Reviews on the shematic before building the PCB of my synthesizer

[CrackersX]

Hey,
I’m a French student learning electronics, and I’ve been working on creating my own synthesizer design.
It has a VCO, VCF ,VCA, MIDI and Voltage input.
I’ve already gotten help from you guys for the VCO and VCF parts .
The goal is to keep it as analog and accesible as possible.
I’ve added many things to the project, and I’d really appreciate it if you could review it, as there might be some parts I haven’t fully understood and that might be just stupid for you.
After that, I’ll start learning PCB design since this is my first project.

MIDI to CV
A simple MIDI to CV converter using an octoplexer and an ATtiny202 as the microcontroller (I’m not familiar with programming it yet, but I’ll figure that out later; for now, I’m focusing on the hardware).
PWM in output that is fixed with a RC filter.

Expo Converter
This design is from here: DIY Synth Series VCO - All About Circuits. https://www.allaboutcircuits.com/projects/diy-synth-series-vco/
I’m having trouble adapting the component values for my circuit—I'm not sure how to do that.

VCO
For more info, check the forum below: VCO Issue - EEVblog.https://www.eevblog.com/forum/beginners/issue-with-my-vco-design/
It’s working well (except I get a weird sawtooth wave in the KiCad simulation).

VCF
Extracted from the LM13700 datasheet. https://www.ti.com/product/LM13700
I don’t fully understand the theory behind it yet, but I’m working on it . Someone’s already tried to explain it to me.
The important thing is that it works in theory, though I haven’t been able to simulate it yet.

VCA
Also extracted from the LM13700 datasheet.
Same situation as the VCF—I don’t fully understand it, but I’m getting there.

Kicad shematic here: https://drive.google.com/file/d/1BJskOecOMx52XnV7E1l1ArKJIcwJJSce/view?usp=sharing

Thanks in advance! 

[CrackersX]

I made some changes about the overallsynth.
If you guys see any mistakes let me know
Thank you very much 

[schmitt trigger]

Micro controller’s PA7 port, which is apparently filtered with a simple RC network, goes to…. Where?

[CrackersX]

Sorry, I haven't mentionned it but to the Sum Op amp at the top on the expo converter with a resistor before 

[MarkF]

Why are you not using PA7 (the USART RXD) for MIDI input?



From the MIDI spec:
   The hardware MIDI interface operates at 31.25 (+/- 1%) Kbaud, asynchronous, with a start bit, 8 data
bits (D0 to D7), and a stop bit. This makes a total of 10 bits for a period of 320 microseconds per serial
byte. The start bit is a logical 0 (current on) and the stop bit is a logical 1 (current off). Bytes are sent
LSB first.

MIDI is a serial protocol data stream. 
It looks like you intend to sample the stream via an A/D conversion to decode the MIDI input.

I would try to decode it with the ATTINY UART.  Haven't looked into generating the MIDI clock or if you would need an external clock source.


How many note polyphony do you think you can get out of an ATTINY? 
From the outset, it appears you do not understand what MIDI is.     
It is NOT an analog audio signal that you can just sample.  It is digital commands (i.e. Note_On, Note_Off, etc.)

[pcprogrammer]

How many note polyphony do you think you can get out of an ATTINY? 

With the PWM output and the low pass RC filter to make it a control signal, like indicated in the original post, it is not about polyphony. The output of the ATtiny is fed into the exponential converter to control the frequency of the VCO.

But what is missing in this synthesizer design is an envelope generator. (ADSR) Without this is will just be a crappy sound generator that allows you to play a little with the knobs to control the sound, but won't allow you to actually play it as a proper instrument.

The MIDI to CV converter needs a bit more than just the control voltage output. You also need a gate signal, that indicates a note being on or off. On receiving a note on message, convert the note value to a control voltage, and set the gate signal active. With a second control voltage it is also possible to use the velocity value to control the loudness, but not directly needed. On receiving a note off message set the gate signal inactive.

The gate signal triggers the ADSR to generate the control voltage for the VCA.

There are better MCU's out there to make things more useful. Ones that have build in DAC's to make the control voltage, or get a DAC that can be controlled with SPI or I2C. Something like this for instance: https://ww1.microchip.com/downloads/en/DeviceDoc/20002249B.pdf. The benefit of using something like this is that it will give a stable signal to control the note and have a more direct response to changes. The problem with the PWM is that it can bleed through into the audio path. A first order RC filter will only do so much unless it is set to a very low cutoff point.

[MarkF]

How many note polyphony do you think you can get out of an ATTINY? 

With the PWM output and the low pass RC filter to make it a control signal, like indicated in the original post, it is not about polyphony. The output of the ATtiny is fed into the exponential converter to control the frequency of the VCO.

But what is missing in this synthesizer design is an envelope generator. (ADSR) Without this is will just be a crappy sound generator that allows you to play a little with the knobs to control the sound, but won't allow you to actually play it as a proper instrument.

The MIDI to CV converter needs a bit more than just the control voltage output. You also need a gate signal, that indicates a note being on or off. On receiving a note on message, convert the note value to a control voltage, and set the gate signal active. With a second control voltage it is also possible to use the velocity value to control the loudness, but not directly needed. On receiving a note off message set the gate signal inactive.

So, this is going to be a single note MIDI implementation?

What are you going to do when the MIDI channel wants to play more then a single note?  For instance, playing a piano/keyboard could possibly try to play half a dozen notes at the same time.  Normally each note would be a sample waveform stored in memory and the MIDI implementation would play multiple samples summed together.

Normally you would sample and digitize each note an instrument can play.  Then recall the samples for every note that is ON, scaling each note's amplitude and sum them all together.  For a full keyboard that will be 88 notes that need to be stored.  Each note will NOT be a single tone.

Only being able to play a single frequency (i.e. only ONE note) is going to be vary lame and hardly worth the effort.  Think pianist here.  If they play a chord, which of the three notes are you going to generate.  And what if they use both hands.  The chord in the left hand will play all the time while the melody is played with the right hand.  This is polyphony and it matters a great deal.  Most modern MIDI implementations have a polyphony of 256 voices.

Listening to a beginner piano player practice a song with just one finger gets old real fast.

[pcprogrammer]

There are several monophonic synthesizers on the market that are highly appreciated. Think of the Moog minimoog. These use either selectable or fixed implementation of which note is played. Like always the lowest or the highest of the notes being active. Or the one that became active last.

In the world of instruments monophonic is not uncommon. Think of wind instruments like a saxophone or a trumpet. Only one note is active when played.