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64-KEY MATRIX Part 2 by Tim Blythman In the last issue we described low-cost hardware that you can build to work with MIDI, including a comprehensive MIDI Encoder Shield and a MIDI Key Matrix to drive it. We have developed some more software to make even better use of this hardware, and we’ll show you how you can even use it with Android smartphones and tablets. T he first part of this series showed how to build a simple MIDI Key Matrix, consisting of a grid of 64 tactile switches. These are scanned by an Arduino-compatible Leonardo board, which acts as a MIDI Encoder. It uses its USB peripheral to generate MIDI messages which can be received on a personal computer running synthesiser or digital audio workstation (DAW) software. It’s also fitted with a pair of MIDI-standard 5-pin DIN sockets. These are configured as MIDI-out and MIDI-in ports. MIDI-out ports generate ‘hardware’ MIDI messages which can be fed to the MIDI-in port on a device such as a MIDI synthesiser. Thus, the MIDI Key Matrix and MIDI Encoder together form a generic MIDI output device, suitable for triggering sounds and notes on any number of MIDI-capable devices that have either a USB or 5-pin DIN MIDI-in connection. It also has a very basic onboard synthesiser which delivers notes to a small 1W speaker as key events occur. On its own, it forms a very simple musical instrument. While it’s possible to wire up the matrix of 64 keys by hand, we also showed how to build a PCB-based Switch Matrix, which simplifies this greatly. The Switch Matrix provides 14 support for several differently-sized tactile switches. We expect some people will think of new and interesting ways to use this hardware. This might include custom programming of the MIDI Encoder to perform a specific role in a MIDI setup. Or it might involve using the Switch Matrix to simplify interfacing to hardware in an unrelated project. If you want to know more about MIDI’s background and workings, see the ‘What is MIDI?’ panel on page 19. LED Matrix While we intended to create a cheap and useful MIDI input device, we’ve used the ample space on the Switch Matrix PCB to add extra features. In particular, there are pads to allow illuminated switches to be fitted. These are also connected in matrix fashion back to a pair of 8-way connectors (CON3 and CON4), with a series resistor for LED current limiting on each row. We showed some basic code to drive the LEDs in the first part, but the original hardware couldn’t sense keypresses and drive the LEDs at the same time. We will now address that. Leonardo limitations The problem is that there aren’t many pins spare on the Arduino Leonardo; certainly not enough to drive the LEDs and scan the switches simultaneously. Indeed, there aren’t many Arduino compatible boards around that would allow that and still provide a USB peripheral. The Arduino Mega has enough pins, but unfortunately, its USB support is limited to serial data through a separate USB-serial IC. In the last issue, we noted that if you just want to light up all the LEDs, you can simply connect power rails to CON3 and CON4 on the Switch Matrix PCB. Practical Electronics | May | 2022 But if you want independent control of the LEDs, the simplest approach is to add a second microcontroller board. You might want to light up each key as a prompt to indicate which one should be pressed next. This could be handy as a learning tool, helping to learn a musical piece by rote. Another example would be to light up the LEDs in time with the keys that are being pressed. This is what we’ve done with our example code. A tale of two micros To light up the keys in time with the keypresses requires communication between the two micros, even though we’ve already established that one of them doesn’t have many pins to spare. Our trick is to borrow one that’s already being used. The MIDI specification supports so-called SYSEX (System Exclusive) messages. These messages are intended to allow manufacturers of MIDI equipment to send custom data that doesn’t fit into the standard messages. They can be used (for example) to send audio sample data between devices. Many manufacturers have specific identifiers, but the 0x7D identifier can be used for development purposes, and that’s what we’re doing here. Using this identifier means our data won’t be confused with another manufacturer’s signals. The System Exclusive message that we send consists of a status byte 0xF0, identifier 0x7D, followed by the ASCII codes for ‘SC’. These extra characters reduce the chance that the data could be misunderstood by other equipment. We follow this with any number of data bytes in the range 0x00 to 0x7F, which gives us 128 codes. Codes 0-63 turn off LEDs 0 to 63 respectively, while codes 64-127 turn on one of them. Any data byte greater than 0x7F (ie, with the most significant bit set) ends the SYSEX message, although we send 0xF7 as this is the defined ‘End SYSEX’ command. All MIDI equipment understands this, and thus everything remains in sync, ignoring data inside these packets. We send this data out on the MIDI out port. Most MIDI equipment will ignore these bytes, so it won’t interfere with our note