555 Keyboard

[Growf911]

I just breadboarded one of the simplest 555 oscillators out there and got some nice results. I used two pots to mod frequency and made a pseudo-keyboard out of some temporary pushbutton switches using resistors to extract different pitches.

Anyway, it's time to move on. I disassembled an old MIDI keyboard made for some primitive mac editing software, and it's a good platform for my project.
[IMG]H:\DCIM\121_PANA\P121063[/IMG]
^^ that's the back of it.

Since my existing oscillator runs from 9v DC, I was thinking of +9v to the bottom strip, then the terminal on the other side running to an individual oscillator.
[IMG]H:\DCIM\121_PANA\P1210266[/IMG]

Is there any way I could make this work, with complete polyphony, without having an individual oscillator for each key on the keyboard? If so, how?


Here's the breadboarded oscillator.
[IMG]H:\DCIM\121_PANA\P1210269[/IMG]

Thanks a lot.

[Iain McNeill]

hmmm, couldn't see the pictures - something about me not having an H:/ drive.

Anyway, most likely the only way to get polyphony is going to be with a small uC detecting the note on and note off transitions and assigning them to a pool of oscillators. There's no easy way to demultiplex the rows&columns switch contacts of the keyboard to a particular oscillator. sorry.

[Growf911]

Okay, thanks anyway.

[wakibaki]

You could, in principle, have an oscillator for every key but in practise this would have some problems. The stability of the oscillator is poor and achieving correct tuning is difficult with standard value resistors so each note would probably require a pot of its own.

It is possible to reduce the number of oscillators but the number of tuning resistances per oscillator remains the same, and now there is an additional problem with switching the tuning resistances into the oscillator in use and selecting which oscillator to use at any time, this further degrades the long-term accuracy of the tuning.

Again, the number of tuning resistances can be reduced to the number of notes, but with the addition of further complications to the switching, the right resistance must now go to the oscillator assigned to that key.

It's enterprising to attempt to make something out of an old keyboard in this way, but I don't think the results will be very satisfactory.

There are two ways you can go with this. You need to understand a bit about switching, so you can dig into logic, keyboard decoding and microprocessors, maybe buy a PIC programmer. You also need to understand a bit more about oscillators, tone and envelope shaping, so you could try building a more sophisticated oscillator and learning a bit of analog electronics.

Somewhere in between these is digitally generated sound. You can have a digitally generated sound or a replayed sample which is assigned to a key.

A lot of people have spent a lot of time working on this stuff, it makes sense to try and catch up on what's already been done so as not to reinvent the wheel, although its interesting to think, 'at least I'm as smart as some guy in the stone age,'

w

[Growf911]

Great! Thanks. I was planning on using a trimpot for each key. It's only 37 keys, and shouldn't be too much of a hit on the wallet if I order from Digi-Key.

While I like my square wave, I was wondering where I could find the materials to make a saw/triangle or sine wave. It would be great to have a selector knob to switch between them.

Also, how can you employ effects such as reverb and filters? I've heard of ICs that will do the job, but I'm not too sure where to find them. I envision a slider or even a small X-Y joystick to control the effects.

I'll try and get my images to photobucket. I can't find any way to attach multiple images to the post.

[Growf911]

I've tracked down the name of my favorite sound, supposedly it's made with a series of sawtooth oscillators slightly detuned against one another.

Is there an IC out there that produces a sawtooth wave, similar to the 555 timer? If possible, I'd like to avoid having multiple oscillators for each key.

Sorry, but I'm terribly confused as to how synthesizers can produce such interesting sounds. I understand waveforms, the duty cycle, and the area, etc. I'm also up to date on logic and basic electronic principles. I'm just curious how the simple components such as resistors and capacitors can make the sounds that I hear in music all around me.

Thanks again.

[Iain McNeill]

your 555 oscillator provides a sawtooth - on the timing cap (pin7)

however, you must buffer this with a hi impedance buffer (e.g. jfet opamp) or you'll change the frequency

filtering get very difficult because the filter is different for every key. It quickly becomes easier to do it all digitally.

BTW the old analog synths used a very fast settling voltage controlled oscillators, filters and amplifiers. But to get these to work at the speed of a talented keyboardist requires much skill.

[Growf911]

Would a programming language come into play at any point? I'm somewhat proficient with C++ if that helps.

Maybe I'll stick with a square wave to see what I can do for now and move on from there.

Does anyone know of an IC that can provide reverb or a phasing filter effect?

Thanks again.

[ostripper]

By Iain - BTW the old analog synths used a very fast settling voltage controlled oscillators, filters and amplifiers. But to get these to work at the speed of a talented keyboardist requires much skill.

The one I am posting below is fast , uses discrete devices/741
op-amp and can "follow" the lead perfectly. It is not as good
as the VCO's in the ARP/MOOG but it can make music better than a 555.

For more info on analog synthesis and more circuits go:
http://71.203.210.93/pdf1/Electronic...s/Synthesizer/

Even has circuits for phase shifting..

To go all the way ,ORION 7 + many wonderful VST plugins
can emulate all the classic analog synths in polyphony,
I have one that does 16 slightly detuned sawtooth's for
a truly "phat" sound.
OS

[Iain McNeill]

good stuff ostripper

Not to hijack the thread or anything but I felt very educated after reading this website in entirety
http://home.swipnet.se/cfmd/synths/companies/oberheim/

The schematics show a Z80 with a single DAC multiplexing at least a hundred sample and hold circuits driving all 8 voices of dual oscillators, filters and amplifiers. Oberheim used a pretty much custom analog ASIC that are very hard to find now.

edit: dang missed the exit again
I'd check out what the pedal guys do. I know Yamaha has reverb and effects chips but they're a bitch to find and get data on. Basically any delay chip can produce all the effects you describe