Monday, December 28, 2015

Making a Breadboard Arduino

I'm still working on my OTA Overdrive modification.  My approach is to control the overdrive using an Arduino-controlled digipot.  But, if you saw the last picture from that post, you saw that my Arduino-digipot setup is a total mess.  To fit it inside my Polysix, I need to clean it up.  I think that means that I'm going to make my own custom PCB.  Exciting!  If I'm going to do that, I want to integrate the Arduino processor right onto the PCB.  Fully integrated!  But that's something that I don't know how to do.  Today is my first step in learning.  Today, I figure out how to build my own Arduino from parts.

Recipe for an Arduino: one Atmel 328P (foreground) along with one 16 MHz crystal and a couple of 22 pF caps (background)

Following Instructions:  My goal is to build an Arduino Uno on a breadboard.  Furthermore, I want to be able to program it from the Arduino IDE, just like a real Arduino.  Not surprisingly, the Arduino folks have a pretty good page on how to make a breadboard Arduino.  These are the instructions that I followed, though I'm bad at following directions, so you may see some differences here.

Buying Stuff:  You don't need many parts.  I already had basic stuff like a solderless breadboard, resistors, and jumper wires.  I did have to buy the Arduino processor (an Atmel AVR ATMega328P), a 16 MHz crystal, and a couple of 22 pF caps.  Easy.

Wiring Together my Breadboard Arduino:  Once I got the parts, I started wiring it up.  As shown in the picture below on the left, I started with the power (+5V) and ground connections, along with the 10K pull resistor for the reset line.  Then, as shown in the picture on the right, I added the crystal and its supporting caps.  That's it.  It's done.  [Note that some folks like to add a push button to make it easier to reset the chip.  It's not necessary and I never felt the desire to use it.  You don't need it.]

Wiring the Elements on a Breadboard.  Start with the Atmel chip and wire in the power (+5V and Gnd) along with the pull-up resistor for the reset line.  Then, add the oscillator elements (crystal plus caps).  That's it!

Is it now an Arduino?  No.  My breadboard circuit still is not quite an "Arduino" because it can't yet be programmed from the Arduino IDE.  The missing piece is that the processor has not been programmed with the Arduino "bootloader".  This is the software that tells the chip what to do upon startup, including how to listen to the Arduino IDE after being reset.  So, to turn my breadboard circuit into a breadboard Arduino, I have to put the bootloader on it.

Burning the Arduino Bootloader: Following the Arduino page on this topic, I put the bootloader on my breadboard Arduino using a real Arduino Uno.  I connected the Uno to my PC and (via the Arduino IDE) uploaded the "ArduinoISP" sketch.  Note that this is programming the real Arduino, not my breadboard one.  Once the Uno was programmed, I continued to follow the instructions and hooked up the Uno's SPI pins to my breadboard Arduino's SPI pins (see photo below).  Finally, in the IDE, I set "Programmer" to "Arduino as ISP" and then selected "Burn Bootloader".  I took just a second or two and it was done!

Using a real Arduino Uno (left) as a programmer for my breadboard circuit.  Here, I'm burning the Arduino bootloader onto my Atmel 328P so that it can be programmed as an Arduino.

Programming my Arduino:  Theoretically, my breadboard Arduino (it's an Uno, specifically) can now be programmed with regular Arduino programs straight from the Arduino IDE.  It has no USB jack, however, so how do I actually get the program onto the breadboard Arduino?  The Arduino instructions say that, if you've got the right kind of real Arduino, you can use your real Arduino to act as a USB-to-Serial converter.  Great!  Let's try!

From One Arduino to Another:  The key to doing this transfer is that you need the right kind of Arduino.  The "right" kind of real Ardiuno is one where the main chip is a big DIP, which will need to be pulled out for this trick.  Most of my Arduinos happen to be the SMT version, which won't work.  In my bin of spare electronics, I did find one Arduino that is DIP version (yay!).  So, following the Arduino page's instructions, I popped out the big DIP chip and then connected the board's TX and RX and Reset lines to my breadboard Arduino (see below).  Once connected, I told the Arduino IDE software to upload the "Blink" sketch and VOILA!  The light blinks!  It works!

