage.

Here is the flowchart of the TwinPeak filter:

What if?... A TwinPeak filter bank (more than one module):

A single Twinpeak can be used to filter a single band of variable bandwidth and with 'corner-peaking'. The last means that the Twinpeak is not simply a bandpass filter with one resonance peak frequency in the middle of the band, like a 12dB state variable filter, but it means that the bandwidth can be set between two separately controllable cutoff frequencies and that when resonance is raised two resonant peaks appear, one at the low cutoff and one at the high cutoff of the band. And the cutoff slopes for both the low and the high cutoff slopes of the band are 18dB, which would only be 6dB on a state variable. So, in essence the Twinpeak is a 36dB bandpass filter.

A signal could first be panned in the DualFader module and then the AX/AL OUT and AR OUT pan outputs be fed to the Twinpeak A and B inputs and the CURVE IN A and B knobs set to lowpass response. This would allow the fader to fade from the A lowpass response, through a bandpass response, to the B lowpass response. Modulating the fader with a LFO or envelope can give quite lively filter results on otherwise static sequences and drones. Adding PEAK 1 MOD and PEAK 2 MOD and a little audio rate modulation on the CUTOFF MOD could be the icing on the cake.

If one would have two or more Twinpeaks it would be possible to make a high quality 'filterbank' with 18dB cutoff slopes with several Twinpeaks, using each Twinpeak for one band. They could all receive the same input signal and the outputs could be mixed on the matrix or with a DualFader module. Or, as in the previous example, a DualFader module can be used to feed the total of four pan outputs into the A and B inputs of two Twinpeaks.

One of the most interesting effects of the Twinpeak is when the two resonant peaks are swept independently, as this creates vowel-like effects. The most obvious use for two Twinpeaks would be to sweep four resonant peaks independently and thus get even more pronounced vowel-like effects. For this use a DualFader to pan one audio signal to the total of four pan outputs and patch those to the A and B filter inputs on the two Twinpeaks. Then mix the Twinpeak outputs in the matrix using the two bipolar VCAs on the NodeProcs to get a complex modulateable filter with many modulation points, as the Dualfader can be modulated, the Twinpeaks can be modulated in several ways and the bipolarVCAs can be modulated. Or, instead of using the matrix/bipolarVCAs, connect the Twinpeak outputs straight to two different channels on a mixingdesk and set stereo panning on the desk. 

Quite some patching options here.

Thanks to Todd Barton for providing the picture of the module.

The 24dB Filter/VCA

This is a 4-pole (24dB) multi mode VCF with an unusual topology in that it doesn't have separate outputs for its different responses (LP, HP and BP), but instead it has three inputs and one mixed output. The three inputs are placed upfront of the filters and each input has a manual level control that determines the amount of signal that goes into each filter. 
The 24dB Filter has a 24dB lowpass slope, a 24dB highpass and the bandpass slope is 6dB highpass and 18dB lowpass.

As the input jacks are normalized, the filter can also be used to create a variable slope for one input signal. This way one can dial in specific responses by setting the levels of the respective inputs. As when a signal is connected to only the LP input the normalization passes the signal on to the next HP and BP inputs. But, one could also input a signal to the LP input, another signal to the HP input and a third signal to the BP input. So the filter can be used as a three input mixer with separate level controls. The output would then be a blend of the first signal treated by the LP filter, the second signal treated by the HP filter and the third signal treated by the BP filter. For example, setting the LP knob to 10 and the HP knob to 7 gives you a resonant notch filter with more emphasis on the lowpass section.

One can crossfade one signal to an other with LP input & HP input. When the LP and HP inputs are used for the same signal they can pass this signal unaltered at specific mixer settings, as the LP and HP slope are fully complementary, in this case it will even suppress the resonant peak, although that peak will be present on the BP input signal.

The LP, HP & BP inputs are DC-coupled so that also CV signals can be processed and it means it can also be used as a slew limiter when you feed it CV. With high resonance, it then imparts very interesting behavior on the resulting control voltage!

Both Cutoff frequency and resonance are under manual and CV control. There's a 1V/Oct CV input to the Cutoff frequency and two supplementary FM inputs with attenuators. The filter maintains a steady perceived volume level at all resonance settings. This filter will not go into self oscillation. 

