I’m continuing my look into the ESP32 and PWM. This time I’m adding in some analog control to introduce an element of frequency modulation to the synthesis.
- Part 1 – All the theory and research around PWM and the ESP32.
- Part 2 – Generating different waveforms on multiple channels.
- Part 3 – Introducing analog control and frequency modulation.
Warning! I strongly recommend using old or second hand equipment for your experiments. I am not responsible for any damage to expensive instruments!
If you are new to microcontrollers, see the Getting Started pages.
Parts list
- ESP32 WROOM Module
- 3x 1kΩ resistor
- 2x 10kΩ potentiometers
- 1x 10uF electrolytic capacitor
- 2x 100nF capacitor
- 1x TRS socket
- Breadboard and jumper wires
The Circuit
This expands on the previous circuit to allow me to feed back the output of one of the PWM channels into an analog input controlling the other.
It also allows the use of two potentiometers to control the frequencies independently.
It’s not very easy to see what is going on with the Fritzing diagram, but there are essentially four circuits in play here.
The first is the PWM filter and audio output stage from parts 1 and 2. This takes the 0-3V3 PWM signal and turns it into a more audio friendly +/- 800mV (ish) signal.
The second is the PWM filter stage but without the voltage divider to reduce the pp voltage and without the coupling capacitor to remove the DC offset. The output of this is therefore a waveform with a 0-3V3 sweep. I’m leaving it like this as I want to be able to use it as a control voltage for an analog input stage, which brings me on to…
The third is a simple potentiometer connected to one of the analog inputs.
The last is another potentiometer input but that can also be modulated by the output of the second PWM signal – the 0-3V3 one.
Now I think I’m ok to connect the output of the PWM stage to the input in the way shown above, but at this stage it might be a good time to remind you I’m not an electronics person and it is best to assume I don’t know what I’m doing. I’m using cheap development boards and effectively throw-away amplification, so am feeling quite free to experiment.
I don’t believe there is any way to connect a high signal source to a low signal sink without going through a resistor to limit current, so I think this is ok for this kind of experiment. Basically I don’t think I’m slowly cooking my ESP32, but don’t take my word for it…
Naturally if this was accepting any kind of input signal from elsewhere some kind of protection circuitry would be required and I’ve given no consideration here to things like impedance. This is me messing around – nothing more, but if you can see something wrong in what I’m saying feel free to let me know in the comments.
ESP32 GPIO Usage:
- GPIO13 – Sine wave output – connected to the audio output.
- GPIO12 – Saw wave output – not used above.
- GPIO14 – Triangle wave output – connected to the analog input for the sine wave.
- GPIO27 – Square wave output – not used above.
- GPIO39 – Analog input to control triangle/square wave frequency.
- GPIO36 – Analog input to control sine/saw wave frequency.
The Code
This is still using four separate PWM outputs but I’ve added in potentiometer control for them in pairs. As described above there is one pot to control the frequency of the sine/saw waves and one for the frequency of the triangle/square waves. But as also already stated, the sine/saw frequency can also be modulated by the triangle wave output.
The key feature to get this working is to use an analog reading as an input to the setFreq() function from before for the individual channels.
My loop, which was previously empty, now looks like this.
#define NUM_ADC_PINS 2
int adc_pins[NUM_ADC_PINS] = {36, 39};
uint16_t adcval[NUM_ADC_PINS];
void loop () {
for (int i=0; i<NUM_ADC_PINS; i++) {
uint16_t algval = analogRead(adc_pins[i]);
if (algval != adcval[i]) {
setPotFreq(i, algval);
}
adcval[i] = algval;
}
}
I’ve just added in some mapping via the setPotFreq() function to determine which PWM pins are associated with which potentiometer.
int pot2pwm[NUM_PWM_PINS] = {0,0,1,1};
void setPotFreq (int pot, unsigned freq) {
for (int i=0; i<NUM_PWM_PINS; i++) {
if (pot2pwm[i] == pot) {
setPwmFreq(i, freq);
}
}
}
To change which PWM channels map to which pots, just update the pot2pwm[] array of values. To have four pots and each channel independent is just a case of adding the two extra pins to the adc_pins[] array and then updating pot2pwm to be {0,1,2,3}.
I’ve not gone that far as I wasn’t too fussed about adding the extra circuitry required – my solderless breadboard is getting a little crowded as it is.
I’m using the “simple” analogRead() functionality which is fully Arduino compatible, albeit with a higher resolution (12-bits). The ESP32 also has the option for a faster, continuous reading of the ADCs via a “Continuous mode driver”. More details can be found here. I did wonder about experimenting with that at some point. It is interesting to consider if that could sample at a similar frequency to the PWM output, giving the possibility of some kind of audio processing, but that is a set of experiments for another day.
I’ve also seen mention of using the I2S driver with the ADC or DAC, but I must admit I don’t quite understand what is being said there. Another thing added to the “to be read/researched” pile.
But I’ve kept it simple for now and that seems to have done the trick.
Closing Thoughts
As can be seen in the video it is possible to get quite an interesting range of sounds out of this already!
The obvious question in all this is why the external electrical feedback from one wave output to the other? That is the kind of thing that can be relatively simply done in software. Indeed, the Mozzi FM synthesis code I used in ESP32 and Mozzi does exactly that and more!
In truth, I quite liked the idea of having more of a “virtual modular” feel in that the ESP32 can have several functions, in this case control voltage inputs and voltage-to-digitally controlled oscillators, that could be patched together using jumper wires.
I’m not sure where I’m going next, but an obvious next step might be some kind of envelope generation using the DAC outputs to modulate the PWM audio.
I really ought to do some proper thinking about those component values though. The filters are working at the frequencies I’m messing around with at the moment, but I’ve not really done any proper calculations to see what the optimal values ought to be. And it would be useful to include a little protection on the inputs and possibly some buffering on the outputs. At best I’m being rather simplistic and somewhat optimistic in my approach so far.
But this is the point (as I’ve said before) where I really need some kind of audio experimenters PCB, which I’m already sort of chewing over in the background. Watch this space.
Kevin
Simple DIY Electronic Music Projects 