messages. It’s then a simple case of receiving that data and displaying it on the LEDs. We’re using a second Leonardo board to do this. Because MIDI data is no more than a serial bitstream at 31,250 baud, we set up a state machine to monitor the incoming data. Once it has received the bytes 0xF0, 0x7D, ‘S’ and ‘C’, it assumes that any following data bytes are commands to turn the LEDs off and on as described above, until it receives a byte above 0x7F, which resets the state machine to wait for the sequence again. The LEDs are multiplexed by a timer interrupt, which ensures that each column receives an equal amount of time and thus the LEDs are driven with even brightness. The code scans through each column in turn, lighting up the LEDs according to the previously received commands. LED driving hardware Since our LED Driver uses standard MIDI packets, the deluxe way to assemble this is to use two Leonardo boards, each topped with a fully kitted-out MIDI Encoder. A standard MIDI cable from the MIDI-out port of the unit programmed as the MIDI Encoder is connected to the MIDI-in port of the unit programmed as the LED Driver; each unit is supplied with power via its USB port. The LED connections on CON3 and CON4 of the Switch Matrix are connected to CON2 and CON1 on the MIDI Shield, as described last month in Part 1. So the MIDI Practical Electronics | May | 2022 Here’s the full rig, with an Android phone hooked up to the MIDI Encloder and Matrix. A second Leonardo drives the LEDs on the Switch Matrix. Encoder Shield only needs the MIDI-in, CON1 and CON2 headers to act as the LED Driver. In line with our cheap-and-cheerful philosophy for this project, we have a simpler solution. Fig.5 shows the minimal wiring needed, with the MIDI Encoder Shield at left and the LED Driver board (using the same PCB) on the right. Our photos show this minimal arrangement too. We have soldered a 2-way female header to each PCB at the 5V/GND connections, and these are connected by a pair of jumper wires (red and blue). The LED Driver PCB is simply our shield PCB from part one fitted with headers to break out the connections. There are also headers underneath to connect to the Leonardo. The easiest way to solder these is to insert the headers into the Leonardo’s sockets, slot the PCB onto the headers and then solder them. The pins are thus square and aligned. The Switch Matrix fitted out with a full complement of 3D-printed key caps. 15 Fig.5: this wiring bypasses the MIDI connectors and allows the MIDI Encoder to both power and communicate directly with the LED Driver. Naturally, there is no isolation in this case! Although two wires are shown for data (green and orange), only one is needed as the pins at each end are connected by PCB traces. We used socket headers for CON1 and CON2 to allow simple plug-plug jumper wires to be used, although these could even be soldered in place. Make sure to wire pin 1 of CON2 (Shield) to pin 1 of CON3 (Matrix) and pin 1 of CON1 (Shield) to pin1 of CON4 (Matrix). The data line is connected at the MIDI Encoder end by piggy-backing it onto the TX jumper at JP1, while the LED Driver is connected to the RX header at JP1. Parts l is t – additio nal p arts f o r L E D Driv e r 1 double-sided PCB coded 23101211, 69 x 54mm from the PE PCB Service 1 Arduino Leonardo module 1 10-way pin header (shield headers to Leonardo) 1 8-way pin header (shield headers to Leonardo) 2 6-way pin headers (shield headers to Leonardo) 2 8-way pin headers or sockets (to connect to Matrix PCB) 2 8-way jumper wires (to connect to Matrix PCB) 3 jumper wires (to connect to MIDI Encoder) If you have a bare Leonardo at either end, you could wire from the TX pin (pin 1) of the MIDI Encoder Leonardo to the RX pin (pin 0) of the LED Driver Leonardo. Don’t forget that you will need the Switch Matrix variant with illuminated switches fitted; they should have their anodes oriented towards the top of the PCB. Software We’ve updated the MIDI Encoder software also to output the SYSEX LED data, so upload the MIDI_ENCODER_ LED_SERIAL_OUT to the Leonardo which is acting as the MIDI Encoder. The LED Driver Leonardo should similarly be programmed with the MATRIX_LED_DRIVER_SERIAL sketch. Once that is done, and both boards are powered up, pressing any key should cause the corresponding LED to light up. If they don’t match up, try swapping or rotating the connections to CON3 or CON4 of the Switch Matrix. If you wanted to do something fancy, like have the LEDs ‘radiate out’ from each key pressed, you just need to make modifications to the MATRIX_ LED_DRIVER_SERIAL sketch. 