Mod II is the level input knob of the second cutoff modulation input. If there is no jack in the second modulation input opening this knob will introduce a smooth, subtle and very natural distortion effect on the resonant peak that is similar to tube distortion (all harmonic distortion). When a jack is connected it sets the amount of cutoff modulation. Often the first cutoff modulation input and level knob are used to control the tracking of the filter to a keyboard, and then the second modulation input can set the level of e.g. a filter sweep.

The filter reacts exceptionally well on audio rate modulations signals, which can add a further range of new and interesting timbres to the already quite versatile module.

Next to the main filter output there's also a VCA output for amplitude modulation of the filter (unattenuated CV input to the VCA and no control over initial gain of the VCA). The VCA has an exponential curve.

You can use this filter to get some really interesting slowly evolving waveforms. Example: patch a fat PW modulated square wave into the LP input, and a triangle wave through a slowly sweeping wavefolder into the HP input. Now sweep the filter cutoff with a mixture of very slow sine LFO plus an audio rate sine that is just slightly detuned from the two audio input waveforms... Bizarre spectral 'crossfading' from one timbre to the next, shifting from the PWM osc to the wavefolded triangle, and producing complex combinations at the points in between...

Here is the flowchart of the 24dB Filter/VCA :

Here are some videos of the module. The second one at Novars workshop greatly explains how the filter is designed.

Thanks again to Todd for providing the picture of the module.

Basic Electricity #15 in Berlin

On April 10th 2015 Basic Electricity #15 took place in Berlin as a special event dedicated to Rob Hordijk instruments and musicians playing them.

Rob himself gave a hands-on demonstration of his modular synth with a special emphasis on the Dual Env (Rev 3) and the S&H function. 

You can watch and listen to his explanations in the videos.

Thanks to Richard and Navs for setting up this event.

The Blippoo Box

Ah! The fabled Blippoo Box. There are many adjectives that describes the sonic character of the "bent by design" box : organic, unique, chaotic, unpredictable, etc...
But you really have to play it once to have a small idea of its potential.

Rob also offers a banana version of the Blippoo Box for those who want to interact with their Buchla/Serge/Banana systems.

There is a great page from Richard Scott's website about the Blippoo Box, including an album and videos with and without the Benjolin.

Hans Tammen has an instructive page about the Blippoo Box too with videos of different applications.

But the most complete and deeply instructive article about the Blippoo Box is in the Leonardo Music Journal issue #19 of 2009. Be sure to read and own it. Wonderful explanation about the concept and the design of the box.

Here is the flowchart of the Blippoo Box.

Check the Blippoo Box and Blippoo hashtags on Instagram for short videos of the box.

Also Youtube has some great gastric sounds. Check for more.

EEEM 2012 : the first series of videos from Rob.

This is an "old" playlist of videos from Mallorca. 

The case is different, lot of updates in the modules since, new modules, etc...

But still, this is a nice ressource and there are already some nice patches in it. Enjoy!

Thanks to Pedro Trotz who filmed and shared theses videos.

Modular Meets Leeds synth tutorial

In this video, Rob is showing a great use of the Shaper module. One can create a totally complex sequence using a waveshaper.

And it is also nice to have more modules explanation.

Live performances of Rob Hordijk

To my knowledge there are only 2 videos of Rob performing live. I really like the one in Rotterdam, great set.

So enjoy !!!

Rob Hordijk at the Noodlebar December 22th of 2016.

Rob Hordijk at Modular Meets Leeds 2017.

Rob Hordijk Masterclass November 21th 2015

A masterclass with only Hordijk users took place at Rob's workshop in The Hague in November 2015.

Anyone who had specific questions could ask and Rob would gave the answer.

The masterclass was more oriented in noise and complex random, self-generating patches.

Thanks to Chris who recorded, edited  and shared the videos.

The Novars Workshop 3-4 June 2017

At Novars Research Center in Manchester, UK was held a long workshop explaining the Hordijk Modular in June 2017. 

Philosophy of concept, functions, design were discussed and each module was explored in depth. There was a lot of technical explanations as well as demos. This is a great and wonderful ressource for any synth enthousiasts, sort of a video manual.

Novars Hordijk Workshop

You will find some of these videos in the modules description.

Thank you very much to Mark Pilkington for making the videos and to the Novars folks for sharing this treasure.

A pic of the Manchester Hordijk Modular.

The Hordijk Minutes

Todd Barton, the famous synth wizard, offers a series of small tutorials/demos of instruments in his Instagram account. 

He just wants to share what he is learning about the Hordijk with others. And we are glad he does.

