Saturday 5 January 2013

MIDIfied Tube Screamer

My Tube Screamer clone (Blogs passim) now sports a MIDI interface and can be controlled externally by MIDI "Control Change" messages...

I've played with sending MIDI from PICs a lot - that being the essence of my Virtual Organ Project; both in generating Note On / Note off messages from my MIDI bass pedals and various Control Change messages from organ drawbars, switches and other controls. But MIDI Tx is easy. It turns out that receiving MIDI is only slightly more tricky but interpreting and acting on MIDI messages is a whole lot tougher!

In the aerial photograph above, you can see the new MIDI elements at top right. There's another Breadboard Module with the obligatory 5-pin 180 degree DIN socket. There's also a little bit of electronics to squeeze MIDI into the PIC. MIDI is a current loop interface and needs an opto-isolator to terminate the Rx end correctly. I had some 4N25s in the junk box, but couldn't get these to work sweetly. Fortunately, I also had some CNW 136s and these worked very well.

Here's my MIDI Rx schematic...

After this simple circuit, the MIDI signal goes to the Rx pin on the 16F887 at the heart of my Digital TS. Of course, masochists can use bit-banging methods to receive the serial MIDI data (just as I continue to use Ross Bencina's code for sending MIDI in the Virtual Organ). However, for MIDI reception, you really need to use the PIC's UART (or EUSART, as Microchip insist on calling it).

There are any number of experts out there who will explain how to set up the UART for MIDI reception (standard 8-bit at 31250 baud). They will have you looking at the RCIF flag bit of the PIR1 register to see when data has arrived. You'll soon appreciate that you need to do this in an interrupt service routine, such that you can get on and do other things in the meantime (such as servicing the local control interface with its buttons and display). After some messing around, you'll get a MIDI stream into your controller - but what do you do with it?

The hard part isn't the reception of MIDI data (despite what you may think if you're struggling to get it going). The hard part is making your system react to it. Here's how I went about it...

Currently I have four "controls" in the digital tube screamer, matching exactly the user controls of the original guitar effect pedal. These controls are 'Drive', 'Tone', 'Level' and 'Bypass'. I want to be able to send values for each of these controls via MIDI from a control device.

MIDI has a feature exactly matched to this requirement (no surprise, since this is exactly what MIDI was conceived for), called "Control Change Messages". These messages identify a particular control on a particular MIDI channel (together constituting an "address" for that control) and pass it a numerical value (an integer between 0 and 127). It is the job of the receiving hardware to understand the message and to apply the value to the control (equivalent in this case to setting the position of one of the knobs on the Tube Screamer or operating its Bypass switch).

Here's my interrupt service routine which does the job...
You can modify this code to parse a MIDI stream for your own application - the only thing it doesn't (try to) do at the moment is handle "System Exclusive" messages. In my case, I'm happy to take the (very small) risk that one such message might mess with the controls of my Tube Screamer.

I tested my MIDIfied Tube Screamer by generating MIDI using the superb MIDI-OX software. I guess the next step is to make a little controller to replace the computer and MIDI-OX.

Wait a minute - wouldn't it have been easier to just put knobs on the Tube Screamer? Now there's a thought...

 ...-.- de m0xpd

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