How exactly related FM amount of Bento Box osc to FM amount of FreqBox?

How exactly related FM amount of Bento Box osc to FM amount of FreqBox?

 

Your thoughts?

 

I did some experiments and observed that 100% of the application of fm in Sytrus is about the same as 42% linear thru-zero FM in the Reaktor. How can I find a full understanding of why there is a difference, and how can I find (or calculate) the real FM amount of Freqbox?

 

Why there is a difference:
There is a difference because they were made by different developers who made different design choices.

To find or calculate the real FM amount (whatever that means?):
Either analyse a schematic of a Freqbox taking into account input buffers, amplifiers, filtering effects of caps and stage impedances etc. And also the type of pots used...
Or take the same source signal (simple wave like sine or square would make sense) and put it through Freqbox and Reaktor at lots of different FM amount settings using a scope or frequency analyser of some sort to create a graph for each device, then compare the two graphs.

Remember though that the device works by using the amplitude of the input signal to modulate the frequency of the internal oscillator, so anything in the signal path to the Freqbox that changes the amplitude will have a significant effect. Also note that for this to make sense, before you start you will need to choose a (somewhat arbitrary) mapping relationship between the analog signal voltage of the input signal to the Freqbox and the abstract idea of amplitude in the digital domain of Reaktor. e.g. is -1..1 in Reaktor the same as -1v..1v, -5v..5v or -9v..9v (or some other range) in Freqbox?

 

Hello Colin! Thank you! I really didn't know about the mapping relationship between the analog signal voltage of the input signal to the Freqbox and the abstract idea of amplitude in the digital domain of the Reaktor. How can I do that?
From MF-107 user manual:
-
Freq CV In
accepts a -5 to +5 CV signal, and is calibrated for the VCO
to change one octave for very volt change at the input. The control input
extends the frequency range of the VCO well beyond the 25Hz-1.6KHz
range of the front panel Frequency control. The VCO oscillates from sub-
audio up to 20KHz.
-
Wave CV In
accepts a zero to +5V CV signal. The range of effect from
a zero to +5V CV is 60% of the range of the front panel control.
-
Env. Amount CV
In accepts a zero to +5V CV signal. The range of
effect from a zero to +5V CV is 100% of the range of the front panel
control.
-
FM Amount CV In
accepts a zero to +5V CV signal. The range of
effect from a zero to +5V CV is 100% of the range of the front panel
control.
-
Mix CV In
accepts a zero to +5V CV signal. The range of effect from a
zero to +5V CV is 100% of the range of the front panel control.
CV Outputs:
The FreqBox has two CV outputs which can be used to
modulate other Voltage controlled gear, such as Moogerfooger analog
effects, the Minimoog Voyager analog synthesizer or Little Phatty analog
synthesizer. The following are the details of these outputs:
- Env. Follower CV Out:
This is the unfiltered Envelope Follower CV,
which is generated from the Drive signal. The Envelope Follower CV
is only produced when the effect is active and a signal is present at the
audio input. The level is nominally zero to +5V. This signal carries larger
audio-induced ripple than the signal used for modulating the VCO, or the
VCO’s VCA.
- Oscillator Direct CV Out:
This is the direct output of the VCO’s
waveshaping circuit before it goes to the VCA, so this signal is
always present, and is nominally +/-2.5V. That’s a lot stronger than an
instrument level signal, so we don’t recommend plugging this output into
an instrument amplifier.


The Freq. In is the only control that accepts
a negative input, as it can aceept a -5V to +5V Input. All other control
inputs are designed to accept zero to +5V inputs. Do not exceed these
ratings.

 

So If I understand correctly, there is a panel control to set a base pitch for the oscillator, and then inputs to 'Freq CV' modulate that pitch by an octave per volt.

Looking inside the Bento Box oscillator, it seems to work like this:

It expects a signal within the range -1..1 at the FM input.
Only the Lin Tz mode uses negative values, the lin mode clips them to zero, the exp mode maps them to frequencies below 8.1758Hz.

The closest thing to '1 octave per volt' is going to be the Exp mode as that uses a pitch to frequency converter (P2F) so that the pitch (and therefore octave) changes linearly with the input rather than the frequency changing linearly.

