With a technology for adaptive speech speed conversion, for a given playback speed, such as 1× speed (playback in real time) or 2× speed (playback in half of real time), the speed is not changed by a uniform factor α over the entire input signal, but rather the speed is changed in each section by a factor larger or smaller than the factor α so as to balance the overall playback time to be the same as when speech speed is converted at the uniform factor α. Thereby it is aimed to generate speech speed converted voice that is “slower and easier to hear” for the listener than when speech speed is converted at the uniform factor α.
Some techniques for achieving the above include (1) lowering the speech speed where the fundamental frequency is high and raising the speech speed where the fundamental frequency is low, (2) treating an interval spoken in one breath as a unit, lowering the speech speed at the start of the interval, and gradually raising the speech speed towards the end of the interval in accordance with changes in the fundamental frequency, and (3) shortening a silent interval between intervals spoken in one breath to a degree that preserves a natural sound (for example, see Patent Literature 1).
Another technique treats a silent interval of at least a given length as a pause, and in a voice interval located between pauses, lowers the speech speed at the start of the voice interval, progressively raises the speech speed during a given time T based on a predetermined decreasing function, and after the given time T elapses, changes the factor for lowering the speech speed by taking into consideration the relative magnitude of the maximum fundamental frequency in each voice interval (for example, see Patent Literature 2).
Within the speech speed control disclosed in Patent Literature 1 or Patent Literature 2, another known technique allows for a brief silent interval within a voice interval located between pauses to be shortened to a degree that still preserves a natural sound. This technique also lowers the subsequent speech speed in so far as possible when the speech speed of each section matches, or is only slightly later than, the time assumed when the speech speed being converted at a uniform factor α, and reduces the amount by which the subsequent speech speed is lowered as the speech speed of each section is increasingly later than the time assumed when the speech speed is converted at the uniform factor α. This technique thereby lessens misalignment, in so far as possible, with the time assumed when the speech speed of each section of the speech speed converted voice is converted at the uniform factor α (for example, see Patent Literature 3).
Furthermore, when separating the input signal into voice intervals and silent intervals, lowering the speech speed in the voice intervals, and shortening the silent intervals, the output voice length extends beyond the input signal length per unit time due to the lowering of the speech speed in the voice intervals. It thus becomes necessary to store the voice after speech speed conversion temporarily in memory, yet there is a limit on memory capacity. Therefore, a technique is known for gradually raising the speech speed in the voice intervals and increasing the amount cut from the silent intervals in accordance with the remaining memory capacity (for example, see Patent Literature 4 and 5).
Additionally, a technique is known for determining the speech speed of each section using a coefficient such that the speech speed is inversely proportional to the increase or decrease of the magnitude (power) or pitch (fundamental frequency) of the input signal, or a coefficient such that the speech speed is inversely proportional to the nth power of the value of the magnitude or volume of the input signal (for example, see Patent Literature 6).