I am composing a piece in which there is a modulation between the keys of Db major and F# minor. Is it possible to use the omnibus progression to smoothly execute this modulation? If it is possible, how do I do it?
Let's begin with a quick overview of the omnibus. Let's write it in C major for clarity's sake:
The omnibus takes a V7 chord (the second harmony of the first measure) and, keeping the minor third (chordal fifth and seventh) constant, it moves the major-third interval (the root and third) towards each other contrary motion by half step. This creates a large-scale voice exchange between the G/B, but also creates some nice chromatic harmonies in between.
Let's look at this chromatic harmonies:
- We begin with a root-position V7. Boring.
- With the G♯ in the bass, we can view this either as an enharmonically spelled V42 of ♭III or as a Ger°3 chord that will move to ii.
- With the A in the bass, we have a minor triad (ii) in second inversion.
- If we spell the A♯ as a B♭, we see that the next chord is a V7 of ♭III.
- And finally we have a V65 of the tonic key.
And just to make certain we're on the same page, here's that same omnibus in your key of D♭ major: (listen to this example)
What's great about the omnibus is that you can stop on each of these chords and use it to modulate to anywhere else.
But more importantly, remember that the omnibus begins with a V7 and that it has a different V7 in the middle of it. This means that we can start one omnibus, and when we reach that other V7 in the middle, we can then start another omnibus. Consider the example below:
In the first measure, I have a V7 of my original D♭; this starts my omnibus. But on the third beat of the next measure, I reach the second V7 that's in the middle of the omnibus. At this point, I then start a new omnibus by holding onto the minor third in that V7 and I move the major third (C♭/E♭) towards each other by half step. I then proceed in that omnibus, at which point, when I reach that minor 64 triad of the new omnibus, I just make that a cadential 64 that cleanly modulates to your intended key: F♯ major. (And if you want to get really specific, the chord on the downbeat of m. 3 is an enharmonically spelled Ger°3 in the new tonic.) Listen to this example here.
Omnibus tonicizes a single pitch class throughout, that is, it progresses without modulation. One easy idea is to end the full omnibus (Db to Db) with the modulation to the modal iv (gb minor) and use your enharmonic spelling, f#m for the new key.
Db: V7/iv __ iv
f#m: V7 __ i
Working backwards from there, you might adjust some chords to properly prepare the modulation. For example,
Db: vii6 V7/iv _ iv
f#m: iv6 V7 _ i
I imagine the further back you can prepare the modulation the smoother it will sound.