Thru-Zero FM Explainer

How to get the best results from your analogue thru-zero linear FM oscillator. An explainer in response to questions about the Doepfer A-110-4, but which is also applicable to other TZ-FM VCOs.



The A-110-4 seems to be more prone to pitching artefacts than other thru-zero (TZ) VCOs. That might partly be due to the specific design, but general considerations like the tolerances and sensitivities of analogue designs also play a role. Another issue might be perception, e.g. when certain modulation depths make it hard to discern the original pitch of the carrier.

DIGITAL FIRST

Ignoring the actual implementation (frequency- or phase-modulation), the reason digital FM is easy is because it’s clean. There are no DC offsets or pitch instabilities to ruin the result.

Here is an example using the Clavia Nord Modular G2. The carrier is always heard and only the modulator is enveloped. The frequencies of the two oscillators are fixed but the FM amount is manually increased: 01-TZ_clavia-digital

The carrier was tuned to B3 and the modulator D#5. I chose these pitches by ear to show an effect which might be confused for pitching at higher mod depths. When the modulation recedes, we get a sound similar to guitar feedback. This shows that it’s possible to find combinations of frequency relationships and modulation depths that will sound ‘wrong’ even when patched digitally. In an analogue system, frequency drift or tracking inconsistencies can worsen the problem.

DIRTY SIGNALS

But weird C:M ratios are not the main problem. The real enemy of dynamic linear FM is asymmetry caused by DC offsets. If the modulating wave is not centred about zero, the sound will churn. Here’s the same G2 patch, this time with some virtual DC offset added: 02-TZ_clavia-digital_virtual-dc-offset

To avoid this, some FM inputs are AC-coupled - the signal is filtered by a capacitor. But this is not a magic bullet. If the offset is large enough and being swept dynamically, the capacitor will have a hard time eliminating it. The most likely cause for DC offsets are the modulating oscillator itself, or more precisely its wave-shaping circuitry. I have had best results with Doepfer’s A-143-9 Quadrature sine-core oscillator.

SCIENCE, SWITCHES

Analogue TZ-FM involves switching to generate ‘negative frequencies’. No matter how finely timed, there is a moment of indecision which may be audible as a growl or rumble. DC offset makes this worse as it shifts the switching point. Lower initial frequencies (aka Bias, Symmetry or LFreq) allow more modulation and brighter sounds, but the cost is lower accuracy and more ‘bum notes’.

THE A-110-4

The Doepfer TZ-VCO switches between two oscillators. This method has pros and cons. If the two VCOs do not respond equally to modulation, the resulting FM will sound ‘off’, even if the modulator is clean and the switching point is accurate. This cannot be trimmed by the user. To mitigate pitching artefacts at the point of switching, calibrating the TZ transition can help.

WHAT IS IT?

As Cynthia Webster says in her humorous video showing the difference between thru-zero and normal linear FM, without TZ “half the modulation is gone”. As you’ll hear, the frequency relationship between carrier and modulator also plays a role.



The first VCO you hear in the next two examples is the Zeroscillator, then the A-143-9 which I have modified for standard linear-FM.

If the carrier’s initial frequency is high enough and/or the modulator is higher in frequency than the carrier, the tonal result is very similar:
03-TZ-high-initial-freq_ZO-A-143-9

The real difference between thru-zero and normal linear FM is apparent when the modulator is slower. As you can see above, instead of adding gentle folds, the modulator pushes and pulls the carrier through zero. TZ allows deep levels of modulation, even when the carrier’s initial frequency is low. When this is the case, there is a big difference between normal and TZ linear FM in the possible tonal result: 04-TZ-low-initial-freq_ZO-A-143-9

One last thought on this as the Zeroscillator video is much quoted when the question “what is thru-zero FM?” is raised: toggling the “Through Zero” switch on the ZO does not yield ’typical’ linear FM; normal linear FM does not sound like this: 05-TZ-not-typical

PATCH TIPS

I have concentrated on the audio effects here, but VCOs that can go thru-zero have other uses when they themselves are used as modulators. For an idea, try this patch for a thru-zero frequency shifter and listen to your sounds swap stereo sides.