Using an empty Arduino Uno to program my breadboard Arduino with the basic "Blink" sketch.  My breadboard Arduino lives!

Programming via FTDI:  After this exciting initial success, I found that this setup wasn't ideal.  It turns out that my DIP Arduino would sometimes fail to program my breadboard Arduino.  I then remembered that this unreliability was why this Arduino had sunk to the bottom of my box of electronics...I had seen problems with it previously.  Very annoying.  So, to avoid more annoyance, I purchased a stand-alone USB-to-serial converter.  There are a variety of these devices available.  I bought an "FTDI Friend" from Adafruit because they usually make good stuff.

Connections for using an Adafruit "FTDI Friend" to program my breadboard Arduino.  On the left, the figure shows the names of the connections as labeled on the FTDI friend itself.  On the right, the figure shows the name of the points on the breadboard Arduino where the wires should get connected.

Connecting the FTDI Friend:  While the Adafruit documentation is good on how get started with the FTDI Friend, it does not explicitly show how to connect it to a breadboard Arduino to transfer a program.  Instead, I found this thread on the Arduino forum, which was very helpful.  The picture above shows the connections that worked for me.  On the left are the names of the wires as labeled on the FTDI friend itself.  On the right are the names of where it should connect to a breadboard Arduino.

Don't Forget the Capacitor!  One key detail (as stated in the thread above) is that you have to use a capacitor between the RTS pin on the FTDI Friend and the Reset pin on the AVR chip.  The cap needs to be 0.1 uF or bigger.  If you don't have the cap in series, it doesn't work!  Once i did this, I could reprogram my breadboard Arduino from the Arduino IDE reliably and with ease.  It is very satisfying.

Can my Arduino Drive my Digipot?  The whole purpose of this experiment is to figure out how to build an Arduino to the digipot that's at the heart of my Overdrive Mod for my Polysix.  So, before I declare victory with my breadboard Arduino, I want to make sure that it worked like a real Arduino to drive my AD5260 digipot.

My breadboard Arduino controlling an AD5260 digipot.  It works!

Testing with the Digipot:  I wired up the digipot the same way as in my previous post, though I did need to reference the Arduino pin map to find which pins on my naked AVR chip corresponded to the SPI bus (answer: pins 19, 17, and 16).  Once I got it all connected (see picture above), I loaded my test code for the digipot, which steps through a few resistance values.  As you can see in the picture below, it works just I'm getting a resistance of 8.3 kOhm.  It was very satisfying to confirm that my breadboard Arduino was able to drive my digipot.

Measuring the resistance values produced by my Arduino-controlled digipot.

Adjusting CPU Speed:  Now that it all works, I'm thinking ahead to the PCB that I'll design to fit these elements within the Polysix.  I'm a bit concerned that the addition of this microcontroller, plus the others that I've already added, might be stressing the power supply of my old synth.  My old synth already gets pretty warm.  So, if possible, I'd like to minimize the power consumption of my added elements.  One easy way to reduce power is to slow down the clock speed of the processor on my breadboard Arduino.  Looking at this thread, you can change the CPU speed on an Arduino simply by adding a little code to your sketch.  That's easy!

Measured Current Draw:  Following the example code in the link above, I tested each speed from 16 MHz down to 1 MHz and recorded the current draw for each case.  My results are shown below.  In all cases, I confirmed that the digipot still works, which is great.  So, it appears that I can cut my power draw almost in half by running at 1 MHz.  Great!

Current Draw When Running the "Blink" Program.  The clock speed is changed by changing the clock divider setting.  No is LED connected.  

Lessons Learned:  The purpose of this whole experiment has been to learn what needs to be learned so that I can build my own Arduino-based modification for my Polysix.  Let's summarized the important lessons:
  • I am able to build my own Arduino from a raw Atmel AVR chip.  That is fantastic.  
  • To put the Arduino bootloader on the raw AVR chip, I need access to the chip's SPI pins.  So, on my custom PCB, I need to make those pins available.
  • To program the AVR chip from the Arduino IDE, I need access to the chip's TX, RX, and Reset pins.  So, I need to break those pins out, too.
  • To upload my programs to the AVR chip, I need a USB-to-serial converter.  My Adafruit FTDI Friend works great as long as I've got that in-line capacitor on the RTS line.
  • To save power, I can run my Arduino at 1 MHz instead of 16 MHz.  I confirmed that the digipot still works fine at the slower speed.
Whew!  What a great day of learning!  Now to start with the PCB design.  That'll be a lot more learning!