3D printed keycaps The 16mm spacing on the Switch Matrix is a good compromise between compactness and usability. If the keys were much closer, there’d be a higher chance of pressing more than one key, while a wider spacing would quickly cause the PCB to blow up in size. We made the off-hand comment last issue that 3D-printed keycaps would be an economical way to add finishing touches to an illuminated matrix. While 12mm tactile switches can be found with large buttons which are easy on the fingers, there are fewer options for the smaller illuminated parts. Unfortunately, many small, illuminated switches only support small keycaps (around 10mm), which would look very odd on the 16mm spacing we have used. So we designed a 3D-printed keycap to suit the ILS series switches that we used. We printed some of these in a translucent PLA filament (Jaycar’s Cat TL4274), and it helped diffuse the light from the LEDs, although as they are so small, they were a fussy fit. We’ve made the 3D files available for download if you want to 3D print your own keycaps and try them for yourself. MIDI glossary Note-on and Note-off messages: The Note-on and Note-off messages correspond to events that occur during music playback and are the only messages that the Encoder generates. They include information about the channel, velocity (see below) Channel: Each MIDI message can specify one of 16 channels, allowand note number. ing data to be routed to or from different instruments while maintaining its source or identifying that it should be played back on Status: The first byte of a message is called the status byte and is marked as such by having its most significant bit set. Typi a particular instrument. ur software uses a single, fi ed channel cally, the lower nybble (four bits) of the status byte contains a which can be changed in the code. four-bit value indicating the channel number. Message: The smallest unit of MIDI data is a message and corresponds to a single event (like a piano key being pressed or Velocity: Velocity is how fast the key on a musical instrument is pressed, but is usually manifested as how loud a note sounds released) or a setting that is to be changed. (which is related to how fast, for example, a piano key is pressed). Our software uses velocity 64, which is the default for deNote number: Each musical note is associated with a number in vices that can t detect key velocity. ike ote number, it can the range 0-127. Middle C is assigned to number 60. Our dehave the value 0-127, with 0 also corresponding to Note-off sign implements 64 of these (in an 8x8 matrix), with the range for some devices. start and end defined in the code. 16 Practical Electronics | May | 2022 Here’s the front and rear PCB photographs which match the overlay diagrams at left. Obviously, there’s not much on the rear side of the PCB except sockets, as shown. These plug directly into the ‘sandwiched’ Leonardo board. The accompanying photos show what the result looks like. MIDI on Android While testing our different MIDI hardware variants, including the MIDI Encoder PCB and the Switch Matrix PCB, Screen1: the FluidSynth MIDI Synthesiser App has few controls, but that is part of what makes it easy to use. Once the MIDI Encoder (or other MIDI device) is plugged in, it becomes available from the top of the screen. Screen2: the MIDI Encoder appears as an Arduino Leonardo device when selected. The number shown is different every time the Encoder is reconnected, but this doesn’t seem to cause any problems. Practical Electronics | May | 2022 we wondered if there was a way to connect the MIDI Encoder to a smartphone. With many people possibly having an old mobile phone around that could be repurposed, such an arrangement would be a quick, cheap and easy way to create a useful musical instrument. We only looked at Android devices because that is what most of us use. Our minimal research suggests that this might be possible on Apple devices (such as iPhones), but it’s not something we’ve delved into. Note also that there are many different types of Android phones, and we can’t claim that this will work with all of them. If you can check that your phone supports USB OTG (on-the-go) and has a fairly recent Android version, then you have a decent chance of success. Android is not restricted to mobile phones; some tablets run Android, plus some other devices (for example, smart TVs). What you need One thing that you’ll almost certainly need is a USB on-the-go (OTG) adapter. This allows the USB socket on your phone to behave as a host device instead of a peripheral device. Not all phones support OTG, but it’s quite common these days. The OTG adapter will have a plug that suits the USB socket on your phone (micro-USB or USB-C) and a USB-A socket (such as you might find on a computer). For example, Jaycar Cat WC7725 (micro-USB lead) or Cat WC7709 (USB-C lead) should work. Or use Altronics Cat P1921 for micro-USB or Cat P1924 for USB-C. Smaller adaptors are available which do not have any leads; they connect the devices directly. We like them because they’re small enough to carry around in your pocket or bag. You can see one of these in our Android phone photo. Indeed, an OTG adapter is a handy thing to have these days, with many phones having support for USB key- boards, mice and flash drives. We’ve even seen an app that lets you use one to program Arduino boards! Screen3: we’ve only used the Settings option from the menu. Recordings are a paid premium feature that we did not test as we figured it would be easy enough to connect a 3.5mm stereo aux lead to the phone’s jack to record audio. Screen4: it’s only necessary to download the SoundFont file during initial setup. After this, you will just need to check that the MIDI Encoder is selected on the main screen before using it. 17 A pair of 2-pin header