I hope this explainer helps you with your TZ-FM experiments. The files folder is here. If you’re not sure about the analogue results you’re getting and need a digital ’control’, download the Clavia Nord Modular G2 demo. If you’re on a Mac and 10.7 or higher, you can run the demo under Wine. There is info here or an installer here.

Powwow 4 - Feedback

Here’s the recording of my live set at the fourth Berlin Powwow session which you can buy from my Bandcamp.



For this performance I wanted to explore feedback and the obstacles and modifiers one can place in the loop. The key players in this were delays, filters, non-linear functions and malfunctioning components.



My main analogue patch featured a Cwejman RES-4 in a loop with a Bananalogue Serge WVX. The combination of four frequency bands of oscillation might have been enough, but placing a Cwejman VCEQ-3 in the chain to dampen or boost the feedback added another level of complexity. A stressed coupling capacitor and scratchy Folds pot on the WVX added to the unpredictability and fun.

Live, I love my Clavia Micro Modular for two main reasons: patch recall, which allows me to set a scene or chop between moods, and polyphony. The virtual feedback patches in this set were simple, but switching them to two or three voices added complexity and interest, for example asynchronous panning or the beating of multiple sine-wave oscillators. You can do it with a ’real’ modular, but at a cost!

This set up is flexible and just about portable. It’s still completely live in that each patch either has to be built or played by hand. But the addition and convenience of the Micro Modular does cause me to question how we approach playing live with a modular. With many musicians turning up to gigs with their rigs pre-patched, is there a difference or even a need for the real deal?

Thanks again to Goldwiener, Hainbach, Paul da Fonk and Mark Berman of Powwow for the opportunity to make some noise.

Basic Electricity Concert Tonight - Rastko, Plastiq Phantom

If you’re in Berlin tonight, come along to Basic Electricity, a concert with the Serge Animal tamer, Rastko and Patch Point proprietor, Plastiq Phantom, aka Dr. Wiener.

Doors open at 19:30 for music at 20:30 at the König Otto in Neukölln. As you can see, it’s an amazing venue housed in the old Kindl brewery.

There’s more info here and a Facebook event here.

See you later!

BE#23, Thursday, 19.10.17 @ König Otto
Tickets 10€. Doors at 19:30, music at 20:30
Am Sudhaus 3, 12053 Berlin Neukölln
U7 Rathaus Neukölln or U8 Boddinstr.

Basic Electricity - concert, tomorrow night in Berlin

Basic Electricity returns tomorrow night with a special concert featuring Michael Vorfeld’s Glühlampenmusik and Wouter Jaspers’ Field Trip at the König Otto in Berlin, Neukölln - come along at at 20:00 for what promises to be an audio-visual treat!

There’s more info at the Basic Electricity Blog and here is the Facebook Event.

Here is Wouter performing with some of his Koma gear at Powwow last week:



Look forward to seeing you tomorrow night at Basic Electricity!

BE#22, Thursday, 28.9.17 @ König Otto
Am Sudhaus 3, 12053 Berlin Neukölln
U7 Rathaus Neukölln or U8 Boddinstr.

Tickets 10€. Doors at 20:00, music starts 21:00
Seating is limited to 50, so please come on time.
The bar will be open for drinks and sandwiches.

Take Seven - modular jazz revisited

A longer improv and edit inspired by this modular bebop sketch.



This track started out as a test for my old Boss SE-70 Super Effects Processor - the kitchen sink FX box from the 90’s, loved by The Prodigy - which I had dusted off to check out the pitch shifters.

A medium-paced Wogglebug conducted two Plan B Model 15s and Maths. There was a bit of FM and wave-folding involved. I’ve always felt that the Wogglebug had the jazz so it was a short jump of the imagination to add some Rhodes/ Cwejman RES-4-ish tones from a Clavia G2 Voice Filter patch and some brushed beats.

I guess this was an exercise in re-contextualisation - hope you enjoy it!

Powwow - Thanks 2

A second round of applause for hosts Hainbach & Goldwiener and their lovely assistant Paul Da Fonk for a fab time at Berlin’s 2nd Powwow stream at Patch Point!