Follow-Up:  I designed a custom PCB, including a home-brew Arduino!  You can check out the story here.

Wednesday, December 23, 2015

Arduino-Controlled OTA Overdrive

I'm really enjoying playing my OTA Overdrive Mod for my Polysix.  I also enjoy how simple it is -- I just added one resistor.  If I want a different amount of overdrive (or none), I swap in a different resistor.  So simple.  The question now, however, is how to make this mod be robust and usable day-to-day?  Well, today, I start that process.  My goals are to make it controllable from the front panel *and* to save and recall its setting with the Polysix's patch memory.  It's this second goal that makes this mod a little tricky.  Let's figure it out!  (Not surprisingly, my solution involves an Arduino...)

Controlling my OTA Overdrive with an Arduino and a Digipot.  

Why Not Use a Pot?  As I said, my OTA overdrive is controlled by changing resistors.  So, the easiest solution is to mount a potentiometer (a "pot") to the Polysix face, to wire it as a variable resistor, and to connect it to the correct spot in front of the OTA.  Fit with a nice knob, a pot is easy to use, simple to install, and inexpensive.  Sound great!  The main problems with using a traditional pot are that (a) I don't want to drill a hole in my Polysix to mount the pot and (b) a pot is not programmable, which means that its setting can't be saved and recalled with the overall synth patch.  I'd like to find a way to overcome both issues.

Using a Digipot as a Variable Resistor for my OTA Overdrive Mod

Use a Digital Potentiometer:  While a traditional pot isn't quite satisfactory, I think that a digital potentiometer (a "digipot") would work great.  I first used a digipot to add velocity sensitivity to my Polysix and I found it to be very useful.  A digipot is like a regular pot in that it can be used as a variable resistor, but a digipot has an advantage in that it can be controlled by digital messages sent from a microcontroller.  As a result, I can use buttons and knobs that already exist on the synth to control the overdrive.  And, in theory, I can even change the overdrive settings with each synth patch.

Control by an Arduino:  You don't get something for nothing, however.  To use a digipot, you need a microcontroller to drive it.  Microcontrollers are pretty easy to fact, I've already added two to my Polysix: (1) an Arduino Mega acting as my key assigner and (2) a Teensy 3.1 acting as my velocity processor.  I could use either of these existing devices to control my digipot, but neither is conveniently located to the KLM-368 Effect PCB where the overdrive mod lives.  I hate running long wires carrying noise-spewing digital signals, so I think that I will add yet another microcontroller to my synth.  This time, it'll be something small like an Arduino Micro.

Block Diagram of my Solution

User Control:  While the Arduino can control the digipot, how will the user tell the Arduino what overdrive level is desired?  Since I don't want to drill a hole to add a new dedicated knob, I looked at my Polysix control panel with fresh eyes.  Which knobs ore buttons could I re-purpose?  My eyes fell upon the "Attenuator" knob.  For me, I never use the knob below 0 dB, which means that the whole left half of the knob is basically unused.  I could totally re-purpose part of that knob for adding overdrive.   No drilling necessary!  But how do I sense the Attenuator's setting?

Re-Using the "Attenuator" Knob to Be My Overdrive Control

Look for the Signal, not the Knob Itself:  The key innovation is that I should not try to sense the knob itself.  Instead, I should sense the signal that the Polysix CPU generates in response to the knob.  The reason to sense the signal instead of the knob is because of patch memory.  To enable patch memory (even when patch memory isn't being used), the Polysix CPU scans all of the knobs and switchs on the panel and then generates control signals in response to those settings.  Then, if a patch is recalled from patch memory, the CPU ignores the switches and knobs and generates the control signals based on memory.  If I am successful in having my Arduino listen to the signal generated by the Polysix CPU in response to the Attenuator knob, my overdrive setting will respond to both the knob and to the patch memroy system.  This is what I want!  But which signal in the Polysix corresponds to the Attenuator knob?

