Need help with polyphonic Cmos 40106 project

richardc64 · Posted: Tue Apr 07, 2015 3:49 pm Post subject:

I was hoping Cynosure would check in with his thoughts on how simple or complex something like this could be made, but PHOBoS has pretty much covered that. (Envelope, VCA and filter for every key!)

Below are two of the simplest "electronic organ" schemes. Twelve of either one would be needed. I don't doubt that one or the other was used in more than one cheap-ass commercial product.

The first scheme, on the left, simply routes 4 (or 5,) octaves of square waves through the keyboard's switches. What results is sounds with a "hard" attack and NO decay/release. This can be mitigated somewhat with reverb, but not a lot. (Note that if CLK in is only one octave higher than the first divisor -- because you didn't use a counter that omits its first four divider outputs -- then that's available for a fifth octave.) Worse than the abruptness of each note sounded is that THEY'RE SQUARE WAVES, which become irritating right quick.

The scheme on the right is maginally better: Adjacent octaves are combined to create 4-step sawtooths (teeths?) as PHOBoS suggested. But the sound is still on/off. (Note that if CLK in is more than one octave higher than the first divisor -- because you DID use a counter that omits its first four divider outputs -- then it can't be used for saw generating. Are you getting the idea that I don't care much for the CD4060?) A small value capacitor to ground could smooth out the steps a bit, but not very effectively for low pitches and would attenuate high pitches.

Other options are possible if the keyboard has more than one buss, but for now I have to cook supper.

PHOBoS · Posted: Tue Apr 07, 2015 4:31 pm Post subject:

douthet · Joined: Apr 03, 2015 Posts: 8 Location: Canada

Wow you guys have given me a lot to think about (Which is a good thing, it means I am learning more!)

I like the idea of using a saw wave, to add a bit of variety to the tone. Im looking for something that is sort of soft sounding or has the potential to be soft. So maybe having the potential to flip between the square wave and the saw wave would be nice. And of course filtering will contribute to the achieving the softer tones.

I also think easier might be better; it just means less will go wrong with my wiring. But I am still a bit confused to how 50240 would divide down the octaves. Would it still just use a 4040 to divide down the 8 octaves? Or are they not compatible? I saw that richardc64 used a 74c93 to provide the 4 octaves. Is there any way of getting 8 octaves? I'm also wondering about the capability of some of my features with 50240. If not I may just have to bite the bullet and put in the work.

Things I need/ don't have a source to use as a reference:
1. VCA
2. Switch between square and saw
3.Power Input (AWG Universal Power Cord) and AC Internal Switching Adapter?

What I have for sources:
1. ADSR: http://electro-music.com/forum/topic-54641.html[/url]
2. Simple Low/High Pass Filter:
http://www.fluxmonkey.com/electronoize/passiveDividersFilters[/url]

Also when this synth is all done I would like to be able to plug it into an outlet with an AWG universal power cable. So I am a bit confused as to how to wire power to my circuits with varying voltages required in my sources I am using. And multiple things requiring power. If someone could send me a link on how to setup power for that (a tutorial or something) that would be really helpful. I was also thinking about using it through some guitar pedals as effects, and then just maybe sending it through a guitar amp for amplification, or just a DI box.

Sorry if my desire to build this project has surpassed my knowledge on how to have it come to fruition. But I really do appreciate all the help you guys have been offering me with this project!

PHOBoS · Posted: Wed Apr 08, 2015 5:45 am Post subject:

douthet · Joined: Apr 03, 2015 Posts: 8 Location: Canada

Thanks for all the links.

I just had a sudden thought. If I were to run the signal through the envelope after the mix, would that mean that when ever I played one note that is affected by the envelope (Let's say attack) then played another that note on top of that note after, would that second note not be affected by the envelope (attack) because the signal is already pass through with the first note? So I would have all notes running after the first with no attack in this case? Would that be correct to think that?

PHOBoS · Posted: Wed Apr 08, 2015 4:11 pm Post subject:

Depending on how it is build it's either that or the envelope generator will be retriggered so the first note wil again
be affected together with the second note. So for true polyphony you would need as many envelope generators
as you have keys (which is an awful lot).

JovianPyx · Posted: Wed Apr 08, 2015 4:17 pm Post subject:

I think the answer is yes.

If you have one audio path (with envelope generator and VCA) after the keyed tones are mixed, then all will be affected by the envelope at whatever state it is currently in when each note is pressed. Envelopes can usually be made "re-triggerable", but again, if there are already two notes held down and you play a third one, the whole three note chord will be "enveloped", not just the last added note.

