Waveshapers - some sound!

As a start I wanted to test the basics - the ability of Arduino to create a musically correct tone and to shape that into a useful ramp or square wave with external circuitry.  As a starting point I used two resources. The strategy of sinneb to generate an accurate pulse from the Arduino as shown here. Also from the PicSynth the simple 4520 based waveshaper shown in this schematic.

Here's an obligatory photo of the breadboard.

Interesting huh? Anyway it worked - I had sound! A chunky sounding square wave and a surprisingly chunky ramp. However there was an oddity - the PWM control seem to only have a could of steps so it wasn't possible to get that classic PWM sweep I like.  Taking a look at the output on my scope it soon became apparent why. Here's the ramp wave:

It's a ramp Jim, but not as we know it. If I'd stopped to think a bit about how the circuit works then it would have made more sense. The 4520 is a four-bit counter and essentially divides the input into four square waves that are added together to make this stepped ramp. 

Reading the PicSynth site a bit more closely I realized that the author knew this of course and had an alternative. This circuit based on a 4046 and 4024 gives more conventional sounds and much better looking waveforms. There's also an excellent explanation on that page of how the ramp->square conversion works. This explains very well why the stepped ramp wave can't really be used for PWM. I won't repeat it now, go read it yourself.

Here's the circuit on the breadboard:

 I haven't built the LFO yet but I've simply been using a pot as a voltage divider to control the pulse width. Here's the ramp and the square with the square wave near the minimum width:

You can see the ramp wave looks much better - more or less like the real thing. This is a 256 step conversion so it's pretty smooth. There's a bit of noise, but hey - it's on a breadboard!

Here's another image with the pulse width near maximum:

I don't have any proper input device yet but have simply been using a pot connected up to the analog input of the Arduino and using that to sweep across the notes.

Finally I used my guitar tuner, which has a chromatic mode, to verify the notes were as I expect them. Every single one was within about 5 cents and I confirmed a few with the frequency counting mode of my multimeter as well. So this is a promising start. The next thing I will try is hooking up some MIDI input so I have a more conventional way of setting the current note.