When asked about why he created it Todd says :

"I like short forms: in poetry-haiku, in literature-short stories, etc. so I thought it would be great challenge to offer some short tutorials. Oh! Ans I was new to Instagram and there is a & minute video limit for content :-)"

He had a lot of positive response from both Hordijk practitionners and other analog synthesists that could translate what he was doing to their formats.

But the most amazing and encouragement came from Rob :

"Thank you very much for the ‘one minutes’ [tutorials], they get a really good reception with users. I must admit they inspire me too, especially as I have come to the habit of immediately setting up complex patches and then getting lost in how to play them. These simple examples let me go back to the basics, with often better results. –Rob Hordijk"

There will be more Minutes to come in the future but for now I invite you to check the ones that are online in Todd's Youtube playlist

Some extra videos are in the playlist. Be sure to watch them too.

The OscSync

The OscSync has at its core a sawtooth oscillator with a downgoing slope. On the flank of the sawtooth it syncs a second built in osc that has a waveform that can be morphed between a triangle and a sawtooth.

A hardsync sound basically has two sonic components, one is a formant effect that when swept gives a flanger-like sound, the other is a pulse that is produced by the transient flank introduced on the syncing moment. This transient has a level that depends on the frequency of the synced osc and when swept it goes up and down in level in basically an unwanted way. As this new transient has a lot of sonic energy in the high end of the spectrum it doesn’t sound good on e.g. sine and triangle waves that by nature do not have one or more transient flanks in their waveform. And so it is of interest to suppress only this sync transient effect. For this a sync transient suppressor is built into the osc and this makes the sync sweep sound as smooth as silk, no matter the type of waveform.

Both the original sawtooth and the ‘transient suppressed’ synced waveform are brought out on separate output connectors.

There is a subsquare divider built in that goes into a built in VCA. This signal can be used to modulate the sync sweep. The effect is that on every alternate wavecycle the formant structure caused by the sync is alternated by two settings that depend on the level of modulation. This gives a ‘talkative’ effect on the sound. This subsquare is also brought out on a connector to modulate e.g. a filter that has the basic sawtooth wave as its input.

The sub out can also be used in a mix with either the sawtooth or the synced sound to add a suboctave bottom to the sound.

There also is a built in S&H that triggers on the flank of the main sawtooth osc. It has its own input and output, but by default the input is normalized to the internal transient suppressor and samples the level of the sync transient. The output is normalized to the modulation inputs of the main pitch, the subsquare modulation level and the sync sweep CV inputs. The sync transient depends on the pitch of the synced oscillator in an ‘irregular way’ it can be fed back to the sync sweep input to cause a ‘chaotic’ modulation on the sync sweep. This effect is not unlike the effect of a rungler circuit. When also fed back to the main pitch modulation input it will change the length of the wave cycle with an amount that will be different for each cycle. As the oscillator can go into the LFO range this can also create quite nice effects.

The normalizations can of course be overridden by plugging in a cable at the CV input connectors or close the modulation level knobs.

The S&H can also be used with an external audio signal to ‘bit crunch’ that signal.

The sync transient suppression is what makes this osc unique. The S&H and subsquare make it great for a vast range of different sonic effects.

As this oscillator can do a range from quite traditional synth and sync sounds, but with a very smooth sound, up to rungler-like chaotic sounds the Twinpeak is the prime suspect to do filtering on the several output signals from the osc. The two Twinpeak ‘peaks’ can e.g. be modulated separately by the subsquare output, the S&H output or an audio range signal from e.g. the OscHRM or an LFO and/or envelope.

Here is the flowchart of the OscSync : 

Chris David aka Osc1899 filmed these videos of Rob explaining the module :

Also this video of the Novars workshop is quite nice :

I also recorded a small series of demos when I received the OscSync Rob Hordijk OscSync demos

And there are also more videos and audio available. Check Todd Barton's channels and be amazed ;-)

Thanks again to Todd for providing the picture of the module.

The Harmonic Oscillator (OscHRM)

Let's start the description of the modules with this beautiful oscillator.

 

The Harmonic Oscillator (OscHRM) is a fully featured, wide range, voltage controlled oscillator with simultaneous dynamic control over pitch, timbre and amplitude.

Pitch control law is 1V/Oct and the module uses a platinum element for temperature stabilization of the pitch curve. The scale is perfectly tuned in the middle six octaves up to a pitch of 4.000 Hz. With external CV the pitch range is 18 octaves. When playing pitches higher as 4000 Hz the 1V/Oct scale starts to break down, due to the necessary internal band limiting in the harmonic generators.

