13. A speech synthesis system for synthesizing speech from text, the system comprising: at least one processor configured to: select a first speech segment sequence corresponding to an input text from a speech segment database by using a first cost value calculated based at least in part on a statistical model of prosody variations; determine prosody modification values for the first speech segment sequence, after the first speech segment sequence is selected, by using a second cost value calculated based at least in part on the statistical model of prosody variations, wherein the first cost value is different from the second cost value; and apply the determined prosody modification values to the first speech segment sequence to produce a second speech segment sequence whose prosodic characteristics are different from prosodic characteristics of the first speech segment sequence, wherein the second cost includes at least a prosody modification cost, and the at least one processor is further configured to increase the prosody modification cost of continuous speech segments having a slope likelihood greater than a given value in the first speech segment sequence before determining the prosody modification values in response to detection of the continuous speech segments.
13. A speech synthesis system for synthesizing speech from text, the system comprising: at least one processor configured to: select a first speech segment sequence corresponding to an input text from a speech segment database by using a first cost value calculated based at least in part on a statistical model of prosody variations; determine prosody modification values for the first speech segment sequence, after the first speech segment sequence is selected, by using a second cost value calculated based at least in part on the statistical model of prosody variations, wherein the first cost value is different from the second cost value; and apply the determined prosody modification values to the first speech segment sequence to produce a second speech segment sequence whose prosodic characteristics are different from prosodic characteristics of the first speech segment sequence, wherein the second cost includes at least a prosody modification cost, and the at least one processor is further configured to increase the prosody modification cost of continuous speech segments having a slope likelihood greater than a given value in the first speech segment sequence before determining the prosody modification values in response to detection of the continuous speech segments. 14. The system of claim 13 , wherein the first cost for determining the first speech segment sequence includes a spectrum continuity cost, a duration error cost, a volume error cost, an absolute frequency likelihood cost, a frequency slope likelihood cost, and a frequency linear approximation error cost.