"P-Vol" is the Attenuator Knob:  Looking around the schematic, I found that the Attenuator setting corresponds to a signal called "P-Vol", which presumably stands for "programmable volume".  It turns out that P-Vol is a simple analog voltage signal.  Therefore, via its Analog Inputs, my Arduino can listen to this signal and engage (and increase) the overdrive whenever the Attenuator knob is turned below 0 dB.  Great.  The P-Vol signal is also available on the KLM-368 Effect PCB (at R188), which makes it convenient to the overdrive mod itself, which is likely where I'll mount the Arduino.

The "P-Vol" signal reflects the setting on the Attenuator knob.  I'll control the amount of overdrive by sensing the value of this signal.  I can read the P-Vol signal at R188 on the KLM-368 Effect PCB.

Interpreting P-Vol:  Attaching a wire to the upstream side of R188, I read the voltage using my digital multi-meter for various settings of the Attenuator knob.  As you can see in the graph below, it's a nice, simple, straight line.  I can use this data as part of a lookup table on my Arduino to determine whether to engage the overdrive (if knob is set to less than 0 dB) and how much overdrive to apply (a lower knob setting will engage more overdrive).  Via this lookup table, the Arduino will command the digipot to actually induce the overdrive that I want.  This plan is coming together nicely.

Data that I measured relating the "P-Vol" voltage at R188 to the Value of the Attenuator Knob

The Wiring Plan:  Now I have to figure out how to wire up all of the pieces to put this plan into action.  Luckily, I already figured out how to connect and use a AD5260 digipot.  This time, however, instead of using a full-size Arduino Uno,  I'll use an Arduino Micro.  The pin mapping is the same, so the connection plan is still valid.  The image below shows my plan for wiring it up.

Wiring the Arduino Micro to the AD5260 Digipot.  Lots of signals are needed from the Polysix, including +5V, -5V, Gnd, P-Vol.  These are in addition to the input and output audio signals that'll be manipulated by the digipot to overdrive the OTA.

Both +5V and -5V:  There is one important difference in this new plan: the digipot needs to be supplied with both +5V and -5V.  The -5V is new.  The reason that it needs a bi-polar power supply is that the audio signal that I'm attenuating is a bi-polar signal.  The audio signal from the synth could span +5V to -5V, therefore the power supplied of the digipot needs to span +5V to -5V.  Lucikly, the Polysix makes -5V, so I can grab that voltage rail from the synth.  I had to pull it from the Polysix's power supply PCB (not the KLM-368 Effects PCB), but that's not really a problem, just a bit messy.

Wiring it Up:  I wired up the Arduino and digipot onto a solderless breadboard, as shown in the picture at the top of this post.  I then tacked in a bunch of wires to the KLM-368 PCB and grabbed the +/-5V from the main power supply PCB.  As you can see below, it's quite a mess.  I usually find that messiness is part of the discovery and invention process.  Things always seem to get messier before they get simpler.

Trying new things is a messy, messy business.

Software:  After wiring up the hardware, I need to write software for the Arduino to listen to the P-Vol signal and to drive the digipot.  For the digipot, I simply re-used my previous software that I wrote to drive the AD5260.  Then, I added the logic to read the analog "P-Vol" signal and decide what resistance level to set in the digipot.  It was nothing complicated, though it did take some trial and error to find the right voltages at which to transition between resistance levels.  My code is shared here on Github.

Success!  After firing up the system, and after fixing a bunch of bugs in my software, I found that this Arduino-digipot solution does successfully control the amount of overdrive based on the Attenuator knob setting.  Success!  And, yes, it also correctly respond via patch memory changes.  Yay!  It feels so good when stuff works.

