After having built a x0xb0x this summer which I fitted with Brian Castro’s x0xio back panel mod I decided a good project to make would be to make a midi-cv converter box. Although the x0x comes with it’s own midi in, the firmware is a bit tempremental and I was getting a lot of skipped notes and no control over slides when programming midi sequences for the x0x in Ableton. After having played for more time than I care to admit to with the x0x I’ve come to the conclusion that although the internal sequencer rocks, especially when using the SokkOS firmware (funky pattern randomise not to mention the other extra features) the midi response is a bit naff.
This formed half the motivation for building a midi-cv box, the other came from my practice as a double bass player. The double bass being fretless lends itself to slides just like the 303, with my new pitch CV in could I take the pitch and amplitude of double bass and use it as a control signal for my x0x ? More on this side of the project later. The brief I set myself for the midi-cv side was simply that it should supply at least 5 cv outs with a resolution high enough to allow for some interesting tuning options later on. I also wanted proper midi in although I knew while I was programming the firmware I’d probably just send the midi byte code over USB .
Having looked around the Arduino forums I found a lot of people using the PWM outputs on the Arduino and just running them through a low pass RC filter to get a smooth variable DC voltage out. This has the benefit of being quick to make but the downside of non-linearity across the domain of the duty cycly and the range of voltage out and the small portamento this approach will neccessarily come with. Having decided against using the PWM out and having recently been introduced to the analog devices catalogue by an a electronics friend I decided to get hold of one of their AD5668 DAC chips and dive into the world of surface mount chips and SPI. I’m not claiming to be the first to hook up a DAC chip to the Arduino as there’s a fair amount of evidence people have taken the same approach when building their own cv boxes but the approach worked and I’m pretty pleased with the results.
The AD5668 and an adaptor to make soldering wires on to a 5mm chip slightly easier, came to about twenty quid from Farnell. The rest of the box materialised over a period of about two weeks as I made nightly visits to my long suffering local Maplin (I swear they hate me in there, there’s only so many times you can roll in at ten to eight and ask for an opto-isolator and a bag load of assorted components).
Although optoisolators aren’t technically neccessary for midi input I’m a bit of a stickler for standards and seeming as I’d never used one before I figured I might as well. This Arduino forum post came in handy for the midi in part of the circuitry and the midi byte parsing, I particularly enjoyed the mspaint circuit diagram. As you can see the box is packed pretty tight and the eight outputs which are on switched mini-jacks and quarter inch jacks barely fitted in the enclosure. The pitch and amplitude tracker I built for my double bass is on a circuit board blu-tacked to the inside of the lid so you can’t see it in this photo but I’ll cover that in another post.
The most time consuming part was reading the AD5668 datasheet forwards, backwards and in random access until I’d squeezed all the functionality I wanted out of it. It uses a 32 bit address space with 4 bit padding at either end, 8 bits for commands and 16 for resolution on the output which swings between 0 and 5v. There’s a fair bit of left and right shifting in the arduino source code as well as some bit masking which is kind of funky if you’re into powers of two.
There already exists a fair wealth of material (e.g. here, here and here) on how to get the Arduino to deal with midi byte code so after having successfuly hooked up the Arduino to the DAC chip (SPI really isn’t that hard given the Arduino’s built in shiftOut method, reading this helped) I just started hacking apart kuk’s midi parsing code for my own purposes. I’ve put the source code here so you can get a head start on making your own if you’re interested in using the AD5668 or just seeing a project that makes broader use of the midi specification. There’s a fair amount of code dedicated to getting pitch and amplitude data from the double bass which you’ll have to scroll past.
Having documented the insides of my midiDAC I’ll leave you with two videos showing its implementation with my x0xb0x. I cooked up a little random scale generator in Max to play on 16th notes and synched the whole thing up to my drum machine via Ableton and Rewire (Max 5’s new transport still doesn’t accept MTC as a clock source, hence Ableton and Rewire joining the party). Enjoy.