So, in my electronics lesson at school I'm allowed to do any project I wish - and I've decided to produce a beginner-level synthesiser. As some preliminary research, I'd like to ask you guys what sort of features you'd expect such a thing to have - with this, I'm aiming to fill a gap somewhere between the Monotron and a microKORG. I want to use off-the-shelf components to implement an assortment of features - however, to decide which ones, I need to ask some of you guys!
I'm thinking of having - VCO with square, sine and triangle waves (selectable all at once if need be, but no detune) LFO (with square, sine and triangle selectable waveforms) Low pass filter (cutoff + resonance) White noise generator ADSR envelope 16 step sequencer (can be synced externally with a click track, like a Monotribe) Line out + amplified headphone out
Now, this isn't meant to be barebones like a Monotron. I want this to be something which, whilst not incredible quality, could be sold for a decent price (around £150, maybe even lower) and give beginners a decent taste of synthesis. If you have any other ideas for what should be included - as long as it's realistic! - then please let me know what you think!
make the lfo and the envelope pointable at anything rather than fixed doing one thing make the lfo sync to the click track with the step sequencer midi sync would be nice and it really needs more than one oscillator
make the lfo and the envelope pointable at anything rather than fixed doing one thing make the lfo sync to the click track with the step sequencer midi sync would be nice and it really needs more than one oscillator
i'd rather have the lfo sync be switchable... my theory is always the more options the better, since that allows for more creativity in sound design
Oh yeah, I was gonna have the LFO sync-able with the step sequencer!
Also, could you like, post lists of basic features? Like LFO, then say maybe extra bits to be able to do with it? I need to justify every feature I put in this, sooo... :3
ADSR is always good. are you building this to be keyboard controlled or step sequencer or maybe ribbon (like the monotron)? I had a similar idea for a very basic synth. nothing fancy but good enough to sample from. also different wave forms. dual oscillator. I am getting greedy....
I think it's most likely going to have a stylophone-style interface, simply because it would be far more practical for me to make than anything else.
The chip I'm using for an oscillator produces all three waveforms at once, so it SORT OF has three oscillators, though you can't detune them from one another. But you will be able to mix between square, sine and triangle to your heart's content!
I suggest keeping your project simple so that you don't bite off more than you can chew. A 1-osc synth is okay if it can be versatile, some examples from history are Roland SH-1000 and Arp Soloist series.
Borrowing the "keyboard" of a stylophone is okay, but see if you can create a modulation source with a light sensor or some other control for the non-stylus hand.
You say you don't want detune, but you'll have it anyway- you have to put the instrument in tune somehow, right?
If you want the instrument to stay versatile, have the modulation sources connect with banana plugs and borrow monotron's input to the filter action. Anything more (to me) is a bit beyond a school project. Good luck!
Ooh yeah, something with an LDR sounds great, can't believe I hadn't thought of that before!
By detune I mean that often with synths with more than one oscillator, you can tune them separately (effectively play harmonies on a mono synth). Despite the fact my synth produces three different waveforms simultaneously - almost like it has 3 oscillators - you can't tune them independently.
Hmm, I'm not too sure on how I'd make the filter patch in-able to - I think I'd have to put it after the VCA with the envelope, but I'd need to make sure there is no feedback and protect the filter from getting overloaded.
Really, the most complex part is going to be the sequencer, as it's gonna be digital and I've gotta program it and figure out which digital to analogue converter I need to use and all this crazy stuff - the rest of the synth is the more simple part
BUT ANYWAY What I really need for my research is for you guys to list features. Like, the features you think a beginner level synth should have - not what it would be nice to have, but what you would expect one to have.
So you'd say like: 2 oscillators LFO Low pass filter Arpeggiator
Etc etc etc
If you can do that, that's what I need! Though I'm certainly happy to discuss my project too
I think the controller is the most important thing, and what can kill beginner synths (or other instruments): monotron, stylophone, etc. I would prefer the option of being able to plug in my own controller, like the mebleep or shruthi-1.
I'm not too sure how I could interface an external controller other than via MIDI - and I have no clue how I could implement that. I can't make it too complex, for obvious reasons. Making it as simply a module, without any kind of controller, would be totally counter-intuitive to what I want to achieve, so the controller is whatever I can make, really.
Nothing is set yet, so whatever I can actually utilise is will what I make - most likely a stylophone style keyboard and that's by far the simplest to do, and will let me have a wider range of notes in a smaller amount of space. If it's for a newbie, it can't be MASSIVE, so it's something I need to take into consideration
Oscillator: You can either go analog, or use a microcontroller like an Arduino for the intelligence + the oscillator.