sockets soldered back to back and with their pins bridged can be used as a jumper that provides extra sockets to tap into. We used this arrangement to break out the MIDI signal from the MIDI Encoder board from JP1, with an extra jumper wire leading to the LED Driver board. Our basic setup allows access to myriad options through musical instrument choices and SoundFont files. The Encoder PCB here is only fitted with headers to connect to the Switch Matrix. Having said that, writing Arduino sketches on such a small screen isn’t the easiest thing in the world. Note that your phone and apps need to have support for the devices you want to attach. Fortunately, MIDI is supported as a Device Class Definition by the USB standard, meaning that any compliant USB MIDI device should work, without needing specialised drivers. MIDI Apps We don’t have any affiliation to the following Android MIDI Apps; we found them by searching the Google Play Store. The first one we tried was called MIDI Keyboard, which was able to recognise the attached Leonardo, but was limited to a single piano instrument. So we kept looking to see what other options were available. The second App, called FluidSynth MIDI Synthesizer, gave a few more options. At the time of writing, it generally had positive reviews, and it also appeared to support downloadable SoundFont (.SF2) files. There is a paid upgrade available to allow recording on your device, but we were well entertained by simply playing sounds back through our phone speaker. A connection using a 3.5mm stereo aux cable should be sufficient to connect the audio to other equipment for amplification or recording. Installing the App was quite simple. It works with Android 6 and later. We found that it needed permission to access device storage; this is required to access the downloaded SF2 files. Screen1 shows its initial screen. When a MIDI device (like the MIDI Encoder) is plugged in via an OTG adapter, it appears in the dropdown menu at the top of the screen. Screen2 is shown when such a device is selected, giving control over the MIDI device. From the menu icon at top left, select Settings (Screen3) and choose ‘Download a SoundFont’. The Chaos V20 option (Screen4) is the smallest of the download options, and gave many different sounds. Returning to the main screen (Screen2) then allows the various channels and instruments to be configured. Having done that, you can use the MIDI Encoder to play through the phone’s speaker. The Channel and Instrument dropdowns allow specific instruments to be allocated to different channels. Once each channel/instrument combination is set, press the ‘SEND PROGRAM’ button to activate it. By default, the MIDI Encoder only delivers data on channel 1, but we have created a software variant that outputs on four channels instead. The Arduino sketch is named MIDI_ENCODER_4_ CHANNEL, and when uploaded to the MIDI Encoder and Switch Matrix hardware, will generate events on channel 1 when S1-S16 are pressed, channel 2 for S17-S32, channel 3 for S33-S48 and channel 4 for S49-S64. You might find that this combination works very well for triggering sound effects. Look near the bottom of the instrument list, among the percussion instruments. Conclusion With the addition of a second Leonardo board and not much else, we can add controllable LEDs to our MIDI Encoder and Matrix. And since our system uses the existing MIDI hardware, our software could be modified to give MIDI control over other things too. Using our MIDI Encoder and Switch Matrix with an old Android phone is a very economical way to access a full range of playable sounds. Fig.6: the arrangement of a typical ‘Note-on’ message. The first byte has its most-significant bit is set to flag that it is the start of the message. The high nybble of 9 means that this is a Note-on message (8 would mean Note-off), with the low nybble containing the channel number. The following bytes carry 7-bit values for note number and velocity. 18 Practical Electronics | May | 2022 What is MIDI? MIDI stands 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 original MIDI standard was agreed on by a group of musical instrument makers in 1983, and has been used and extended since then. The last time we looked at MIDI was before the Arduino phenomenon had fully developed. Arduino has made it very easy to interface electronics to MIDI equipment. People have created an assortment of controllers, instruments and even synthesisers with a variety of sounds using Arduino. A MIDI 2.0 standard was released in January 2020, and is intended to be backwards compatible with the original I I specification. The new standard is not yet in widespread use and is still undergoing testing. Electrical protocol The standard specifies events that occur such as the start or end of notes being played), which are encapsulated in messages that are transmitted between devices. The ‘original’ MIDI relies on serial data communication at 31.25kb/s using asynchronous 5mA current-loop signalling, with the current provided by the transmitting end. This means that each byte of a MIDI message takes only 320µs to be transmitted (counting start and stop bits). Since MIDI messages are either one, two