A simple MIDI keyboard for PC. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Musician
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As you know, at present, almost all sound cards for personal computers (PCs) have an MlDl-interface (Roland MPU-401) that supports UART functions, to which any external MIDI device can be connected. More about this is described in [1]. With a software sequencer (such as Cakewalk Pro Audio), the PC can play a stream of MIDI commands. whose source is a file or an external controller. External MIDI devices and PC communicate with each other via an interface. Its official specification can be purchased at midi.org/, and a description in Russian can be found in [2].
The device is made on the i8751H microcontroller and assembled according to the scheme shown in fig. 1. SA1 adopts hexadecimal output drum switch (from CNC machine). It will completely replace the usual one with 16 positions and two directions with the K155IV1 encoder. One direction of such a switch is used to control the state of the P2.3 line of the DDI microcontroller. the other - the inputs of the encoder. In this case, the program must invert the data read from port P2 (in the blocks for forming the first byte of the MIDI message). Socket XS1 - ONTS-VG-5/16-R (SG5). Instead of J8751H, you can use a domestic single-chip microcomputer KP1816BE51.78L05 replaceable by KR142EN5A. KR142EN5V (at input voltage 8...15 V).
Key design - any. Read about the features of the mechanical device of factory-made keyboards in [3].
The text of the MID / -keyboard program is given in Table. 1, its codes are in table. 2. The program implements only the basic functions of the keyboard, but since only 2 bytes out of 200 KB of built-in ROM are used, if desired, you can add a key velocity sensor, a pedal, a transposition function (shift a few semitones up or down).
(click to enlarge)
An adapter is required to connect the keyboard to a conventional sound card. The need for it is due to the fact that there is nowhere to place MIDI connectors in a PC, so the optocoupler isolation required by the standard and the interface converter to the "current loop" interface have to be moved to an external MIDI adapter. Schematic solutions for this node can be found in various newsgroups dedicated to MIDI and sound cards. It can also be assembled according to the scheme shown in Fig. 2.
As U1, it is permissible to use any diode optocoupler with a turn-on and turn-off delay of no more than 2 μs, a sensitivity of at least 5 mA and a TTL output (for example, K2S2KP1A). Connector XP1 - cable plug D15 (it is connected to the "Joystick" connector of the sound card). XS2 and XS5 - D15 sockets for printed wiring, necessary to maintain the ability to connect joysticks to the card (if this is not necessary, they are not installed). For the keyboard to work, you need to install the MIDI input driver that comes with your sound card.
Author's versions of the described devices are assembled on breadboards, printed circuit boards were not developed. The cable for connecting the controller to the adapter (the "MIDI OUT" sockets of the first and the "MIDI IN" of the second are connected) is made according to the 4-4, 2-2, 5-5 scheme (using ONTS-VG-5/16-V (SSh5) plugs) .
The check showed that with serviceable parts and no errors in the installation, the devices do not require adjustment and start working immediately after turning on the power. They were tested with Gravis Ultrasound Classic rev.3.71 sound cards. Gravis Ultrasound MAX. Gravis Ultrasound PnP and Creative SoundBlaster Live! value.
Literature
- Application Note AN27. - http://crystal.com/.
- Muzychenko E. Description of the MIDI interface. - ixbt.stack.net/.
- Izhaev P. Home studio: MIDI keyboards. - midi.ru/.
Author: I. Oborotov, St. Petersburg
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