First of all the Amnt control value is multiplied by 135. The result is then multiplied with the modulator signal. This means that with the control at max=1, the range from a -1 to 1 input modulator signal is from pitch -135 to pitch 135 (roughly 0.0034Hz to 19912.13Hz after P2F conversion)
This pitch value is then added to the base pitch of the oscillator , then the whole lot is converted to a frequency value.



(the Sel module just routes the modulator signal depending on Mode. The bottom output is the one for EXP mode which is what we are interested in)

Assuming that the input modulation signal ranges from -1 to 1, the maximum modulation would be 270 (135-(-135)) semitones wide = 22.5 octaves. If you wanted to match the Freq box, you would want a 10 octave range...
If the amt is 0.4444, then that would give a pitch range -60 to 60 which would be your 10 octave range.

Of course, this is all guesswork based on assumption. It also still leaves the problem of ensuring that your test signal is -1..1 in Reaktor and -5v..5v in FreqBox.
If you can get close, and my assumptions are correct, then setting the FM mode to EXP and the FM knob to 44.4 (use shift to fine tune the knob value) in Bento Box might get you closer to the FreqBox FM behaviour.

Col

 

Thank you very much! That's was interesting to read! But I'm not clear 100% why EXP mode we are interested in (Freqbox provides linear FM)?
From Freqbox manual:
the linearity of the FreqBox CV response is good in the lower and mid-
range frequencies, but may not be perfectly linear at higher frequencies.
These considerations aside, a wide range of musical usage is available
to the analog musician using the FreqBox as an oscillator in a voltage
controlled system.

 

1v per octave means that it is linear in pitch - addin 1 octave per volt increase.
But each time you add an octave, you double the frequency, so linear in pitch is exponential in frequency. The modes in bento box refer to frequency, so Exp mode in bento box is roughly equivalent to the 'linear' volt per octave setup in FreqBox. FreqBox is analog though, so there will be variation from absolute linearity which means that to model it accurately, you'd need to take lots of measurements.
Just switching the Bentobox osc to EXP mode and FM amnt to 44.4 would be a good start though

 

Hello Colin!
Obtained that Freqbox isn't truly linear FM, as I understand linearity is present only in the control voltage applied to the internal oscillator, while pitch (on basis of 1v/oct) obtained at the output of the Freqbox is exponential...
About this may indicate the words of the manual:
The "voltage controlled" part of a VCO refers to the fact that in this
circuit a control voltage (CV) determines the frequency of the oscillator.
A steady CV will result in a steady pitch, while a changing CV will cause
a change in frequency. The FreqBox has a front panel Frequency control
changing that generates a voltage that increases as the control is turned
clockwise. This causes the frequency of the FreqBox VCO to rise.

FM Amount CV In
accepts a zero to + 5V CV signal. The range of
effect from a zero to + 5V CV is 100% of the range of the front panel
control.
I.e. there are 5 octaves, instead of 10 octaves? Or I didn't understand correctly? It is also not clear from the question of the influence of the amplitude at FM.

I've made this changes with Bento Box osc
1.base frequency range now is 25Hz-1.6kHz
2.pitch range -60 to 60, instead of 44.4% of FM amnt=) I think that would be better solution for using this parameter more comfortly

 

I'm not sure what you mean by the output pitch being exponential?

Freqbox has an internal oscillator which generates the sound you hear at the output. You set the base frequency using a front panel knob. Inputs to the FreqCV input modulate the frequency of the oscillator.
The bit you seem to be getting stuck with is the concept of linearity with respect to pitch.
FreqBox FM is linear with pitch, ie. pitch changes linearly with control voltage.
When Bento Box says 'lin' it means linear with Frequency - not pitch. When it says 'exp' it means exponential with Frequency.
Exponential with frequency is the same as linear with pitch!!!
What you have to understand that Pitch and Frequency are not the same thing. They are related exponentially. Linear changes in Pitch equate to exponential changes in Frequency.

This highlights a problem with their documentation. Particularly the line "A steady CV will result in a steady pitch, while a changing CV will cause a change in frequency."
While in this context it is technically correct, IMO using the terms pitch and frequency interchangeably gives the false impression that they are the same thing. I suppose that whoever wrote the manual possibly didn't have a technical background?

