How does it work?

An upcoming article discusses a new educational freeware utility, the MiniMIDI Recorder. With this program, you can record the MIDI signals of a keyboard performance and save them as a general MIDI file. There are two differences from the built-in REC function of the PSR E423/433:

  • The file is created on your computer, not trapped inside the keyboard.
  • The controls are logical and straightforward. To see what I mean, try making a recording by pushing buttons following the instructions in the E423 manual and see how many tries it takes.

To understand how the MiniMIDI Recorder works, it’s useful to have a basic understanding of what goes on inside the keyboard and how operation is affected by some of the function settings. In this article, I’ll describe the keyboard organization and how the KBD-Infinity program fits into the scenario[1].

Block diagram of the PSR E423 attached to a computer

Figure 1. Block diagram of the PSR E423 attached to a computer.


Figure 1 shows a block diagram of the keyboard. Although externally it appears to be a unified device, it actually contains three relatively independent microprocessors that communicate through standard MIDI signals. The physical keyboard is a set of 61 pressure-sensitive switches connected to the relatively simple keyboard processor. The sole function of this processor is to send out MIDI messages consisting of three bytes: a status byte and two data bytes.

  • The status byte designates whether a key has been pressed (NOTEON) or released (NOTEOFF).
  • The first data byte gives the note.
  • The second data byte gives the note volume if TOUCH is ON. Otherwise, the data byte has the maximum value (&hFF = 255).

I expect that the keyboard processor also monitors the setting of the pitch-bend wheel (the thing on the left side of the keyboard that you would have to operate with your nose if you are playing with both hands) and sends appropriate messages.

When the keyboard is in LOCAL mode, the MIDI signals from the keyboard are transmitted to the main processor. This internal computer performs functions controlled by the rest of the buttons, knobs and dials on the keyboard. Performance functions include the following:

  • Add information to the MIDI stream to tell how the notes from the keyboard should sound, depending in the VOICE settings (e.g., flute, saxophone, piano,…).
  • Add information from a style file to the MIDI stream, with harmonic offsets controlled by keys pressed in the lower part of the keyboard.
  • Calculate harmonies and add additional notes to the melody line when HARMONY is active.

The main processor has other functions, like playing songs, adding metronome marks to the MIDI stream and driving the LCD display. The output from the main processor goes to the MIDI interpreter processor, a digital-to-analog converter. This device takes the specifications of the MIDI messages and generates a complex waveform with information on attack, overtones and other features to represent an interesting instrumental voice. The waveforms are ported to the internal amplifier and speakers or to external devices.

With this general sketch, we can understand how an external computer with a program like KBD-Infinity fits in. First, we should recognize that the differences between a basic keyboard like the PSR E423 and an expensive model with lots of features reside mainly in the components enclosed in the dashed red box of Fig. 1. The keys on the E423 are reliable with a good touch and the keyboard processor isn’t called on to do much, so there’s not much difference in the input. The MIDI interpreter supports XG voices and makes a good sound. My guess is that Yamaha uses a standard interpreter in a variety of keyboards, so there’s not much difference in the output. On the other hand, the main processor is probably a venerable16 or 32 bit device with a small on-board memory, just fast enough for the modest demands of music performance. The functionality is purposely limited by the ROM program. Bringing out a new model (like the E433) is simply a matter of changing the ROM and adding a button or two. There is therefore an opportunity to make major improvements by bypassing the main processor.

The key to expanding the power of the keyboard is the USB cable connection to a computer. The cable carries a MIDI output port with signals from the keyboard (and optionally some signals from the main processor). There is also a MIDI input port connected directly to the interpreter. The idea of KBD-Infinity is to eliminate the components in the dashed red box by turning off the LOCAL switch and performing all their functions on the computer. There are several potential advantages:

  • A 64-bit Intel or AMD chip will definitely outperform the funky internal processor of the keyboard.
  • With 8 GB of RAM and a terabyte hard disk, you could store all the presets and voices you want.
  • It’s much easier to implement complex setups using a color monitor, a mouse and a computer keyboard than through the haphazard assortment of buttons on the E423.
  • The main processor of the keyboard is a proprietary device hidden in the black plastic box. In contrast, the computer is an open system to which we can add unlimited capabilities. In fact, the computer can speak with the entire world through the Internet. Hence, the infinity in KBD-Infinity.


[1] Detailed technical information on the E423 is not available, so I am making some educated guesses about the keyboard organization based on my experience as an electrical engineer.

[2] Yamaha PSR E423 and PSR E433 are trademarks of the Yamaha Corporation. This site has no affiliation with the Yamaha Corporation.

[3] Find out more about KBD-Infinity: Home page.

[4] If you have comments, please contact us at

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