CMOS workshop part 3 - octave down

6. Octave down, Flip Flops and Latches
Lets have a look at a few different ways to create octave down using CMOS chips.

In all of the examples below something called a flip flop will do the frequency dividing. The flip flop/latch is one of the fundamental logic building blocks. It's used to store data and is how RAM memory in our computers works. One flip flop can store 1 bit of data, which means that it can hold a high or low state until we tell it otherwise. This can be very useful, for example when we want to use a momentary switch to turn something on or off until we press it again. Flip flops can also be used as oscillators, counters and much more.

But now let's focus on frequency dividing. What it does, to put it as simple as possible: Each time the clock (our input signal) goes high, the output changes state. Since the output only changes on the positive transitions and holds that state on the negative transitions we get exactly half the frequency at the output, like this:

Example of a basic S-R flip flop using NAND gates

Example of a flip flop latch

If you want to learn more about the logic math behind the flip flop, I recommend this video:
https://www.youtube.com/watch?v=PCT76PsDr6g

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Pre-filtering
Making a octave down circuit with CMOS is easy since we are now working with 1's and 0's. Getting good tracking is harder, but there are a few simple measures we can take to get acceptable tracking. The first thing to keep in mind is that we can improve tracking significantly by using the neck pickup of the guitar and the treble rolled off. But we also need to add a lowpass filter after our schmitt trigger since it adds alot of high frequency content. There are many different lowpass filters we can use, varying in complexity, but for this article we'll use a simple passive RC filter.

RC lowpass filter

Here is a sound example of octave down with: 1. Bridge pickup, no filter 2. Bridge pickup with RC filter 3. Neck pickup with RC filter.

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Let's get the breadboard ready!
We'll start out with the circuit we made in part two, then add the new section before the output capacitor. To keep it simple, the schematics will not show the "cmos'ifier" part or the output part. Lets add the RC filter first (R6/C4). 10K and 22nF-100nF is a good starting point. Then we'll add the octave down part.

No decoupling capacitor after the schmitt trigger is needed. CMOS chips are make to interface with each other directly so decoupling caps are rarley needed, except for linear amplifiers, filters or special schmitt trigger operations. It's very forgiving this way.

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1. Dual D-Flip Flop (CD4013)
This chips is used in the MXR Blue Box, MadBean Lowrider, The Rocktave and many others.

This chip has two D-flip flops. The D prefix means that it changes output state on positive transitions only, exactly what we need. For two octaves down, we'll just put both flip flops of the chip in series so that the second flip flop will divide the output of the first flip flop.

• Input goes to Clock
• The NOT Q output (Q with a line) goes back to the data input (D) via a 10K resistor
• Set and Reset goes to ground
• Q or NOT Q goes to output. NOT Q is the inverted output
• VCC to V+ and VSS to ground

CD4013 pinout

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Output controls
You can use pots/trimmers as variable resistors or voltage dividers to have control over each octave or a switch to toggle between them, a blend pot/trimmer between straight square wave and octave down ect... Experiment with the controls that you want. Here are a few examples:

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2. Binary Ripple Counter (CD4024)
This chip is what I used in the Arcadiator. it's very common in DIY synth stuff and it's also used in the Slacktave (by Slacker, the designer of the Echo Base) and the Bit Commander by Earthquaker Devices. It's very simple to use and require no extra external components.

The CD4024 chip is a counter that advances one count every time the clock changes from high to low. The first output is divided by two, and the next output divides the preceding output by two and so on, up to output 7 that has the input frequency divided by 128. This means that each output is one octave lower.

• Input goes to Ø
• Reset goes to ground
• Q1 is the one octave down output
• Q2 is the two octaves down output
• VCC to V+ and VSS to ground

CD4024 pinout

Part of the Arcadiator schematic

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3. NAND Gates (CD40106, CD4093)
Let's look at how it's possible to make a D-flip flop out of NAND gates.

As you can see, it requires many gates and even one 3-input gate, which means we have to use at least two chips. But remember that I mentioned that schmitt triggers can be configured to make NAND and NOR gates?

A schmitt trigger gate can be made into a multiple input NAND or NOR gate using 1N4148 diodes and a resistor, with as many inputs as we want. This is something I picked up from Ray Wilson, he calles it "mickey mouse logic". This makes the CD40106 a very versatile chip, and using this neat trick we can make a flip flop out of a single CD40106 chip.

"mickey mouse logic"

The same mickey mouse logic can be applied to the 2-input CD4093 schmitt trigger by first connecting both inputs together (which makes a NOT gate).

A good way to test these octave down circuits on the breadboard is to connect a LFO with a LED to the input and another LED at the output. That way we can easily see if it works. I tested the NAND schematic first with a freeware program called Logic Circuit just to make sure it would work, then I breadboarded it. This is the result.

For this video I used another CD40106 for the LFO. We'll cover LFO's in a later part. (Ignore the extra parts on my breadboard and the non decoupled inputs.. I cheated alittle to make it quickly...)

NAND equivalent of a D-flip flop

CD40106 flip flop schematic

Transistors bi-stable multivibrator (equivalent to a D-flip flop) from the Colorsound Octivider schematic

It works. :) I actually used this flip flop version for the sound example in the beginning of the post. This was very confusing to breadboard and I won't even try to make a breadboard diagram. It's not a very convenient method, unless you have a huge stash of schmitt triggers and diodes that you want to put to good use... I just wanted to show you what is possible.

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Post-filtering
It can be a good idea to have a RC lowpass filter on the octave output if you want it to sound less synthy or harsh. It's common to filter out anything above around 100hz to make it blend better with a clean guitarsignal. Experiment!

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I'll stop here. This post became massive and took me a whole day, and I didn't even mention octave down by using gated oscillators, phase locked loops or shift registers. Maybe more about that in a later part. But in the next part I will cover LFO's and oscillators.
We will analyze the Flawed Logic Fuzz and we will breadboard a drone synth.

UPDATED 2023-10-31: Some minor spelling fixes.