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MIDI-Mate An easy-to-build MIDI interface for PCs OU’VE PROBABLY HEARD of Want to use your PC to control Y the MIDI system because it’s been around for quite a few years now. That acronym stands for “Musi­cal InMIDI synthesisers, key­boards strument Digital Interface” and it was originally designed to allow electronic & instruments? With the right musical instruments, keyboards and other “controllers” to communicate with each other. Then when computsoftware, most PCs make good ers and dedicated music sequencers came along, it was logical to use MIDI sequencers for electronic music to hook them up as well. Essentially, MIDI is a fairly low-tech based on serial data commumaking. You’ll also probably need system, nications at 31,250 bits per second, current loop circuit. It’s a this little MIDI interface, because inbit alike5mAa faster version of the system used to connect teleprinter machines, most sound cards don’t provide a about 40 years ago. You can find out more about MIDI and how it works in the accompanying panel – “MIDI In full MIDI port. A Nutshell”. By JIM ROWE 26  Silicon Chip PCs & sound cards Modern PCs make good sequencers Fig.1: the circuit uses two low-cost ICs – a 6N138 optoisolator and a 74HC04 hex inverter. The optoisolator provides the correct isolation for the MIDI IN socket, while the inverter stages buffer the signal outputs from the sound card. Please note: It has been found that the MIDI-in port does not work with all PC sound cards. The simplest solution is to increase the value of the resistor in series with LED1 to 680Ω (from 330Ω) and then fit a 470Ω resistor on the underside of the PCB in parallel with the series combination of LED1 and the 680Ω resistor, ie, from the +5V rail directly to pin 13 of IC1. for electronic music. That’s because they have plenty of memory and “crunch power” for editing and manipulating music files, plus lots of hard disk space to store them. All you need for a PC to control a bunch of instruments is a MIDI port – and once you have this, you can also use it to hook up music keyboards, drum machines and other “controllers” to feed music into the PC as well. Ironically, most PC sound cards already provide what’s usually called a MIDI port (also known as an MPU401 port) but it’s really only “half” of such a port. It includes an addressable UART designed to send and receive data at the correct 31.25kb/s, for example. However, the UART’s serial input and output operate at logic voltage levels and the connections are simply made available at two pins on the card’s game port. There’s no attempt to provide the correct current-loop interface or the standard MIDI connectors needed to communicate with normal synthesisers, instruments or keyboards. So that’s what this little “MIDI-Mate” project provides: the extra circuitry needed to provide a PC sound card with a full MIDI port. It gives your PC and its sound card a properly isolated MIDI input, two standard MIDI outputs and also a MIDI THRU output which provides a buffered replica of the MIDI IN signal (for driving other instruments). There’s also a couple of LED indica­tors on the front panel, to let you easily monitor the activity on the MIDI IN/THRU and MIDI OUT sides of the interface. The complete circuit uses only two low-cost chips, plus a handful of resistors and other parts. Everything fits on a small PC board, including the MIDI connectors, and it’s all housed in a compact low-profile plastic instrument case. The interface circuitry needs only a few tens of milliamps at 5V DC and this is sourced “free” from the PC itself – via the ribbon cable which connects the MIDI-Mate to the sound card’s 15-pin game port socket. Incidentally, a duplicate 15-pin socket is provided on the end of the ribbon cable, which extends back out from the MIDI-Mate case. This allows you to still use the game port to connect a joystick or similar for playing games, even when the MIDI-Mate is connected. Circuit details Fig.1 shows the circuit details for the MIDI-Mate. As you can see, there are only two chips involved: a 6N138 high-speed optocoupler (OPTO1) and a 74HC04 hex inverter (IC1). As stated above, both chips are run from a +5V supply rail which is obtained from the PC sound card – via pins 1, 8 and 9 of the game port connector (via the ribbon cable and CON1). The 10µF capacitor is used to provide a local peak current reservoir. Optocoupler OPTO1 is used to provide the correct isolation for the MIDI IN socket (CON2), which is wired in the standard way with pins 4 and 5 connected to the optocoupler’s LED via a 220Ω series resistor. Diode D1 protects the LED against accidental polarity reversal (from a wrongly wired cable, for example). By the way don’t be tempted to try substituting another optocoupler for OPTO1. Most other optocouplers don’t have the switching speed of the 6N138 and won’t reliably transfer MIDI signals. FEBRUARY 2001  27 Parts List 1 PC board, code 01201011, 117 x 112mm 1 low-profile plastic instrument case, 141 x 111 x 35mm 4 5-pin DIN sockets, 90° PC board mounting 1 16-way DIL pin strip 1 DB15 male IDC connector 1 DB15 female IDC connector 1 16-way DIL socket, IDC type 1 2m length of 15-way IDC ribbon cable 4 small self-tapping screws, 6mm long Semiconductors 1 6N138 fast optocoupler (OPTO1) 1 74HC04 hex inverter (IC1) 2 3mm red LED 1 1N4148 or 1N914 diode Capacitors 1 10µF 10VW tag tantalum Resistors (0.25W, 1%) 7 220Ω 2 330Ω Fig.2: install the parts on the PC board as shown here. Make sure that all polarised parts are correctly orientated and note that the two 330Ω resistors go in the centre of the board (ie, they connect to the cathodes of the two LEDs). The PC board is very simple and should only take a few minutes to assemble. It is secured to the base of the case using self-tapping screws which go into integral plastic standoffs. 