JovianPyx · Posted: Wed Apr 08, 2015 4:23 pm Post subject:

True polyphony can also be attained with N voice paths, let's say 4, where each voice path has an oscillator, a filter, two envelope generators and one VCA. So 4 VCOs (or 4 different selected notes from a TOG and dividers), 4 filters, 8 ADSR and 4 VCAs.

However, in this particular case you would need a microprocessor to select and manage voices. In this case, each note pressed will independently go through it's own envelope processing - up to 4 notes at a time.

Cynosure · Posted: Wed Apr 08, 2015 4:54 pm Post subject:

Hello. Sorry for joining the discussion so late, and for not getting back to your question on my blog. I have been very busy the past couple of weeks getting my house ready to sell and I haven't had much time to check emails and forums.

I am very happy that my project has inspired you to create something of your own. Unfortunately, it was my first project and I barely knew how to read schematics let alone create them. Everyone here has already offered great advice. Here are some things that I can add:

1. this is a bigger project than it seems. If this is your first electronics project, then you might want to put the plans on hold until you do some smaller builds first. I wish I had done that.

2. Listen to JovianPyx. Use a dual rail power supply. It is required for a project of this size.

3. Approach the build in a modular fashion. Get the oscillators working, then the dividers, then the filters and so on.

4. A 7555 can be used to make a simple AR envelope. There are a few different versions here on this site, including a recent one by Thomas Henry. The trick with this type of synth is converting the audio signal into a gate. It can be done with a comparator and low-pass filter. http://electro-music.com/forum/topic-61297.html

5. If I was to do this type of build again (which I have thought about), I would do as Richard suggests and use a microprocessor. Someone already did an MK50240 clone with the arduino. You could even use a second proc with a multiplexer to do the key switching. Then you would have a midi controlled top octave synth.

Let me know here if you have any other questions. However, there might be a bit of a delay in my reply time. I will be busy for the rest of the month. Damn personal stuff and life getting in the way of building and playing synths all the time :p

Good luck!

richardc64 · Posted: Wed Apr 08, 2015 5:14 pm Post subject:

Yes, as key depressions are added those new notes don't get an Attack while the still held down keys keep sounding. Conversely, as keys are released they just go off without any Decay while held down keys still sound.

Some "polyphonic" instruments were in fact made that way. It's not a totally worthless structure, but such instruments can best be described as "synth-like." Some instruments included the typical monophonic VCO-VCF-VCA structure and the more-than-one-note-at-a-time section was called Strings because it could play chords.

If the keyboard you want to use has more than one buss, then one could be used to create a GATE signal to trigger an Envelope Generator. If not, there are other ways to detect keys being pressed.

It wouldn't be a bad start toward developing a truly polyphonic synth-organ.

JL · Joined: Jun 25, 2012 Posts: 20 Location: New England

Sounds like this project has the end-goal of something very similar to 1970s polyphonic string synths. My old Roland RS-202 and Solina String Ensemble both come to mind (really wish I still had those!). Might be worth checking out their schematics to get some ideas.

I'm thinking instead of the keyboard directly switching the notes, it should send gate signals to the corresponding EG>VCA>filter for that note. Like richardc64 said. So I'll assume the polyphony is unlimited because there is no keyboard scanning. That means each and every key needs a waveshaper (for square and saw at least?), VCA, A/D envelope, and possibly a filter or means of selecting say, 16' or 8' (or both?). That's a lot of circuitry to build for just one key. Regardless of your experience douthet, you better make damn sure the circuit for one note works perfectly before you build it 43 more times.

I've thought about how I'd build this sort of thing before - making each key's voice on it's own separate board, so one PCB for each key that just plugs in to a long motherboard. There's pros and cons to that, but it could make servicing and debugging a little more manageable. And if you make a good layout for the board, building it many times is easy, albeit time consuming.

If you are OK with paraphony it makes things *much* easier (like others have already mentioned). If you're determined to build a keyboard, that's the route I suggest you take considering your experience with electronics. Seriously, this is not an easy project at all. I built a 49-key combo organ last winter and although it was fun and sounds cool, it took at least three times longer than it should have and was very frustrating at times. I overlooked a lot of details at first, and even a minor error that ordinarily wouldn't be much trouble get multiplied due to the amount of repetition in the circuitry.