I read a dissertation in which the author describes the octatonic scale as 'equal interval chromatic'. I thought that the octatonic was a mixture of semitones and tones?
Moreover the dissertation is in relation to Rachmanioff's association of it with 'intensification, destabilisation and climax'.
If the dissertation in question is the one linked to in guest's answer, "equal-interval" is the general term here, referring to structures that divide the octave equally. This tends to be especially applied to division of the octave into major thirds or minor thirds. (Equal division into seconds also produces both the standard chromatic scale, as well as the whole-tone scale, which is discussed periodically in the dissertation too.)
However, such divisions don't tend to be very useful by themselves -- they just produce static diminished or augmented chords. (Or a whole-tone scale by itself, which generally sounds pretty static harmonically.) Instead, a lot of times these notes (and others) are used to produce progressions around such division points within the octave.
One possible extension of equal division is to superimpose two cycles of equal division next to each other (as Michael Curtis describes). If one does this for augmented triads, one produces a hexatonic scale. If one does this with diminished seventh chords, one produces an octatonic scale. Hexatonic and octatonic scales can be used to produce progressions that move between the major/minor thirds that the "equal division" structures are originally based around. Hexatonic and octatonic scales also allow one to build simple major or minor triads within them, which isn't possible with a simple equal division of the octave by itself (again, just an augmented or diminished seventh chord).
Thus, hexatonic and octatonic scales can be used with standard major and minor triads (as well as more dissonant chords) to produce progressions that move chromatically across equal intervals (i.e., major or minor thirds). Hence equal interval chromatic (which is more about the type of progression possible than a description of the pure scale). This is opposed to normal functional chromaticism, which tends to be organized around fifth and fourth relationships with dominants (and secondary dominants) progressing to tonics and generally isn't organized around progressions of equal intervals.
EDIT: Also, if the dissertation linked is the one OP is discussing, I'm not sure where the term "equal-interval chromatic" is used to describe the octatonic scale. Rather, octatonic pitch collections are used in the dissertation at times as a basis for finding "equal-interval chromatic structures" (again, mostly based around progressions that involve symmetry by minor third intervals). If there's a place where OP sees a specific designation of "equal-interval chromatic" referring to the scale itself, it might be easier to interpret the question.
I guess you are referring to this PhD thesis: https://deepblue.lib.umich.edu/handle/2027.42/63739. Certainly the phrase "equal interval chromatic" occurs twice in the abstract, and once adjacent to the word "octatonic".
Anyone who can make sense of the word salad in the summary is welcome to post an explanation - e.g.
I show that different harmonic components have different rhetorical associations in the works studied; that different components are generally associated with different locations in form; and that acknowledging the interaction of different kinds of harmonic structures in a work contributes significantly to an understanding of expressive trajectory and large-scale organization, and—especially—to exegesis of climax events.
Maybe reading the complete 305-page document will produce enlightenment. Or maybe not.
Obviously the scale is not all equal steps. It's a combination of whole and half steps.
You need to look at some other interval relationship between tones beside step by step intervals.
You can generate an octatonic scale by superimposing two diminished seventh chords each of which is composed of equally spaced minor thirds.
Ex. take a B diminished seventh chord and superimpose a C# diminished seventh chord and you get an octatonic scale.
Here's one writer's description of the octonic scale: 'The octatonic scale alternates between whole and half steps, and is therefore a combination of the principles behind the chromatic scale and the whole-tone scale.'
http://www.musiccrashcourses.com/lessons/scales_more.html
Maybe that's what your guy was getting at? Give us some more context, it might make more sense.
