MIDI keyboard. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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MIDI keyboard. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Musician

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A characteristic feature of the keyboard described here is that its output is not sound, as in conventional EMI, but the output of a digital music interface MIDI. Various musical instruments can be connected to such a keyboard, of course, if they have an input of this interface. Many industrial synthesizers, such as Yamaha's PSS series, have smaller keyboards that are not always comfortable to play, and a MIDI keyboard with full-size keys allows you to significantly expand their performance capabilities. In this case, of course, there is no need to make any alterations or design improvements in the instrument - you just need to connect the keyboard with the MIDI IN cable to the synthesizer's "MIDI IN" input and program it to receive MIDI signals.

The proposed device can also become part of your home MIDI studio or be used in conjunction with sound modules and samplers in a keyboardless "Rack" version.

(click to enlarge)

The MIDI digital music interface, which appeared in 1982, made it possible to separate the performing (master) and sound generating (performing) parts of musical instruments [1-3]. The information transmitted between them has the form of simple commands, common for keyboards, fretboards, wind and other EMPs. Commands reflect the influence of the performer on the instrument, for example, "TURN ON A NOTE", "CHANGE THE POSITION OF THE REGULATOR". Despite the fact that the methods and techniques of playing the instruments differ, MIDI commands are universal, which allows you to connect different types of EMP and get unprecedented combinations of performance and sound possibilities.

The task of performing devices is to perceive and convey the most subtle shades of musical performance as fully and correctly as possible. Modern EMP keyboards are called dynamic, because they record not only the fact of pressing a key, but also the force or speed of its pressing [5], which can affect both the volume of the sound and its timbre - depending on how it is programmed AMY. What's more, velocity can control the activation of sound effects and even switching tones. For example, with "soft" playing, the instrument sounds with one timbre, and with a more "hard" one, with another. This technique is often used in sampler instruments. Let's say they record the sound of a piano with strong and weak strikes on a key in a sampler, and then the sounds are mixed in a proportion depending on the velocity of pressing. This allows you to simulate the dynamic and timbre range of a natural piano and flexibly control it while playing.

However, many acoustic instruments have at least one more degree of timbre control. Fretboard, wind instruments allow you to vary the sound to a large extent after taking a note. Playing bowed instruments, the electric guitar consists in the art of braces, vibrato and other performance effects throughout the sound of a note. And how tempting it is for a keyboardist to use not automatic, but finger, natural and lively, “vibrato” or revive a chord by entering modulation without lifting his fingers from the keys) A touch-sensitive keyboard allows you to do all this, because it measures the force the pressure of the musician's fingers throughout the sound of the note. Perhaps in the future there will be keyboards that measure the pressure on the keys in all three directions - musicians will certainly want to use all this.

Of course, all measured parameters must be transmitted over the interface and control all connected EMR. To do this, the MIDI standard provides the means, the essence of which is shown in the table below. 1. The command "ON NOTE" carries information about the force or speed of pressing each key of the keyboard. All dynamic keyboards measure and transmit this parameter in the range of 0 (no sound) to 127 (fortissimo). Similarly, the command "TURN OFF NOTE" tells the speed of the release of the note. Few keyboards measure this parameter, so the "ON NOTE" command at zero speed is often used instead, which is not forbidden by the MIDI standard. The command "PRESSURE ON THE KEY" is transmitted only by some dynamic keyboards - those that perceive the pressure of the musician's fingers on the key after the note is struck. The "WHEEL POSITION" command is transmitted when the tone change wheel is rotated, if, of course, there is one.

Table 1
Command (in hex code) Status byte (n - channel number) First data byte (0...7Fh) Second data byte (0...7Fh)
ENABLE NOTE 9n Note number Velocity (1...7Fh)
TURN OFF NOTE 8n
9n Note number
Note number Release speed
Speed=0
KEY PRESSURE 0An Note number Force of pressure
WHEEL POSITION 0in Lower 7 bits High 7 bits
CHANGE OF MANAGEMENT 0bn 0 - regulator 0
1 - regulator 1
(mod wheel)
*
*
*
1Fh - regulator 31
20h - controller 0
21h - controller 1
(mod wheel)
*
*
*
3Fh - regulator 31
40h - switch 0
41h - switch 1
*
*
*
5Fh - switch 31
High 7 bits
High 7 bits ---- High 7 bits
Lower 7 bits
Lower 7 bits
----Lower 7 bits
0-off, 7Fh-on
0-off, 7Fh-on
---0-off, 7Fh-on

A large group of commands "CHANGE CONTROL" is reserved for any, both discrete and continuously operating controls. The binding of specific switches and controls to the codes of transmitted and received commands is individual for each instrument. They also include the modulation wheel, with which you can control the vibrato effect during the game.

The proposed MIDI keyboard is the simplest. It transmits only two commands - "ON-OFF NOTE" in any of the sixteen MIDI channels. Since the keyboard is not dynamic, the press-release speed is always transmitted at the same speed of 63. In addition, an "ACTIVE TEST" command (OFEh byte) is transmitted every 300 ms, sometimes used to test the MIDI electrical connection.

