1. Field of the Invention
The present invention relates to a data reproduction apparatus and data reproduction method, and is preferably applied for reproducing high frequency components, which were lost after original music data, a source of Compact Disc Digital Audio (CDDA) data, was compressed in a digital compression format such as MPEG-1 Audio Layer 3 (MP3) format, for example.
2. Description of the Related Art
There is an audio signal reproduction apparatus that performs a so-called oversampling process, in which a sampling frequency for Pulse Code Modulation (PCM) digital audio signals read from a storage medium is multiplied by n (n: an integer greater than or equal to 2) to interpolate new sampling points, to produce higher harmonic components that the original signal components do not have. The audio signal reproduction apparatus then superimposes the higher harmonic components, whose bandwidth is greater than or equal to the audible bandwidth, on the original signal components in order to reproduce more natural sound (see Patent Document 1: Jpn. Pat. No. 3140273, for example).
In addition, there is an acoustic reproduction apparatus that clips a waveform of the original signal components by using a nonlinear circuit including a silicon diode to produce higher harmonic components, whose bandwidth is greater than or equal to the audible bandwidth. The acoustic reproduction apparatus then adds the higher harmonic components to the original signal components to reproduce sound, whose quality is close to natural sound spectrum (see Patent Document 2: Jpn. Pat. Laid-open Publication No. H8-2119, for example).
By the way, MP3 players have become popular for listening to music. MP3 players reproduce music data, which have been compressed in a digital compression format, such as MP3: the format is used to compress original music data like CDDA.
The higher the compression rate, the more music data a storage medium, such as a hard disk or a flash memory, can store. That is why a user wants a higher compression rate. However, the higher the compression rate, the more original signal components may be lost.
As shown in FIG. 1, as the compression rate increases (i.e. the bit rate decreases), the upper limit of a reproduction frequency range becomes lower. Accordingly, the original signals'high frequency components are gone. This is explained in FIGS. 2A and 2B: It is evident from FIG. 2A, which illustrates a reproduction spectrum for CDDA music data, that output signal components are covering the entire bandwidth up to 22.05 kHz, which is a half of a sampling frequency Fs (44.1 kHz); and it is also evident from FIG. 2B, which illustrates a reproduction spectrum for CDDA music data compressed in a bit rate of 64 Kbps, that output signal components are not covering high frequency components greater than about 8 kHz.
In that manner, by increasing the compression rate, music data is reduced in size, allowing a storage medium to store more music data. On the other hand, the higher the compression rate, the more high frequency components (which are greater than or equal to a predetermined frequency bandwidth) are lost, decreasing sound quality compared to the original uncompressed music data.