Modulation : Vibrato and tremolo

Vibrato and tremolo
Vibrato는 pitch 즉 높이가 떨리듯 변화하는 것이고
tremolo는 볼륨이 떨리듯 변화하는 것임
실제 노래할때 둘중 하나만 독립적으로 이루어지기는 힘듬

설명
The terms vibrato and tremolo are sometimes used interchangeably,
although the strict definitions of each describe them as separate effects:
vibrato is a periodic variation in the pitch (frequency) of a musical note,
whereas tremolo usually refers to periodic variations in the volume (amplitude)
of a musical note.

In practice, it is difficult for a singer or musical instrument player
to achieve a pure vibrato or tremolo (where only the pitch or only
the volume is varied), and variations in both pitch and volume will
often be achieved at the same time.

출처 : http://en.wikipedia.org/wiki/Vibrato