I have been learning "Rage Over A Lost Penny", and there is a part about one third in where the left hand quickly plays in 16th notes at about the same tempo the fingers are 2,1,5,1,2,1,5,1 and is supposed to seamlessly change the notes that 5 plays each time. Then I play 5,1,2,1,5,1,2,1 and about 6 more variations seamlessly with only two short pauses.
Here is what I have figured on my own after 6 months of practice and I have about got it mastered.
- Vary your tempo during practice: Practice slow and steady, as well as at tempo (even if it sounds bad). Switch back and forth. (Your brain can get a better idea what is going on if you play slow once in a while)
- When performing, you might be able to cheat the tempo: It is possible to gradually speed up or slow down the song to accommodate your skill, as long as it sounds like part of the effect. Make sure it sounds like part of the effect, if the real purpose occurs to one of the audience, it may be a distraction, and if change in tempo only happens in one place, there could be a problem and if the audience has heard the song before there could be a problem. On the other side, if done right, a tempo change can be refreshing. I currently practice tempo changes temporarily for Rage Over a Lost Penny, until I sound good with the correct speed. In this case, I cut the tempo in half for a certain part, which does not mess with the beat (Actually, this is just before I move into a transition to the first section so I can make an end about half way, I am not yet able to play the whole thing). Also, in The Entertainer, I have (don't anymore), slowed down the song when playing the first section the third time, that way I can preserve the slower tempo for the second half. This does not work for every song. (Perhaps see below comments that object to this suggestion)
- Interesting case from a well known composer:
Michele McLaughlin has a song called So Long Mr. Selfish, and she has composed in an alternating octave similar to what you have described, in her CD, she plays it surprisingly inconsistent, although it does not sound like a struggle to me. This however is not written out as 16th notes, but rather as a Tremolo between two whole notes an octave apart.
Now that I know you mean both notes at once, the answer mostly applies, but whatever you do, do not sound like you are struggling. :) Anything to make it sound natural.