1. Field of the Invention
The present invention relates to a decoder for decoding compressed audio data obtained through encoding audio data such as music data.
2. Description of the Background Art
Recently, memory cards are increasing in capacity, while hard disks of larger capacities are being reduced in size. There is an increasing demand for a portable audio player enclosing therein a recording medium, such as semiconductor storage cards or magnetic recording disks, on which there may be recorded encoded audio data, such as music data. An encoder system as MP3 (MPEG 2 (Moving Picture Experts Group) Audio Layer 3) or MPEG2/AAC (Advanced Audio Coding) has been standardized as a technique for encoding audio data.
With the AAC encoding system, audio data are divided into temporal units and the audio data of each temporal unit are encoded for compression in accordance with the PCM (Pulse Code Modulation) system. The temporal unit corresponds to a unit time of the PCM data decoded from compressed, encoded audio data. The unit time of PCM data is recorded in the form of frame. In decoding, each frame of compressed audio data is decoded as a unit, and the PCM data thus obtained through decoding on the frame basis is reproduced continuously.
The compressed audio data obtained through encoding are decoded by a conventional decoder, in which the compressed audio data to be decoded may be a bitstream of the AAC-LC (Low Complexity) profile, referred to below simply as AAC-ADTS (Audio Data Transport Stream) format, in which a header including information, such as a synchronous code or a frame length, is annexed to each frame composed of compressed audio data, in turn composed of plural elements. If an error has occurred during the decoding operation, data for interpolation is generated by an output data interpolation circuit and applied for regulating the bitstream decoding operation. This is described, for example mainly in paragraphs 0068 to 0083 and FIGS. 2 and 4 of Japanese patent laid-open publication No. 2002-335230.
There is also known a technique which includes the power information as side information of the MP3 system bitstream having a header on a frame basis, and adjusts the sound volume from one music piece to another on the basis of the power information in order to dispense with adjustment of the sound volume at the time of continuous playback. It is stated that this technique may be applied to a bitstream of the ADIF (Audio Data Interchange Format) format of MPEG/AAC in which there is only one header at the beginning of a data file. This is described, for example mainly on page 6, paragraphs 0037 to 0045 and FIGS. 1 and 3 of U.S. patent application publication No. US 2005/0147004 A1 to Kumagai, et al.
In the techniques of the aforementioned patent publications, it is a bitstream of a data file of the AAC-ADTS format or the MP3 format that is decoded. In the bitstream of a data file of the AAC-ADTS format or the MP3 format, a header is appended to each frame. Hence, if an operator, such as a user of a portable audio player, instructs a fast feed operation by way of a seek operation in which part of a piece of music under playback corresponding to a predetermined period of time, such as five seconds, is skipped from the position being reproduced to reproduce the piece of music recorded in the forward position, then a jump is made to a forward frame ahead by the predetermined period of time, and a header of the next frame following the forward frame is detected to re-initiate the decoding to reproduce the music of the next frame. If the operator instructs a rewind operation by way of a seek operation in which reversion is made from the position being reproduced by the predetermined period of time to reproduce again the already reproduced part of the music piece, then a jump is made to a backward frame by one frame in rear corresponding to the predetermined period of time before, and a header of the next frame following the frame reached by the backward jump is detected to re-initiate the decoding to reproduce the music from before the predetermined period of time.
However, if a bitstream of a data file of data encoded in accordance with a file format such as ADIF format of the MPEG2/AAC-LC profile, referred to be low as AAC-ADIF format, in which the bitstream in the file format including plural frames connected to a sole header is decoded as stated in the publication No. 2005/0147004, then there is only one header in each data file. Hence, it is not possible to detect a header appended to each frame, so that the aforementioned seek operation cannot be made.