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It inevitably starts with a piano voice, then electric piano voices. Chromatically pitched percussion, guitars, strings, and so on, ending with drums. Even the most limited of keyboards have this layout in a highly diminished form. I am aware that this has to do with the MIDI standard, but

  • I am under the impression that MIDI is a communication protocol for devices, not necessarily having to do with the sounds those devices make

  • I do not understand why it has to be those voices: why are there always vibes? A slap bass? A synth pad? A reed organ?

Is it known why these voices made it to the standard, and why a communication protocol committee was responsible for picking them?

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    Having a defined standard set of sounds is very useful for communicating things like "piano sound", don't you think? :) You can compare General MIDI to the highest layer of the OSI model or something. It's indeed about communication, but riding on top of lower-level things.
    – piiperi Reinstate Monica
    Commented Jun 1, 2020 at 18:08
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    @piiperiReinstateMonica: another useful analogy for General MIDI is the ASCII character set. It’s a standardised list of common characters, including only a tiny fraction of all the characters that exist; some of the selections are obvious, but others rather more surprising, and the whole thing is (inevitably) fairly arbitrary and tied to the specific cultural background of the people who fixed it.
    – PLL
    Commented Jun 1, 2020 at 22:46

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Yes, it's correct that there is a MIDI standard which relates to your question, but what you are talking about predates MIDI and has more to do with the history of sample-based keyboard instruments. The MIDI 1.0 Specification was published in 1985, but didn't include specific instrument voices like you are talking about, and was only an abstract communication protocol. The Instrument List was introduced with General Midi in 1991. (Wikipedia)

The MIDI General Instrument List likely just immortalized an arbitrary list of sounds available on existing popular keyboards. This article on the Casio SA-1, a keyboard from 1989, shows the list of 100 preset sounds available on that keyboard, which have considerable overlap with the MIDI Instrument List. The decision to choose 100 sounds works well with a two-digit selection interface which is still used to this day, and is pretty close to the MIDI list of 128 instruments that fit within a one-byte integer. We see a possible bias towards Japanese traditional instruments, such as shamisen, koto, and taiko drums, which would have been familiar to Casio as a Japanese company, as well as the mostly American and Japanese creators of the MIDI standards.

We can go back even further and see sounds that carried over from the original digital sampler keyboard, 1979's Fairlight CMI, which included a similar mix of orchestral and popular instruments, some sound effects such as glass breaking, and the "orchestra hit" sound that it originated.

There's certainly a lot more that could be written but I that's as far as I'll go researching this. It's an interesting historical question for sure.

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Yes, MIDI is primarily a communications protocol. But it also includes specifications for the General MIDI sound set. 128 sounds. Here they are, with their Program Change numbers.

https://en.wikipedia.org/wiki/General_MIDI#Piano

It wasn't ALL that long ago that sound files were unmanageably large for sending over the internet. So a lot of music was distributed as MIDI files. General MIDI ensured that something close to the intended sounds would come out whatever device was playing them.

There were advantages other than file size. Tempo changes and transpositions were easily done without the horrible side-effects from pitch-shifting audio. Tracks could be easily edited and modified.

The General MIDI instrument list survives today mainly in the 'home keyboard' type of instrument, probably to facilitate the auto-accompaniment features.

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    Well, back when General Midi was specified Internet didn't really exist, compressed audio formats didn't exist at all and you would have needed a supercomputer to uncompress one in real time, and hard drives, not to mention floppies, were too small for any kind of audio files beyond short low resolution samples.
    – ojs
    Commented Jun 1, 2020 at 7:43
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    @ojs General MIDI just lists the sounds and doesn't specify whether they are synthesized or sampled (or even a real instrument like an organ). In the early days almost all of them would be synthesized (mostly really badly).
    – PiedPiper
    Commented Jun 1, 2020 at 8:22
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    @ojs To nitpick: General MIDI was published in 1991, when the Internet had been around for over 20 years, and widely used for a decade; even the WWW had been around for a couple of years. MP3 was drafted that year, and digital audio players had been invented. — However, your main point is still valid: the WWW didn't take off until a couple of years later, and commonly-available hardware wasn't quite up to handling compressed audio of any quality. The first MP3 player was released in 1997. I remember recording (uncompressed) audio to a Jaz drive in the late 1990s.
    – gidds
    Commented Jun 1, 2020 at 11:48
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    @ojs, MIDI was older. But MIDI and GM are not the same thing. MIDI was an electrical standard—Originally meant to allow a musician to remotely play one or more digital instruments from a central keyboard. GM was a file format that filled the same purpose as 19th century player piano rolls: It was a medium for distributing pre-recorded performances.
    – Solomon Slow
    Commented Jun 1, 2020 at 12:42
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    General MIDI survives on MIDI sound modules too. Typically they are low quality compared to the other sounds available, but you'll still find them there because they form part of the standard. A manufacturer can add whatever sounds they want with other banks, but they still have to support the basic GM sounds in order to say their module is a General MIDI module.
    – Graham
    Commented Jun 1, 2020 at 13:32
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Just answering a part of your question:

I am under the impression that MIDI is a communication protocol for devices, not necessarily having to do with the sounds those devices make

That is correct. Sort of. MIDI in its original form basically is a protocol that allows you to say, in a device-independent, abstract way: "Please, play note 43 with velocity 76." It also allows you to say, in a way that would be understood by every instrument: "Please, play the following with instrument 56."

But: what is instrument 56? And that's where the Instrument List of the General MIDI standard comes in: it specifies that a certain set of instrument voices is available at a certain set of instrument numbers.

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