It’s very interesting to see the different takes on interpreting this change. And I tend to think there’s some degree of validity in all of them.
Enamuel, regarding the quote above, I tried out your F minor version and it does sound more classical that way, than when thinking of it as C major. Nonetheless I like it in C major as well. By way of an explanation, I came across this when transcribing a portion of Goldsmith’s main theme for Star Trek from memory, and specifically a measure that uses several planing chords. When I went back to listen to the original piece, it turned out my transcription of the penultimate chord was incorrect and the actual chord was more straightforward, but to be honest I liked my transcribed version a little better (I know … what chutzpah!) because I’d extended the planing through one additional chord and ended up with the Bbm7 to C major change that I asked about. It absolutely helps to see it in Fm, but I don’t think I’ll shy away from using it in C major too.
it can be in any key. Normally in theory we choose the easiest and more natural explanation. Without other chords is hard to tell which is the most simple explanaiton. This can perfectly be a modal cadence in a C major piece, because of the neapolitan tones..
Hearing now the music i see that the cadence is in G, and there is no Bbm i can hear. I hear a Bb on bass,possible of a Eb4/6. IN this case a plagal cadence where the subdominant chord is on C, but sounds like a sus4. This all sounds very fast, I would have to check again.
Now this is what i found for the keys, in a short look:
The music starts in Bb, with a clear dominant statement of F Major at the end of the phrase (half cadence). Typical opening setence in the classicisism, with the diference that hear the composer is smart guy and just repeat it straight forward.
This short theme is answered with abrupt modulation into major chromatic mediant. The answer of a theme using a key in a relationship of 3th to the tonic comes from minor themes in the classicism and dance movements in the barroque, when a section in the minor tonic preceds another in the rel. major. . Later romantic composers use it, but they used a lot conserving the same minor or major mode. This was part of the "expansion" of sonata form. So you have the chromatic mediant playing a role in the form, as in this music. I guess you find in Schubert and Beethoven the first of such examples.
This answer is repeated a half step above, on the subdominant of Bb. Another very smart and efficient move. He knows that the abrupt modulation gave richness to the harmony without losing the tonal coherence. This means, he can tension one step more, away from tonic now, and gives a basic development (i would rather call it short term "distraction") that makes the reprise of the theme more satisfatory. This part ends on the dominant of Bb.
Considering parts in the key of subdominant is a great way of giving the feeling of distraction/development, but also knowing where you are: you are close to the dominant, and so you are just few steps of a modulation back to tonic (Ruckmodulation).
It is a tri-part form, where the contrastant section is one-part, or "direct", and the first and last are parts based on alternation like theme/soft contrast. For a moment I thought he would do a kind of rondeau, what is not a bad idea for such situation! The bass remain very solid and coherent in the first and last parts, so I guess the composer singed over a chord progression he liked.I guess he had a very solid and grotest insight based on bass line. It is structural and steady for the whole piece, despite the contrast section. To not say, maybe he composed this in the organ? It is a perfect organ theme.
I can´t continue, but It is a great piece. thanks for reminding me.
If you transcribe some bars of the Robcop theme, I offer a free lesson