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基于LINUX操作系统的PMP评论

发布时间:2005-11-16 上午11:51

作者: 又摘桃枝

等级: 黄金少侠

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目前在PMP上采用LINUX操作系统的还有多见,不过国外已经出现了。下面是相关评论:

How codecs work

The WAV file format is a Microsoft and IBM standard for digital storage of audio. It uses the RIFF file format structure, and as such is closely related to the AIFF and IFF formats. Multimedia applications use a wide variety of data, including bitmaps, audio data, video data, and peripheral device control information. RIFF provides a way to store these varied types of data. The type of data a RIFF file contains is indicated by the file extension. Examples of data that may be stored in RIFF files are:

# Audio/visual interleaved data (.AVI)
# Waveform data (.WAV)
# Bitmapped data (.RDI)
# MIDI information (.RMI)
# Color palette (.PAL)
# Multimedia movie (.RMN)
# Animated cursor (.ANI)
# A bundle of other RIFF files (.BND)

RIFF is a binary file format containing nested data structures. Each data structure is called a chunk. Chunks do not have fixed positions within a RIFF file. Therefore offset values cannot be used to locate fields. A chunk contains data such as a data structure, a data stream, or another chunk called a subchunk. Every RIFF chunk has the following structure:

typedef struct _Chunk
{
DWORD ChunkId; /* Chunk ID marker */
DWORD ChunkSize; /* Size of the chunk data in bytes */
BYTE ChunkData[ChunkSize]; /* The chunk data */
} CHUNK;

ChunkId contains four ASCII characters that identify the data the chunk contains. The characters RIFF are used to identify chunks containing RIFF data. IDs smaller than four characters are padded on the right using spaces. Since the file structure was originally defined by Microsoft and IBM for the PC, RIFF files are written in little-endian byte order. Files written using the big-endian byte ordering scheme use the identifier ‘RIFX’.

ChunkSize is the length of the data stored in the ChunkData field excluding any padding added to the data. The size of the ChunkId and ChunkSize fields are not included in this value.

ChunkData contains data that is WORD-aligned within the RIFF file. If the data is an odd length in size, an extra byte of NULL padding is appended.

Subchunks have the same structure as chunks. A subchunk is simply any chunk that is contained within another chunk. The only chunks that may contain subchunks are the RIFF file chunk ''RIFF'' and the list chunk ''LIST''. All other chunks may contain only data. A RIFF file is one RIFF chunk. All other chunks and subchunks in the file are contained within this chunk.

While WAV files can store numerous formats of data, the common vernacular usage of ''WAV'' typically refers to audio encoded with pulse-code modulation (PCM). Because the WAV format arose out of the Windows/Intel environment it is stored in Little-Endian byte ordering.


Figure 1 Canonical form of WAV files. WAV files include a FMT and DATA sub-chunk. DATA includes the length of the audio data and the data itself.

Pulse-code Modulation

Claude E. Shannon of Bell Labs published "The Mathematical Theory of Communication" in 1948 laying out the basis for modern telecommunications. That same year, PCM was invented by Bell Labs engineers. Although revolutionary at the time, today Pulse-code modulation is seen as a very simplistic lossless encoding format in which the audio is sampled at regular intervals and quantized into frequency band values. Other common applications that utilize this encoding method include telephony and compact discs. There are three main variants of PCM: standard PCM, Differential pulse-code modulation (DPCM), and adaptive DPCM. In standard PCM the frequency bands are typically quantized into linear step frequency bands, storing the absolute magn
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