The single-chip microcomputer K1816BE35 (DD6) made it possible to greatly simplify the circuitry of the device. ROM K537RF5 (DS1) contains the codes of the program executed by the microcomputer (Table 2). Gaps in the table can be filled with any code. Register K580IR82 (DD7) is used to store the lower part of the ROM address [4]. The signal at the "MIDI OUT" output is generated by software, so the transmission rate directly depends on the frequency of the ZQ1 quartz resonator. Its frequency can be in the range of 8820 ... 8990 kHz - it is quite suitable, for example, for a resonator used in PAL decoders at a frequency of 8,86 MHz.

       Table 2 Address 0 1 2 3 4 5 6 7 8 9 abcdef K.S. 0000 23 ff 3a 00 00 00 00 00 00 00 00 00 00 00 00 00 5d5c 0010 b00 00 ba 00 bb 00 f00 00c 00 00c 00 00 00 00 bc34e 80 f64 b74 7с еb 0020 a00 8 ea 20 ef 08 b08 fe 0 2 39 2f97c 36 50e ff ff ff ff ff ff ff ff ff ff ff ff ff ff ff 67e1f 0030 a6 0 e1 29 0 18 26 ff ff ff ff ff ff ff ff ff ff aca20 9 34d b00 04f f9 9 0040 a1 1 0 fc 0050 7 a67 6 80 b04 32b0f 0080 2f 9 3 1d 03 90 ff ff ff ff ff ff ff ff ff ff d9cc 34b00 03d b24 9f f34 00 9 a8 3 0090 fc 3 34 a00 2 04 b32 6b00f 0c2 9f 3 1 03d 80 9 ff ff ff ff ff ff ff ff ff d34cc 00 03 24 9a f34 e00 e9 e7 e2 b00 0 3 34 00 2 04 32 b6b0100 23 00 3 9 7 7a 7 7 9 08 00 00 00 00 00 00 8 b3b0110 00 00 b00 00 3 e77 00 bb 00 fb 00a00f 00 00 00 00 00 eb 00f 1 b1 0120f a00 00 00a 00 ff ff ff f9f14

The SB1 "Reset" button of the microcomputer is used to program the MIDI channel number. Keyboard contacts S1-S64, connected to multiplexers K155KP1 (DD1-DD4), are interrogated sequentially to detect pressed keys. The outputs of these microcircuits are switched by the K555KP2 (DD5) multiplexer. The status of the polled contact is fixed at the test input TO (pin 1) of the micro-computer DD6. The keyboard is powered by a 5 V DC source and the current consumption is not more than 0,5 A.

The printed circuit board for the device was not developed. To reduce the length of the harness multiplexers DD1-DD4 are located directly under the keyboard. Instead of mechanical contacts S1-S64, reed switches with magnets attached to the keys can be used. Contact S1 corresponds to the bottom key (note C to the big octave), S64 - to the top one. Connector X1, through which a MIDI cable is connected to a MIDI keyboard, is a standard five-pin socket СШ5 (DIN-5) with unused pins 1 and 3.

The K1816BE35 chip can be replaced with K1816BE39, K573RF5 with K573RF2, and K555KP2 with K555KP12. Multiplexers DD1-DD5 can be any similar series K133, K555, K533, K1533.

With known good parts and error-free installation, the keyboard starts working immediately after turning on the power. If, however, this does not happen, first of all check the presence of supply voltage on all microcircuits. Using an oscilloscope, observe the clock pulses at pin 11 of the DD6 chip. The absence of oscillations is a sign of the malfunction of this microcircuit or the ZQ1 resonator.

Next, the output conductor of the 1-chip DD6 disconnect from the pin 9 of the DD5 chip and connect it to a common wire. Connect headphones to slots 5 and 4 of connector X1 - clicks should be heard in them with a frequency of about 3 Hz - this is the bytes OFEh "Active check" are transmitted. If they are not there, then one of the DD6, DD7 or DS1 microcircuits is faulty. Then reconnect pins 1 and 9 of DD6 and DD5. Now clicks in phones should be audible every time you press and release keys. If this is not the case, the fault is in the DD1-DD5 microcircuit assembly, or there are permanently closed contacts among the keyboard contacts.

Connect the keyboard to the instrument in accordance with the recommendations given in [3]. When the power is turned on, the keyboard is automatically tuned to the first MIDI channel. To change the channel number, press the SB1 "Reset" button and, without releasing it, press the key with the serial number of the desired channel. Release the button first, then the key. After such switching of the device circuits, all commands will be transmitted over the channel with the desired number.

Literature

Subbotkin M. MIDI. - IN/OUT, 1992, N1, p.18.
Kudeev Yu. MIDI-oobsobyaiya. - IN / OUT, 1992, N 2, pp. 66, 67.
Studnev A. What is MIDI? - Radio, 1993, N 1, pp. 32, 33; No. 2, p. 23-25.
Stashin VV, Urusov AV, Mologontseva OF Designing digital devices on single-chip microcontrollers. - M.: Energoatomizdat, 1990.
Kuznetsov L. A. Fundamentals of theory, design, production and repair of electrical musical instruments. - M.: Light and food industry, 1981.

Author: Alexey Studnev, Zhukovsky; Publication: cxem.net

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