28  Silicon Chip OPTO1’s output transistor is connected as a simple switch, with its emitter grounded and its collector connected to the +5V rail via LED1 and a 330Ω series resistor. When MIDI information arrives via CON2, the transistor in OPTO1 switches on and off and its collector current causes LED1 to blink in sympathy. At the same time, the collector voltage at pin 6 swings up and down between 0V and +5V and it’s this logic voltage signal that’s fed to the UART in the PC’s sound card via pin 15 of CON1. This same signal is also used to produce the circuit’s MIDI THRU signal, by passing it through inverters IC1f and IC1a in series. Pin 2 of IC1a therefore provides a buffered version of the incoming MIDI signal. The two associated 220Ω resistors are used to regulate the 5mA current in any MIDI load circuit connected to CON3. The two MIDI OUT signals are produced in a similar way to the MIDI THRU signal. However, in this case they use the voltage signal from the sound card’s MIDI output instead of the signal from OPTO1. This signal comes from pin 12 of the sound card’s game port connector, via CON1. It’s passed first through inverter IC1e and The 15-way connecting cable passes through a slot that’s filed in the top of the rear panel. Also show here is the 15-way female IDC connector at one end of the cable. This allows devices such as joysticks to be connected to the game port, without disconnecting the MIDIMate. Fig.3: here’s how to assemble the connecting cable. Note that the red conductor goes to pin 1 of each connector and that pin 16 of the 16way header socket is unused. then through inverters IC1c and IC1d. These then drive the MIDI OUT sockets (CON4 and CON5), again via their own pairs of 220Ω series resistors. The remaining inverter (IC1b) is used to drive the MIDI OUT indicator LED2, using the logic signal from the output of IC2e. Construction All the components used in the MI- The 16-way IDC header socket plugs into a matching 16-way DIL pin strip on the PC board. Power for the unit is supplied via the PC’s game port, so no external supply is needed. FEBRUARY 2001  29 Fig.4: this is the full-size etching pattern for the PC board. DI-Mate are mounted on a PC board coded 01201011 and measuring 117 x 112mm. A 16-way section of DIL connector strip on the PC board is used as input connector CON1. This mates with a 16-way DIL socket on the 15-way ribbon cable, which links the MIDI-Mate to the PC sound card’s game port. The 16-way DIL socket is mounted about 100mm from one end of the cable, while the DB15 female IDC connector is mounted at the adjacent end. The DB15 male IDC connector is fitted at the sound card end of the cable (ie, at the far end). Note that all three connectors are IDC types for easy fit­ting. The 16-way DIL socket is simply fitted with the cable located over the connector teeth for pins 1-15. This is easy to do if you use the coloured side (red stripe) of the ribbon cable to indicate the “pin 1” conductor. Be sure also to connect the conductor with the red stripe to pin 1 on all the connectors – see Fig.3. There’s virtually no physical wiring inside the MIDI-Mate box, Fig.5: be sure to set up the options in either Multimedia Player or your sequencer program so that they’re talking and listening to the sound card’s MPU-401 MIDI port. because even the four DIN sockets and the indicator LEDs are mounted directly on the board. Fig.2 shows the layout on the PC board. The parts can be fitted in any order – the only things to watch are that you fit the polarity sensitive parts the correct way around. These in­clude OPTO1 and IC1, the 1N4148 diode, the two LEDs and the 10µF capacitor. Both LEDs are fitted with their cathode lead closest to CON3. It’s easy to identify the cathode lead – it’s always adja­cent to a flat section on the plastic collar of the LED body. It’s also shorter than the anode lead (see Fig.1). Initially, the LEDs can be fitted in the upright position, with their leads straight. Later on, the leads can be bent for­ward at a right angle about 11mm above the board, so that they protrude through matching holes in the front panel. Note that the board must be fitted with two short wire links. One is just above the 10µF capacitor and IC1, while the other is just behind CON4. Final assembly The assembled PC board is secured inside the case using four 6mm-long self-tapping screws. These mate with four of the moulded mounting pillars. The case rear panel has a 20 x 4mm rectangular notch cut into the top centre. This allows the ribbon cable from the PC to loop in and connect to the MIDI-Mate and then loop back out again. The front panel has six round holes – two 3mm holes for the LEDs and four 16mm holes for the DIN sockets. These holes can be marked out by using a photocopy of the front panel artwork as a template. The best way to make the holes for the DIN sockets is to first drill small pilot holes. These holes can then be carefully enlarged to the correct size using a tapered reamer. Trying it out No adjustments are required for the MIDI-Mate; if you’ve assembled it correctly, it should be ready for use Fig.6: you can use this fullsize artwork as a drilling template for the front panel. Drill small pilot holes for the MIDI sockets first, then carefully enlarge them using a tapered reamer. 