Tons. Nearly everything. Sheet music is very expressive. So is MIDI. It’s kind of like asking, “What information isn’t captured in the movie but is captured in the novel?” They’re just radically different ways of expressing music, and much is lost when you translate from one to the other.
If you are curious, try flipping through a book on orchestration at the library sometime, or a book on music theory. Even at its most basic, a score records which notes to play, but MIDI doesn’t even do that—MIDI records the note values only, so you can’t tell the difference between G# and Ab. Then add in all the articulations, dynamics, and arbitrary instructions that a composer can put into a score.
For a keyboard instrument, played conventionally, MIDI does a fairly good job of capturing everything that a composer could tell a performer (and vice-versa).
Slightly less true for percussion, but still somewhat true.
Rather untrue for any instrument where technique can be (ab)used to alter timbre significantly (e.g. most reeds, most strings)
Very untrue for any instrument capable of continuous pitch generation (e.g. unfretted strings).
Obviously "arbitrary instructions" are out, but then they are also not formally part of "sheet music", but more "additional written material from the composer", which can accompany MIDI in various ways too.
In a piano, the relevant acoustic properties controllable by the player is key down (with differing amounts of force) and key up, with the pedals effectively acting as a way to delay key up (although one pedal causes the hammers to hit two instead of three strings, which largely manifests as quieting the piano). The simple note on/off + note velocity of MIDI models this well, and it's basically the use case MIDI was designed for.
By contrast, the technique involved in playing the violin family is very varied:
* Which string are you playing the note on? You get different sympathetic vibrations depending on which one you choose.
* Are you moving the bow up or down? Direction matters!
* How are you changing bow movement between notes? Keeping the same direction, or changing? Resting your bow on the string the entire time, or bouncing the bow?
* Where you are playing the bow? Down by the bridge, or up by the fingerboard?
* Are you even using the bow to the play notes? You can pluck it instead!
* Fingering too, you can make many small motions with your finger to give it a vibrating quality.
* On a related note, the pitch you play is a continuous quality: you can slide from one note to the other and hit all the notes in between. Unlike a piano (and MIDI), where notes are discrete pitches.
* You can also adjust tuning of the strings (though not on the fly), or damper them with a mute (which can be done during a long rest in a piece).
There's probably a few more expressive techniques I've forgotten, and I've definitely forgotten all of the fancy Italian names for these techniques.
You can look at the violin sections of Saint-Saëns' Danse Macabre (https://www.youtube.com/watch?v=71fZhMXlGT4) to see how different bowing techniques can produce rather stark effects.
In general, think of this: You are touching a musical instrument. You are physically touching the strings, and you can undoubtedly imagine many different ways of touching the strings that would have different effects on the sound produced. If you were to sit down and imagine different ways to play violin, you would probably come up with techniques that already have standardized names (in Italian, of course) and perhaps even standardized symbols in notation. The possibilities are AMAZING and definitely not limited to simple pitch/loudness (I mean, obviously!)
If you want to see a reference, go to your local library and find a book like The Study of Orchestration (https://www.amazon.com/Study-Orchestration-Fourth-Samuel-Adl...) and flip to the section on strings. I own this book, but I don’t know where it is at the moment, so… off the top of my head, here are other dimensions besides pitch and loudness:
- Notes can be connected in different ways. Legato, détaché, martelé, staccato, spiccato, sautillé, jeté/ricochet, tremolo, pizzacato, louré, marcato
- “Extended techniques”—altering the tuning, col legno, etc.
These all affect the sound. Note that the difference between staccato and legato is NOT accounted for solely by the length of the notes as in a MIDI file. You also might be surprised how many of these have really boring, everyday notational conventions. As in, a violinist would look at sheet music and say, “obviously, technique X is used for this note”, but that would not be encoded in the MIDI file at all.
All of the above techniques are explained and demonstrated in YouTube videos if you are interested.
Not a violin, but on mandolin I can greatly change the tone (which is somehow the overtones) by changing here and how the pick hits the strings. A good violin player can do a lot with the bow.
Not relevant to the line quoted above, but ... you can apply more or less bow pressure, resulting in different timbres. You can pick/pluck the strings rather than bowing them at all.
That's a problem with MIDI-the-standard, not MIDI-the-idea. In the 1.x standard, it's not just a simplistic definition; the "known workaround" (pitch bend) for when you actually need to express this difference in a played note, is global per instrument. But it's nothing that can't be fixed with a protocol that follows the same principles, but has a richer data model (like OSC).
> What information isn’t captured in the movie but is captured in the novel?
This, absolutely this. And it also doesn't mean we shouldn't be getting better cameras.
If you are curious, try flipping through a book on orchestration at the library sometime, or a book on music theory. Even at its most basic, a score records which notes to play, but MIDI doesn’t even do that—MIDI records the note values only, so you can’t tell the difference between G# and Ab. Then add in all the articulations, dynamics, and arbitrary instructions that a composer can put into a score.