Issues:  This setup does have one small problem, however -- the lowest setting of the Attenuator knob (-10 dB) isn't detected by the Arduino.  The -8 dB setting and the -10 dB setting give the same behavior.  The problem should have been obvious earlier.  As shown earlier in my graph of voltage vs knob setting, note that both the -8 dB and -10 dB knob settings yield a P-Vol voltage that is negative.  The Arduino can't measure negative values.  To the Arduino, they both look like zero volts.  As a result, the Arduino can't tell the difference between the two knob settings.  I totally should have realized that earlier.  Bummer!

Next Step:  There are two easy solutions here: (1) shift the DC level of P-Vol with an op-amp prior to sending it to the Arduino's analog input, or (2) abandon P-Vol and, instead, sample the four digital signals that the Polysix uses to generate the P-Vol.  The latter approach definitely looks easier.  Next time, I'll give that a try instead.

Follow-Up:  I'm thinking about doing a custom PCB with the digipot and Arduino integrated together onto a single board.  To help me learn how to do that, I created my own breadboard Arduino from raw parts.  You can check it out here!

Follow-Up:  I've completed the PCB design.  It was actually pretty fun!  You can check out the story here.

Thursday, December 10, 2015

Polysix OTA Overdrive

I still chase my dream -- to get a decent electric piano feeling from my Korg Polysix.  Adding velocity sensitivity got me a long way there, but the sound itself needs to be dirtier, with more compression, more grit.  I tried adding diodes to the signal path to generate some distortion, but it sounded bad.  Too fizzy.  Now, I've modified my Polysix to overdrive one of its OTA circuits.  Having a soft onset that eventually leads to a deep, warm saturation, I think that it worked out pretty well!

OTA Overdrive:  After the failure of my experiment with diodes, I looked for other places in the Polysix circuit where I could generate distortion in other ways.  I noticed that the Polysix has a number of LM13600 chips, which are operational transconductance amplifiers, or OTAs.  Seeing them there reminded that a number of newer synths (e.g. Moog Sub Phatty) say that they offer "OTA Overdrive" as a means of user-controllable distortion.   I don't know how these synths overdrive their OTAs, but I was certainly game to figure out how to overdrive mine.

Pre-OTA Attenuation:  A key requirement of the LM13600 is that the input signals have to be very small.  If the inputs are too strong, the out signal will become distorted.  Since I want distortion, this looks like a good place to make my modifications.  Looking at the Polysix schematic, I see voltage dividers in front of every LM13600 to cut the signal down.  One example is shown below, which shows the elements around IC20.  The signal comes in on the right and exits on the left.  The voltage divider in front of IC20 is formed by R155 (10 kOhm) and R163 (100 ohm).  This cuts the input signal voltage by a factor of 100.  That's 40 dB of pre-OTA attenuation!

Adding a resistor in parallel to R155 reduces the attenuation prior to IC20 enabling the LM13600 OTA to be overdriven

Reducting the Pre-OTA Attenuation:  If I modify the circuit to have less pre-OTA attenuation (ie, to allow the signal to be stronger), I will likely overdrive this OTA.  I can easily reduce the attenuation by adding a resistor in parallel with R155, which will reduce the effect of the voltage divider.  For example, putting a 1 kOhm resistor in parallel to R155 will result in the signal being cut down by only a factor of 10, instead of the factor of 100.  This means that the signal is attenuated by only 20 dB instead of the 40 dB.  In effect, I'm slamming the OTA with a 20 dB stronger signal.  This seems like an easy path to overdriving IC20.  That's what I want.

Connecting the Resistor:  Looking inside the synth, I first looked for IC20 on the KLM-368 PCB.  I found R155 nearby.  To easily try different resistors in parallel to R155, I attached clip leads on either side of R155.  See the pics below.  I also attached my oscilloscope to C75 so that I could visualize the output as well as hear it.

Clipping in on either side of R155.
Adding a resistor, via the clip leads, to be in parallel with R155.

Trying a Range of Resistors:  As you saw in the video at the top of the post, I tried a range of different resistor values.  For gritty electric piano sounds, I liked the 3.3K and 1.1K values the best because the effect was simply warmed up the signal and only added some grit when I played hard.  At the other end of the spectrum of resistor values (ie, when I shorted across R155), I unexpectedly enjoyed the unpredictable chaos of the heavily saturated distortion.  It was really fun!