The FM amount CV doesn't directly modulate the frequency, so it being 0..5v is not important. The FreqCV input is -5v..5v, at 1 octave per volt that is 10 octaves. I assume that you only get the full 10 octave range with amount set to max. (I don't know if the amount control is linear or log, but that doesn't change the sound, just the ease of selecting a setting, so it's not nearly as important).

 

Thank you, I begin to understand! But I am also interested in this question:
How to make thru-zero linear FM?
in the same manner as in these oscillators
(I'm trying to understand the difference of implementations of frequency modulation modes between the reaktor and hardware):
1. Doepfer VCOs provide Exp and Lin TZFM
http://www.doepfer.de/A100_QVCO.htm
http://www.doepfer.de/a1102.htm
http://www.doepfer.de/A1104.htm
2. Schippmann CS-8 (Thru-Zero FM/Thru-Zero PM)
http://www.schippmann-music.com/downloads/CS-8_OmegaPhi_man_Rev104_EN.pdf
3. Intellijel Rubicon
http://intellijel.com/wp-content/uploads/2012/12/Rubicon-Manual-V1.01.pdf
from Rubicon's manual:
TZFM EXP/LIN switch
- Selects the type of FM modulation the TZFM input will be.
This switch determines whether the response of TZFM is linear or exponential. The exponential setting is not really 1V/oct, and really just gives a
more extreme response to the modulation signal
but A100's manual talks: The most common exponential scale is 1V/octave. In this mode thru-zero is not possible as the exponential function cannot become negative.
Intellijel were wrong?
Also I found these

• Switchable linear and (for the first time in any module) exponential tzfm
• Linear index vca for dynamic control of tzfm depth

4. HERTZ DONUT MARK 2
Dual digital oscillator with dynamic-depth thru-zero frequency modulation (what means dynamic-depth?)
http://www.theharvestman.org/9791mk2.php
5. Make Noise DPO
http://www.makenoisemusic.com/content/manuals/DPOManual.pdf
Dynamic FM, Circular FM - what is this?
DPO provides EXp and Linear FM

 

here a little example:
the oscillator runs at 300Hz and the fm amount is 500Hz so the oscillators frequency is between -200Hz and 800Hz. This means you get negative frequencies which theoretically don't exist. The way the oscillator handles these negative frequencies makes the difference of these FM modes. Thru-Zero just means that you get these negative frequencies and the waveform starts to play backwards. The normal linear mode in the Bento OSC is quite useless cause it simply cuts the negative frequencies. Another way some synthesizers came with is to mirror the negative frequencies so in our example the oscillator would go down to 0Hz and goes back in the positive range up to 200Hz. In exponential mode the FM amount is in semitones so the frequency of the upper band halves with every octave the fm amount rises. In other words we never get negative frequencies. Exponential FM is also known as pitch modulation so its just modulating the pitch input of an oscillator in audio rate. The good thing here is that the fm amount is relative to the oscilattors pitch and rises when higher notes are played so the overtones does not change.
I have no clue what Circular FM is but i think its something similar to Dynamic FM. Modulate the FM amount and give the child a name for better marketing - Dynamic FM This is why FM synths got plenty of envelopes - to make it dynamic. You wont get these nice fm bells with a static tone, thats all the secret here. Do not get confused by all these nice terms used for marketing purposes.

some interesting links:
http://www.dragonflyalley.com/constructionFritzTZVCO.htm
http://pugix.com/synth/teezer-through-zero-vco/

 

Hello sellotape! Test it please. It sounds crazy and wild. The sound is sometimes close to the original, but still a long way from the original. Envelope follower and drive don't work as it should. Or maybe something else? I think the input signal must be very great (because my Ohmicide sounds very quiet). How many dB usually provides DI guitar signal? I don't have a guitar to check this, I use Shreddage.
For a reference sound I took this (57:11)

I've got this sound (carefully - Loud!)
https://drive.google.com/file/d/0B-qd1kodWxtyaEJwbU5UZmx2aVk/view?usp=sharing
Here the patch (actually I've used Ohmicide for post distortion too)
https://drive.google.com/file/d/0B-qd1kodWxtyVFBOdE9WVG1fb3c/view?usp=sharing