Here’s the recording of my set which you can also buy from my Bandcamp:




Get ready for the 3rd Berlin Powwow event, also on Youtube, on Friday, July 21st with music from Ashley M Puente, Hainbach, Goldwiener, Lindred and Navs and featuring the knock-out visuals of Luma //Chroma!

Navs Live on Berlin Powwow Stream Tonight!

I’m taking part in the second Berlin POWWOW live video stream this evening, Thursday, 8th June. It’s a live modular improvisation over the internet also featuring Andrea Lange, Hainbach, Goldwiener, a special surprise act and visuals by Luma //Chroma.

The first Berlin session was loads of fun - join us for part two tonight!

A-147 LFO Wave Skew Mod

A simple modification to gain voltage controllable wave shapes from Doepfer’s A-147 VCLFO. Learn how to turn this old stalwart into a morphing modulator with just a couple of resistors.



The A-147 is a triangle/square-wave oscillator built around the pairing of an integrator and comparator. They work in a loop with the one feeding the other, back and forth. You can see them in action here.

The result of integrating a square wave is a triangle. But if we independently change the rate at which the integrator charges and discharges, we can skew the waveform from a rising to falling sawtooth via a triangle. Because the integrator and comparator are related, the square wave becomes a variable width pulse.



The easiest way to achieve this is to hijack the square-wave on its way to the integrator, split its path with two diodes and then set the balance with a potentiometer. You can see this manual arrangement in Ray Wilson’s Variable Skew LFO or Ken Stone’s Utility LFO.

To get proper voltage control over this ‘re-balancing’ might take a considerable amount of extra active circuitry, more than I felt the effect is worth, especially given the digital options available today. Here’s my simple kludge:



The idea is to massively reduce the size of the square wave going into the integrator so that the injection of an external voltage is enough to shift the bias of the square and hence alter the shape coming out of the integrator.

Resistor R14 (15k), which is to the left of the quad opamp (TL084) forms a voltage divider with the 1k resistor which also marked in yellow in the picture above. The output of this divider goes to the CA3080 OTA for integration. I found that swapping the 15k for a 1M resistor was enough to allow the bias to be shifted with a +/-5V CV. For the CV input I ended up using two resistors in series to give me 670k rather than the 1M in the picture.

Reducing the amplitude of the square wave to the integrator also reduces the overall frequency of the LFO, so I also swapped the 0.1uF capacitor C2 for a smaller 15nF to bump up the speed.

Below is how it looks and here are some construction pictures.



I like simple mods - minimum change for maximum effect. Skewing the wave shape with this method also changes the frequency of the LFO, which you might not want. There is a way to compensate for this, but again, I decided it wasn’t worth the extra circuitry as I like the effect of the LFO slowing down as it changes direction, almost like a thru-zero oscillator. But you could try the following schemes. One is relatively simple, the other might need an expander module to provide space for the extra PCB. Neither is perfect.

The idea here is to inject a fraction of the skew CV to the A-147’s FCV input. For example, given a skew CV of 5V I found I needed about 5-600mV FCV to correct the frequency. You can use the attenuator on the module for this or send your skew CV via a ca. 560k resistor to the output pin of the 50k FCV potentiometer (ahead of the FCV buffer). The ‘gotcha’ with this method is that it only works for positive offsets as we always need to speed up the LFO when skewing.

What is needed is an active full-wave rectifier to ensure that the compensation voltage is always positive. This is a simple enough circuit to add but for the lack of space behind the panel. If you want to try this, have a look at Jim Patchell’s Ideal Diode Tutorial hosted at René Schmitz’ Schmitzbits site.

The usual DIY disclaimers apply - you do this at your own risk. Have fun but take care of your module and yourself!

Thank you Tim Stinchcombe for identifying the relevant parts and for your thoughts and suggestions.

Patch Tips #28 - Wavetable Clock Multiplier

Driving a wavetable oscillator to get reliable clock multiplication, regardless of changes in tempo. A follow-up to these posts (1, 2).



Clock division is easy and instantaneous. Getting faster pulse streams from a master clock is an imperfect science, even with dedicated solutions. Here is a multiplier patch that tracks changes in tempo and avoids the lags and glitches that can affect modules which try to average or predict incoming clocks.