If you go digital, you can still do the waveforms you want, but possibly with NES/chip style bit reduction for the saw/triangle. What you want to do is probably to use a passive resistor ladder DAC, which allows you to oscillators with a few bits (as in binary bits of data) with only some microcontroller outputs and a few resistors. You will still likely want to use a 'duino if you're looking to implement MIDI. And the digital method has the added advantage of being in tune at all times without trimming.
If you go analog, I recommend a design built around the XR2206, like a simplified version of the XR-VCO by Thomas Henry. It can produce sine and/or triangle in its basic configuration. I recommend just going for triangle as sine always be approximated by using a low filter setting, but otherwise you can add a switch to choose between the two. But the interesting part is the skew function. It flips half of the waveform so you first get a triangle /\/\/\ which morphs into something like /-/-/- at medium settings and finally ////// at the highest setting.
As for controlling the synth, you basically have two options if you want to keep it analog and simple. Carbon strip or a resistor voltage divider ladder with a button for each note. Carbon strip is what the Gakken syntesizer is using for example, but I'm not sure whether this can be achieved by on a DIY level. The voltage divider ladder works like this: you have a bunch of resistors in series. One end is connected to a positive reference voltage and the other end to ground. In each step you're going to have a consecutively higher voltage. Each point is connected to a switch which in turn is connected to the voltage control input. Simple example:
___ ___ ___ ___ ___
+5V - |___| - o - |___| - o - |___| - o - |___| - o - |___| - Ground
| 4 V | 3 V | 2 V | 1 V
That way you get a number of evenly spaced voltage reference points that can be used as control voltages. This system fails as soon as you press two keys simultaneously, but hey you can't get everything...
A creative way to this approach is to use a printed circuit board where you use copper shapes as keys and play the thing with a "pen" attached with a wire. This is the approach Cheap, Fat and Open is using, even though it's using a different wiring of the electronics since it's a digital synth.
A nice trick you can use to almost detune the oscillator is a sawtooth phase shifter. There's a schematic for a really simple one here: http://rubidium.dyndns.org/~magnus/synths/schematics/ I think that one flipped the + and - of one of the opamps, however. Here's a demonstration of this effect unit in my modular homebuilt modular system, with an oscilloscope view and annotations explaining what I'm twiddling with. Vhen modulated with an LFO, it fattens up the sound a bit compared to a simple sawtooth.
As for a sequencer, what are you hoping it will control? Pitch? Other parameters? Make sure this isn't shooting over the target and that it will actually be useful.
And then there's the envelope, VCA, filter and LFO design aspects. I don't really have time to write more about this right now, and I also have less useful advice on those parts of the synth. But before I conclude, let me warn you of feature creep, i.e. adding too much shit and not being able to finish the synth, especially if you're on a deadline, which you probably are since it's a course. With complexity, the number of things that can go wrong increases exponentially. I'm of the opinion that having to debug your stuff is the best, if not the only way to gain practical knowledge. However, respect deadlines and know your limits.
1 osc 1 filter 1 amp 1 envelope 1 lfo 1 stylus controller 1 modulation controller (light, pressure, wheel, whatever) Banana plug/jacks to connect pitch and mod CVs (audio signal should be hardwired for simplicity) Mix input for outside source so it goes through the filter
I'm not too sure how I could interface an external controller other than via MIDI - and I have no clue how I could implement that.
Gate+CV (control voltage)! This is what an analog synth would be using internally, only that now it's coming from the outside through a couple of jacks. Many old synths will provide it.
What I'm looking at using for the oscillator is a function generator chip - I can't for the life of me remember the name, it certainly has a 3 in it though! The advantage of it is that it produces square, sine and triangle simultaneously, and all at the same frequency. I can put a variable resistor on each output (well, it will probably be more complex to avoid overloading the filter etc but yknow) and you can combine waveforms thusly.
I was looking at, for the sequencer, having a PIC (most likely a GENIE20 as my school uses a lot of their stuff and I already have experience with the programming software) reading input voltages (as in, the voltage for each note played). It would store up to 16 of these - a skip step button and end button would also be included - and play them back when a pulse is recieved on another leg of the chip. I'll most likely use a 555 astable to trigger this, but it also means you can have a sync input like the Korg Monotribe and trigger it from a click track. It would then output the voltages in binary, to a digital to analogue converter, which would go back to the oscillator and give it the appropriate voltage, meaning I get the right notes. It's low tech, I know, but that's part of the point of it.
Also, I'm gonna have the LFO sync-able to the sequencer. And hopefully have the LFO trigger the envelope too? I'm not sure. I found an ADSR schematic which uses a 555 timer which I may well be using, but until I get a chance to breadboards stuff I'm not too au fait on how it works.
And I'm hitting a bit of a wall in terms of finding a decent explaination of building a LPF - a diode ladder seems a good choice, but I'm not too sure where to go with it or what to do!
Oh also I'm planning on using the same function generator for the LFO, so you can select it's waveform too