The OscHRM uses a biquad sine/cosine oscillator at its core and through a process of linear FM feedback harmonic series of overtones are generated. By gradually opening the knobs that control the amount of recursion more and more harmonics are generated. This arrangement allows all sorts of filter-y effects without using a filter.

The first circuit adds odd harmonics morphing the sine wave into a square wave.

The second circuit adds all harmonics, modulating between a down-saw, through a sine, to an up-saw. The center position is basically whatever the first circuit is currently doing to the waveform. The waveforms have an exceptionally warm sound and when dynamically modulated have a deep spatial and organic character. The HRM MOD CV input has an attenuator. PWM can be created by turning up the ODD knob and changing ALL knob setting.

Dynamic wave shaping is available under full voltage control and can be modulated from slow LFO speeds to fast audio rates to create FM timbres. When the waveforms are modulated there is a negligable amount of detune (less than 1 cent), though when modulating at audio rates an asymmetry in the modulating waveform can cause detune effects on deep modulations.

The OscHRM also includes a VCA with manual initial gain control and an attenuated CV input. This is useful for manipulating FM levels. Having a VCA at the end of the signal chain does not affect timbre, this arrangement does allow timbre changes, as well as ring modulation type sounds. It can be used simply to control the level, like other VCAs, or patching the VCA out to one of the harmonic modulation inputs makes the module's waveform modulate itself under control of the VCA. The velocity parameter can be controlled with MIDI. The advantage is that sometimes effects like distortion are depending on loudness. Direct oscillator output (Full out) is doubled for convenient patching to two destinations (i.e. to have two separate signal paths sourcing from one oscillator).

The final output signal can be taken from a point just before the VCA and at the output of the VCA. This enables the module to be easily used in a situation where one wants to modulate another module by an audio rate signal and have the modulation depth under voltage control using e.g. a LFO waveform, an envelope voltage signal or a play controller that produces a control voltage, while still having the full output level signal available on the full output to serve different purposes.

There is a Sub Out to the OscHRM. This suboctave is a pulse one octave below the osc that inherits its pulsewidth from the EVEN and ODD settings and modulations. Also the output level is controlled by the VCA LVL and VCA MOD knobs and input signals. The curve of the Sub out is a bit louder as the VCA out at a lower level and a bit less loud at full level.

The switch has three positions. In the upward position the pitch knob can tune the osc between +/- 0.5Hz and 25kHz, so over the full range. In the middle position the pitch knob is disconnected and the osc tunes to middle C, so it can be easily played by a 1V/Oct MIDI CV without the possible chance of severely detuning the osc by accidentally twisting the pitch knob. The fine knob still works, of course. The third position tunes the osc an octave below middle C.

The Fine knob course is about two half notes up and two half notes down. It is limited for the reason that it should be able to precisely set the detune between two oscillators To make a detune of e.g. a fifth it is more interesting to use the quantizer from the NodeProcs, as this module can add the correct voltage to e.g. a 1V/Oct signal for transposing a particular note interval, up to five octaves up or down and optionally quantized to a chosen scale.

The Chain Out jack is like a multiple when chaining two or more oscs to one 1V/Oct signal. It is handy when e.g a MIDI->CV box or analog keyboard has only one 1V/Oct jack, in which case a cable is connected between the first osc chain output and the second osc 1V/Oct input.

Here is the flowchart of the Harmonic Oscillator :

Check these videos for sounds and more explanations :

And this one explains very well how the harmonics are created in the Harmonic Oscillator.

Thanks to Todd Barton who provided the picture of the module.

Greetings!

This blog is meant to share informations about the Hordijk Modular synthesizer.
This IS NOT an official website/blog. This is made by Hordijk enthusiasts.
In this blog, you will find videos, audio files, pics and sometimes patches.

If you want to comment, correct or add something, please feel free to email me at thehordijkmodularblog@gmail.com
I welcome any constructive comment.

If you'd like more information, please contact Rob at rhordijk@xs4all.nl

You can also visit the topic about the Hordijk Modular here in Muffwiggler : https://www.muffwiggler.com/forum/viewtopic.php?t=70363&start=all&postdays=0&postorder=asc&sid=ae57170e258117ee6a0be3f3c9ecfafc
It is full of informations.

I hope you will enjoy this blog.

Regards.