1. A mobile terminal, comprising: a receiver configured to receive a content signal comprising video data and audio data; a display configured to display information; and a controller configured to control the display unit to: display video based on the received video data; display subtitles simultaneously with the displayed video, wherein content of the subtitles is based on the received audio data and at least a portion of the subtitles is displayed in a first size; gradually increase a size of the at least the portion of the displayed subtitles to a second size in proportion to an increase in a difference between a sound characteristic of the received audio data corresponding to the at least the portion of the displayed subtitles and a reference value while the subtitles are displayed when the corresponding sound characteristic is greater than the reference value; gradually decrease the size of the at least a portion of the displayed subtitles to a third size in proportion to an increase in the difference between the corresponding sound characteristic and the reference value while the subtitles are displayed when the corresponding sound characteristic is less than the reference value; and provide the at least a portion of the displayed subtitles with a movement effect while the size of the at least a portion of the displayed subtitles is varied when the amplitude of the received audio data increases at greater than a preset rate.