Please explain for me the difference between chromatic root movement and modulation in the chord progression of a song?
Chromatic root movement would typically refer to a progression of chords--all in the same key signature--where the root descends chromatically or ascends chromatically. For example, this can be achieved using a tritone substitution:
| Dmin | Db7 | CMaj |
The root descends chromatically from
C. The entire progression is in the key of
C, and the chromatic movement doesn't imply a change of key/tonal center. As a more dramatic example, we could have something like this:
| CMaj | GMaj/B | BbMaj | FMaj/A | E7alt/G# | CMaj/G | D7/F# | G7 |
Again, this entire progression is centered around the key of
CMaj. While there may be some passing chords that don't fit perfectly in
CMaj, nothing here qualifies as a key change to a different tonal center.
By contrast, modulation implies moving from a first key to a second, different key. For example, the song could modulate from a tonal center of
| CMaj | FMaj | G7 | Ab7 | DbMaj | GbMaj | Ab7 | GbMaj | DbMaj |
In the third and fourth bars, there is ascending chromatic movement in the bass, and this is due to a key change from
So in short: chromatic root movement typically implies chromatic ascension/descension in a single key, whereas modulation implies moving to a new key. Both can involve movement along the chromatic scale.