If you play a chromatic scale on a valved brass instrument, played with no pitch adjustments via embouchure, you end up with a series of pitches that are of questionable temperament, since some intervals are achieved by jumping between registers which utilizes the harmonic series (integer multiples of frequency) while others are achieved through valves which don't necessarily utilize integer ratios. [0] To make unoptimized keys sound good, the player can sharpen or flatten with their mouth and I suspect this is considered unnatural.
Contrast with instruments that don't use the harmonic series and are always in equal temperament, like woodwinds and keyboards. Also contrast with fretless stringed instruments, for which there is no inherent temperament whatsoever if you avoid open strings, since the tuning of each note occurs by normal fingering, which I suppose is considered natural.
Slightly adjusting intonation is no problem at all for trained wind players, "played with no pitch adjustments via embouchure" is just not something that happens. Much less difficult to learn than learning to play the violin at all, if Quartesixte's description is anything to go by. Not being able to play all 12 notes with natural tones does not in any way result in a lack of versatility for wind instruments in a modern orchestra setting.
PS woodwinds aren't "always in equal temperament" either, the player has to adjust for tuning compromises in these instruments as well. Not that being in equal temperament is even something an orchestra aims for.
I know it's no problem for trained players, but I suspected that an instrument's ability to have its player worry about pitch exclusively on the fingerings might be what Quartesixte meant in their claim that a violin plays in all keys "naturally."
Interesting about woodwinds not being equally tempered! I see another commenter points out the same, so I take that back as an incorrect assumption. So I wonder why they wouldn't build them as such, given that it would reduce the worst-case required bending to achieve arbitrary-key just tuning. I think the octave key is typically the only critical use of the harmonic series (altissimo aside), and that's a perfect 1:2 in all systems so it's moot.
Yeah I guess what I was trying to get at is the instrument isn’t “naturally biased” towards a specific key in the same way woodwinds and brass instruments are.
But that's most true only if you forbid playing open strings. Or if you demand that they be tuned to equally tempered fifths rather than just fifths, and that the player finger accordingly! Neither is likely, in which case it's biased toward keys that include just-tuned open strings, and a player would need to make a conscious effort to avoid open strings when other key-appropriate intonation is desired. That leaves us with only keyboard/mallet instruments as being unbiased with respect to key, but their equal temperament isn't what I'd call natural nor what an orchestra ought to sound like overall (though they could be tuned with bias toward the keys being performed, to help blend with the orchestra...).
Trumpets usually have tuning slides which are actuaded during play. Usually the tone is too high on specific button combinations in certain registers. On those the player moves the slide out a little. It's really a no brainer for the most part.
Alternatively you can give them additional valves which are tuned slightly differently. Which is usually found on Tubas and other deeper brass instruments.
There are more technical solutions like compensator valves too.
Contrast with instruments that don't use the harmonic series and are always in equal temperament, like woodwinds and keyboards. Also contrast with fretless stringed instruments, for which there is no inherent temperament whatsoever if you avoid open strings, since the tuning of each note occurs by normal fingering, which I suppose is considered natural.
[0] https://www.reddit.com/r/trumpet/comments/1zy0w8/trumpet_phy...