I made a stream of consciousness-ish post on another forum regarding some of my thoughts about improvisation in general, and improvisation over pitch axis progressions in particular, and I thought I'd post it here in case anybody finds it interesting or helpful.
I've been playing around with Band-In-A-Box for a few hours today, working on improvising over chord/key changes, and I'm going to share a few vague and random thoughts I had with everyone. Most of these points don't really go anywhere, they're just really general impressions that came to me. Some of you might be able to expand on these or turn them into something useful.
My thought fragments...
1) I was toying with pitch axis progressions, and I began to experiment with different modulations, trying to familiarize myself with which modes complement each other and which sound harsh and unnatural beside each other. Needless to say, it's a matter of intervals. The harshest change by far comes from altering the third, as it changes the entire tonality of the piece. Don't emphasize that third. The change seems to flow better if you refrain from making that third the focus of the melody line. I actually found myself avoid it during the period immediately before and after the chord change, only introducing it once the backing chord had "familiarized me" to the new tonality.
The most pleasing changes, to me, involve the fourth and seventh scale degrees. I actually found that, even without changing the backing chord, I could alter those notes at my leisure in the melody without it sounding unnatural at all. Over a Cmaj chord for instance, I could easily use a 4 and #4 in the melody, even right next to each other, without any apparent dissonance. This isn't entirely true over a maj7 chord, since the 4 is then an avoid note, but even then, the use of both wasn't at all displeasing. I was essentially playing chromatic lines that sounded completely natural in the context of the song.
Experiment with this.
2) Also relating to pitch axis, I constantly found myself trying to create subtleties within the progression that let the melody line flow naturally. Most people are familiar with pitch axis as Satriani applies it in songs like Not of this Earth, which has him quickly modulating between lydian, aeolian, and mixolydian. He's essentially creating rapid and dramatic changes in the overall tonality with an almost chromatic and disorienting effect. It's a very interesting effect, if that's what you're going for, but I also find pitch axis useful for gradual changes in overall mood. One progression that I spend some time improvising over consisted of...
Cmaj7b5 - C6 - C7sus - Cm7 - Cm - Csusb9 - C7 - Cm7b5
My thought process over this progression was basically as follows...
Cmajb5: Lydian mode. Technically, this isn't proper, but the #4 is enharmonic with the b5, making lydian entirely usable.
C6: Would generally call for a switch to the ionian mode, but I found the #4 of the lydian entirely usable here. Eventually, I found I could easily use both without sounding "out". The fact that the switch from Cmaj7b5 to C6 did not specify a new fourth created an ambiguity that let me break free from the diatonic scale, and into "chromatic country"
C7sus: We have now specified a natural fourth, as well as a b7, so the mixolydian mode in the logical choice. The change from a 7 to a b7 is slightly unexpected, but by no means jarring. You can safely emphasize this change without fear of sounding "out". I actually repeatedly found myself leading into the chord change with a chromatic line (octave --> 7 --> (chord change) --> b7) without any negative effect.
Cm7: As far as the chord change goes, it's not entirely unpleasant, but the switch to the dorian mode was just jarring. The flatting of the third here sounds off. There's really no way that I could find to gently modulate between parallel diatonic major and minor scales (but what about non-diatonic scales? More on that later) My suggestion? As I mentioned earlier, avoid emphasizing the flatting of the third in the melody line. Let the chord change establish the new mood before using the new third while soloing. Ideally, avoid the third completely until the switch to...
Cm: There are actually a number of options here, but I chose aeolian because it flows more smoothly into the next chord change (you're only flatting the 2nd). The flatting of the sixth is surprisingly dramatic here, but not necessarily unwelcome (I hate the sound of a natural sixth paired with a flat third). The sixth doesn't really "work" as a chromatic note, but a b2 works surprisingly well. By this point, the flat third felt comfortable, so I resumed using it in the melody line.
Csusb9: A flat 9 can only mean one thing (Ok...several things, but I'm using phrygian). The second sounds damn good here, and the change from aeolian isn't as apparent as you'd think. Don't think that the chordal ambiguity will let you toy with the third for a phrygian dominant effect, it just sounds off.
C7: You have a huge number of options here (mixolydian, altered, lydian dominant, pentatonic etc), but I kept right on with phrygian, for two reasons. One, it provided the smoothest transition to the next chord, and two, it was suggested to me that the b3 of the phrygian mode would act as a #2 over a dominant chord, giving an altered sound. Loving the altered scale as I do, I gave it a try, and lo' and behold, the phrygian mode works very well here. Similar in sound to locrian, without the dissonance. Don't think this is restricted to jazz, either. This sound is very pleasing to the ear; it's not even necessarily "dark".
Cm7b5: Obviously, locrian. The b5 isn't as jarring as it looks, but overuse of the b2 in the melody seemed to make the progression want to resolve to the relative major (more on this in a second). If the b5 doesn't appeal to you, you can safely avoid it in the melody and let the backing chord do the "locrianizing" for you. An interesting thing I discovered about locrians tendency to resolve upwards to the second degree is that it sounds entirely natural in the context of the progression. It would be very easy to eliminate C as the tonal center here and resolve up to C#. You could then repeat the progression in C, starting from C#6, and keep moving up the fretboard chromatically. An interesting idea for a song (or a series of songs, each in a different key).
The progression then repeats itself, from Cm7b5 back to Cmaj7b5. Contrary to what I said earlier, the shifting of the third here does not sound off. In fact, the switch from locrian to lydian seemed completely natural, probably because the focus of both modes is, enharmonically, the same note, making the change in the third seem almost secondary. Actually, resting on the #4/b5 during the chord change results in a very interesting effect, almost like "coming out from a tunnel"
Essentially, instead of rapidly shifting moods, I slowly altered the generally feel of the song by changing single intervals at a time, allowing the piece to float naturally into a dramatically different atmosphere. The point is to stop fixating on scales in a traditional sense, and instead use the structure of the progression to your advantage, allowing you to pick individual notes based on the effect you want to get across, and the effects that those notes give when paired with the chords.
Experiment with this.
I'll post more nonsensical nonsense when it comes to me.