In a sense, it’s an analogue rather than digital approach: instead of counting pulses or square waves from a master clock (tap tempo etc.), we use a sawtooth wave to voltage address a suitable waveform in a wavetable module.

If we scan a wave that contains four equally placed pulses (see image above) we can quadruple our ‘clock’. If the amplitude of our sawtooth remains constant - i.e., we read out the entire contents of the page - we will always get four pulses for each single sweep of the saw, regardless of how fast or slow that saw moves.

Here’s how it sounds: wavetable-clock-multiplier.mp3

For this patch, I used a sawtooth LFO to drive a Richter Megawave. I multed this saw and used one copy to trigger one envelope (master clock, left channel voice) and patched the other to the input of the Megawave. I patched its output to a second envelope and voice (multiplied clock, right channel). I used the sine waves in ’Miniwave’ (Bank 14, Wave 3) as they are more regular than the pulses in Bank 7. Fortunately, these sines were enough to trigger my envelopes. I kept the multiplication at x4 but modulated the frequency of the LFO (and the pitch of the audio VCOs) with a voltage offset and a clocked random voltage.

To get voltage control over the multiplier e.g., jump from quarter notes to 16-ths, we can do two things: either select a new wave via CV or manually select the wave with the most pulses and control the amplitude of the sawtooth with a VCA, as you would with a wave-folder. Much like the old Beat Folder patch, we are re-assigning voltages. But instead of using an analogue transfer function, the wavetable module allows us to look up a series of digitally stored arbitrary values.

If you want to read some more about the Megawave and its many uses, head here. The Megawave is full of maths, a ‘best-of’ transfer functions (see the Socket Rocket manual for ideas). This is my sort of module. It’s multifunctional - not because it has many modes but because its principle function can be re-imagined according to the desired use.

Powwow - thanks …

… to our hosts, Hainbach & Goldwiener and to Luma //Chroma for the mesmerising visuals! The launch of Powwow Berlin was loads of fun and I’m looking forward to doing it again.



Here is a video teaser of my set:



And here is a proper recording which you can also buy from my Bandcamp:



I patched a 6U case live, with back-up provided by a Clavia Micromodular and EHX SMMH as delay and looper. Given the short set-up and play time, this was a good selection which provided enough options. It was nice to get back to basics after a couple of virtual modular gigs and I think there’s enough play in this caseful of modules for another outing.

Navs on Powwow Live Stream Thursday

I’m taking part in the first POWWOW Berlin Live Stream this Thursday, 4th May from 19:00. It’s a live modular improvisation over the internet featuring Hainbach, Lindred, Navs, zv_k and Goldwiener.

Details are here.



I’ve never knowingly played to a virtual audience before and I’m looking forward to making music with these lovely people, so tune in on Thursday night!

Basic Electricity on Friday - Joker Nies, Gino Robair, Richard Scott

Join us on Friday, April 21st for an evening of hands-on live electronics with Joker Nies, Gino Robair and Richard Scott. It’s the second Basic Electricity concert in our new, super venue, the König Otto in Berlin, Neukölln.

There’s more info, pics + video here and the Facebook event is here.

Here is a video of the re-opening concert with Wolfgang Seidel, Hilary Jeffery and me:



direct link

We're looking forward to seeing you on Friday!

BE#20, Friday, 21.04.17 @ König Otto
Am Sudhaus 3, 12053 Berlin Neukölln
U7 Rathaus Neukölln or U8 Boddinstr.
 
Tickets 8€. Doors open 20:00, music starts at 21:00.
Seating is limited to 50, so please come on time.
The bar will be open for drinks and sandwiches.

March 24 - Navs Live @ Basic Electricity Berlin

I’ll be playing live with Wolfgang Seidel, Hilary Jeffery & Hainbach this Friday, March 24th, at Berlin's best live electronic night, Basic Electricity.



We’re very happy to have a new venue, the wonderful König Otto in the old Kindl brewery in Neukölln.

There’s more info, pics and video here; the Facebook event is here.

Doors open at 20:00, music kicks off at 21:00. The address is Am Sudhaus 3, 12053 Berlin Neukölln.

Look forward to seeing you on Friday - until then, here are some ideas I have been working on for the show:



Cheers, Navs.