30  Silicon Chip MIDI In A Nutshell: What It Is & How It Works MIDI is an acronym standing for “Musical Instrument Digital Interface”. It’s a standardised system for communicating between electronic musical instruments, keyboards, controllers and se­quencers (including PC-based sequencers). The MIDI standard was agreed on by a group of musical instrument makers in 1983 and has been used and extended since then. MIDI uses serial data communication at 31.25kb/s (kilobits per second). This involves using asynchronous 5mA current loop signalling, with the current provided by the “transmitting” end. Each byte of a MIDI message takes only 320µs to be transmitted (counting start and stop bits). Since most MIDI messages are 2-byte control codes, this means that over 1500 such codes can be sent each second via a single MIDI cable. Each MIDI cable carries only one signal, so for bi-direc­tional communication, two cables must be used. The cables them­selves use shielded two-conductor wire. All MIDI cables are fitted with standard 180° 5-pin DIN plugs at both ends. However, only pins 4 and 5 are used for the actual current loop signalling (wired 4-4 and 5-5). Pins 1 and 3 are left unconnected, while the shield braid is connected to pin 2 at each plug. Inside MIDI equipment, pin 2 is connected to earth only on MIDI immediately. You need only connect it to the sound card of your PC via the ribbon cable and game port connector, and fire up the computer. To try it out you will need to have some sort of synthesis­er or other MIDI instrument to hook up to one of the MIDI OUT sockets and also a way of playing MIDI files. This could be just the Windows 95/98 “Multimedia Player” accessory program, playing almost any handy “.MID” music file. Of course if you have a more elaborate sequencer program like Windjammer, Cakewalk or MidiSoft Recording Session, these would be even better. The main thing you need to watch OUT sockets. This allows correct earthing of the cable shield braids, without creating earth loop problems. Unlike most other current-loop signalling, current only flows in a MIDI link when data is actually being transmitted. This allows MIDI cables to be plugged and unplugged without any problems, as long as data is not actually being transmitted at the time. To prevent equipment damage due to wiring errors or compon­ent faults, all MIDI inputs are provided with 3kV of galvanic and electrostatic isolation via an optocoupler. For correct MIDI communication between equipment, a MIDI OUT or MIDI THRU socket at one end must be connected to a MIDI IN socket at the other. In most MIDI systems there is a single main controller or sequencer (often the computer), from which most of the MIDI messages originate. When these messages must be sent to more than one instrument, they can be distributed in either “star” or “daisy-chain” fashion, as desired. There’s no need to worry much about the actual code messag­ es sent over the MIDI links, because nowadays this is all handled by sequencer or other software running on the PC, and by firmware running in the other instruments and keyboards. It’s probably enough to know that most MIDI messages are short commands to allocate a is that you set up the options in either Multimedia Player or your sequencer program so that they’re talking and listening to the sound card’s MPU401 MIDI port, instead of its inbuilt FM or wavetable synthesiser. Otherwise, the fact that you’ve connected up MIDI-Mate will be ignored and your external synthesiser will remain silent. Assuming that you have a synth­ esiser or other MIDI instru­ment connected to the MIDI-Mate, getting it to “play” should now simply be a matter of loading a MIDI file and clicking on the “Play” button. And while the file is playing, MIDI-Mate’s MIDI OUT particular instrument to a particular channel, to tell it to start or stop playing a particular note, to change the instrument’s attack/decay or other performance parameters, and so on. As mentioned earlier, these commands are generally in the form of 2-byte codes. Using a PC-based music editing and sequencer program (and perhaps a MIDI music keyboard to feed in the actual notes), you can assemble a complete sequence of MIDI commands to play a piece of music – on say the “instruments” in a synthesiser. The synthe­ siser can then be made to “perform” that piece of music simply by sending the sequence to it, via the MIDI link. When you’re happy with the result, you can save the se­ quence on disk as a MIDI music file. These have a standardised format and are identified with the “.MID” extension. Disks with collections of pre-composed MIDI music files are also available and you can download them from the Internet as well. Finally, it’s important to realise that although a MIDI music file may look superficially similar to a .WAV file of a digital sound recording, it’s really quite different. It’s more like an electronic equivalent of sheet music – simply a sequence of instructions describing how to play the music. In this case, the instructions are for electronic instruments rather than for human players. LED should blink away merrily as the MIDI commands stream out to the synthesiser. Similarly if you have a MIDIequipped music keyboard or other controller, you’ll now be able to hook its MIDI OUT to the MIDI IN socket on MIDI-Mate and record your own music on the computer’s hard disk – after clicking on the sequencer program’s “Record” button, of course. And that’s really all there is to it. With MIDI-Mate and a sound card, your computer will have all the hardware it needs to become a powerful MIDI sequencer. The rest is up to you SC and your musical creativity! FEBRUARY 2001  31