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.

I'm partial to glissando.

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

- Bowing direction: up/down (they sound different)

- Adjust bow position: sul ponticello, sul tasto

- Natural and artificial harmonics

- Use of mutes

- “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.