Is it possible to approach hidden 5ths & 8ves by contrary or oblique motion?
Put differently, every time there is contrary or oblique motion between voices, are direct 5ths & 8ves allowed between soprano and bass? In yet different words, is the problem with direct 5ths & 8ves essentially the similar motion, so when there is no similar motion it's ok?
Is it possible to approach hidden 5ths & 8ves by contrary or oblique motion?
No. It is impossible because, by definition, if the motion is contrary or oblique, there is no hidden fifth or octave. From Wikipedia (with added emphasis):
So-called hidden consecutives, also called direct or covered octaves or fifths, occur when two independent parts approach a single perfect fifth or octave by similar motion instead of oblique or contrary motion.
From Dolmetsch online (with added emphasis):
hidden consecutives occur when two independent parts approach a single perfect fifth or an octave by similar motion instead of oblique or contrary motion. Conventional style dictates that such an interval, often called an exposed fifth or exposed octave, be avoided. But this is sometimes permitted under certain conditions, such as the following: the interval does not involve either the highest or the lowest part, the interval does not occur between both of those extreme parts, the interval is approached in one part by a semitone step, or the interval is approached in the higher part by step. The details differ considerably from period to period, and even among composers writing in the same period
approaching fifths by similar motion can produce the same effect as approaching fifths by parallel motion. Adding a passing tone to a hidden fifth produces a parallel fifth, for example. Since the parallel fifth is implied by a missing note, approaching fifths by similar motion are called hidden fifths
approaching octaves *by similar motion& can produce the same effect as approaching octaves by parallel motion. Adding a passing tone to a hidden octave produces a parallel octave, for example. Since the parallel octave is implied by a missing note, approaching octaves by similar motion are called hidden octaves
in another meaning is the Problem of direct 5ths & 8ves essestially is the similar motion so when there is no similar motion it's ok?
Yes, that is correct.
Allen Chou's answer mentions some circumstances under which hidden fifths or octaves are permissible, but note that they all involve similar motion, so are not strictly within the scope of this question. (Also note that the acceptability of various exceptions is different in different times and places, much more variable than the prohibition of parallel fifths and octaves.)
SIMILAR MOTION:
CPE Bach, while discussing thoroughbass in his Essay on the True Art of Playing Keyboard Instruments, writes that although one should try to avoid direct perfect 5ths and octaves, there are some progressions in which they may be used (Figure 223):
One may also need to use hidden 5ths/8ves (even in the outer parts) in a cadence. In Figure 235, we see that the progression is faulty because the B, which wants to lead up a half-step to C, instead leads to G. The corrected example in Figure 236 resolves the B to a C, creating a hidden, but acceptable 8ve between the soprano and the bass.
PARALLEL MOTION:
Edit: Regarding parallel 5ths and 8ves, we can turn to Fux's Gradus Ad Parnassum:
...the progression at B... in direct motion need not be considered a mistake because of the difficulty of this species.
However, the editor points out writings of Beethoven in the footnotes:
"Such liberties are more acceptable in a descending than in an ascending motion." However, in his Introduction, we find Beethoven's comment on this example and this particular instance (the succession occurring between the outer voices): "The second progression, at B, would never be excusable for my ear."
Here we see that the use of parallel 5ths and 8ves in the outer voices can be decided by necessity. Beethoven implies that they should be avoided altogether, while Fux avoids them but allows a filled out form to be used as a sort of last resort if no other progressions can be found.
No. Because then they wouldn't be 'hidden 5ths'.
The issue with 'hidden 5ths', as with 'parallel 5ths', is based on similar motion. Otherwise, think about it, ANY movement that resulted in a 5th would be prohibited!
We spend a lot of time on teaching smooth, step-wise voice writing. (Possibly because it's easy to grade :-). There are probably enough 19th century style hymn tunes already. We SAY that each voice line should have melodic interest, but object when one gets TOO interesting. Interesting melodic lines have leaps! But OK, if you're stuck in 'Harmony 101' and want your exercises to be graded well, better stick to the rules.