or three bytes in length, this means that over 1000 such messages can be sent each second via a single MIDI cable. Each MIDI cable carries only one signal, so for bidirectional communication, two cables must be used. The cables themselves use shielded two-conductor wire. ll I I cables are fitted with standard pin I 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 OUT sockets. This provides shielding via earthed cable shield braids without creating earth loop problems. nlike most other current loop signalling protocols, current only ows in a MIDI link when data is being transmitted. This allows MIDI cables to be ‘hot’ plugged and unplugged without any problems, as long as they are not in active use. All MIDI inputs are provided with 3kV of galvanic and electrostatic isolation via an optocoupler to prevent equipment damage due to wiring errors or component faults. For proper MIDI communication between equipment, a MIDI OUT or MIDI THRU socket at one end must be connected to a I I I socket at the other. That is what we call the ‘hardware’ MIDI implementation. There also exists a USB implementation that allows for much faster communication. This is not merely a USB-serial type of translation (although software exists to use USB-serial converters for this purpose). Jaycar’s Cat XC4934 USB MIDI Interface is one example of hardware for translating between these two protocols for device interconnection. Since MIDI is not much more than a series of data bytes, various other hardware transports have been used. These include ire ire, and even wireless transmission methods. Logical protocol ach message starts with a byte that has its most significant bit set, and the remaining bytes for practically all messages has the most significant bit cleared. This means it is very easy to serialise and packetise the data for conversion over other transports. Of course, you’re not limited to using our MIDI Encoder with this App. You might want to modify the Arduino sketch to provide your own interface, or even plug in another USB MIDI device. Practical Electronics | May | 2022 There is a single main controller or sequencer in most MIDI systems from which most of the MIDI messages originate (often the computer, or perhaps a keyboard or digital audio workstation). When these messages must be sent to more than one instrument, they can be distributed in either ‘star’ or ‘daisy-chain’ manner, as desired. It’s possible to combine two MIDI streams. However, a device to do that is not trivial to implement, as it must handle the case when two messages arrive at the same time and queue them for consecutive output without delaying them excessively. There’s no need to worry much about the actual code messages sent over the I I links. owadays, that is handled by a sequencer or other software running in the 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 particular instrument to a specific 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 three byte messages see ig. . owever, some configuration and or system management messages are only one or two bytes long. Longer, equipment specific configuration messages also e ist, for e ample, to load digital audio samples into a sampler. File format Using a PC-based music editing and sequencer program, and perhaps with 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 – eg, on the ‘instruments’ in a synthesiser. The synthesiser can then be made to ‘perform’ that piece of music by merely sending the sequence to it, via the MIDI link. When you’re happy with the result, you can save the sequence on disk as a MIDI music file. These have a standardised format and are identified with a ‘.MID’ extension. The . I file format is essentially a series of I I messages after a header, stored in chunks in a similar fashion to . file chunks and intermingled with timing data to ensure that the messages can be played back correctly. It s important to realise that although a I I music file may look superficially similar to a . 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 detailed instructions describing how to play the music. In this case, they are instructions for electronic instruments rather than for human players. And depending on the instrument that is providing playback, the sound that is output can vary substantially. Software The advent of Arduino-compatible boards with an integrated USB peripheral such as the Leonardo (based on the ATmega32u4 microcontroller) makes it easy to implement a USB MIDI interface. And there is much PC software around that can work with USB MIDI devices. While we were testing our design, we experimented with MuseScore (https://musescore.org/en), an open-source score-writing program and Anvil Studio (www.anvilstudio.com), a digital audio workstation program. Both can act as a synthesiser, producing sounds based on incoming MIDI messages. If you would like to delve further into the technicalities of MIDI, then take a look at the MIDI Manufacturers Association website here: www.midi.org/specifications Links SoundFont files https://bit.ly/pe-may22-sfz Reproduced by arrangement with SILICON CHIP magazine 2022. www.siliconchip.com.au FluidSynth MIDI Synthesizer App https://bit.ly/pe-may22-fsms 19