Seeing the Effect:  The video demonstrated what this modification sounds like.  The figure below shows what it *looked* like.  The figure shows the output of IC20 for different resistor values across R155.  As you can see, with no resistor (which is the same as a very large resistor), the output signal is a nice sawtooth, but it is relatively small.  Then, as I add a large resistor (3.3 kOhm) the signal gets quite a bit stronger due to the extra gain.  As I make the resistor smaller (1.1 kOhm), the signal gets stronger.  Continuing to make the resistor stronger (500 ohm and on), the shape of the waveform starts to deviate from a sawtooth -- the top and bottom are becoming rounded.  This is signature of the OTA being overdriven.  It's a softer, more rounded, distortion than seen in my diode mod from my last post.  Finally, when I short across R155, the distortion becomes so heavy that the signal is a square wave.  Under certain contditions, as seen in the video, that can be cool in its own way.

Output of IC20 recorded for different resistors placed in parallel with R155. Note that decreasing resistance results in increased gain, which eventually leads to overdriving IC20, causing the sawtooth to become distorted.  Note that, for these images, I played two notes at the same pitch and waited for the two voices to naturally phase into alignment.

More Than Just Gain:  Clearly, the main effect of changing the resistor is to boost the level of the signal going into the OTA, which then overloads the OTA and causes the output signal to be distorted,  But, gain and distortion is not the resistor's only effect.  Looking back at the Polysix schematic, I see that R156 and C80 are also in parallel with R155.  This resistor and capacitor act to boost the treble frequencies.  By adding my own resistor around R155, I will also be reducing the effect of this treble boost.  So, maybe the "warmth" that I felt was actually a result of changing the frequency response, and not necessarily due to overdriving the OTA.  To confirm this theory, let's analyze the circuit for different resistor values and see what happens.

Circuit Simulation:  To simulate this part of the circuit, I used 5Spice Analysis, which is a SPICE-based circuit simulation program with a nice graphical interface for Windows.  Like most graphical versions of SPICE, you start by drawing the schematic of the circuit that you want to simulate.  The screenshot below shows the schematic that I made to represent the elements leading into the IC20 OTA.  For this simulation, the input signal is generated by "SigIn" on the right.  "TPv1" on the left represents the output signal, which would normally go to IC20.  In between the elements of the Polysix's voltage divider, the R-C treble boost, and my added overdrive-inducing resistor.  Now I can run the simulation and see the expected frequency response.

Using "5Spice Analysis" to model the frequency response of the pre-OTA voltage divider network.

Expected Response, Unmodified Polysix:  The graph below shows the modeled response of the circuit.  On the bottom (in blue) is the the circuit response with no added resistor around R155.  As expected, the signal is 40 dB down, which is what we expect based on the 10 kOhm / 100 Ohm voltage divider.  Also note, however, that the treble frequencies are boosted due to the effect of R156 and C80.  At 5 kHz, the signal is boosted by about 5 dB.  While not a huge boost, this would definitely sharpen the sound.

Expected Response of the pre-OTA voltage divider network.  The different lines show the effect of different values for the resistor that I placed in parallel to R155.
Expected Response, Modified Polysix:  When the parallel resistor is added, I get the black traces shown in the figure above.  The primary effect is that the signal level goes up.  The 1.1 kOhm case, for example, shows that the signal is at -20 dB instead of at -40 dB.  This is the 20 dB gain that I mentioned earlier.  A secondary effect of adding the parallel resistor, however, is that the boost to treble frequencies becomes much less.  Perhaps this is the increased "warmth" that I perceived.  Hmm.  I think that this requires a little more exploration.  Maybe there are additional modifications that I can make to shape the frequency response to make the OTA overdrive sound even better!

Next Step:  I liked the sound and feel of this OTA overdrive much more than the diode distortion.  I think that, if I can figure out how to make this OTA overdrive controllable from the Polysix's front panel, I'll enjoy having this modification.  So, that's my next step: figure out how to have a controllable amount of overdrive for IC20.  Stay tuned!

Follow-Up:  How to make this mod controllable without clipping in new resistors?  In this follow-up post, I start designing this mod to use an Arduino and a digipot.