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5-1-2 Sound Processing

Sound processing is a process that analog sound is handled as digital data. The techniques of A/D conversion by which an analog signal is encoded into a digital signal include PCM (Pulse Code Modulation). The procedure of encoding (i.e., A/D conversion) by PCM is as shown below.

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As clear from the above example, the amount of digital data after encoding is determined on the basis of the sampling frequency (i.e., sampling interval) and quantization bit rate. Moreover, D/A conversion that a digital signal (i.e., code) is restored as an analog signal (i.e., sound data) can be implemented by performing the encoding procedure in the reverse manner. At this time, in order to be able to perform restoration at a quality level that is closest to the original analog signal, either the sampling frequency is increased (sampling interval is shortened) and the number of sampling values is increased, or the quantization bit rate is increased to perform detailed staging of sampling values. · 1 question

The typical formats (i.e., standards) of a sound file are as follows:

• • MIDI (Musical Instruments Digital Interface)
  It is a file format that an electronic musical instrument and PC are connected and music data is exchanged. Not sound as such, but the rendition information for playing back the sound is handled.
• • MP3 (MPEG1 Audio Layer3)
  It is a high-quality sound compression and decompression format using the sound

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technology of MPEG (moving images compression and decompression format) standardized by ISO. It is also used in Internet music distribution and portable players.

• • WAV (RIFF Waveform Audio Format)
  It is a sound data format that is mainly used in Windows systems. Various formats of data, such as ADPCM and WMA, can be stored in the container format.
  • • ADPCM (Adaptive Differential Pulse Code Modulation):
    It is a sound signal compression method in which the PCM technique is applied.
  • • WMA (Windows Media Audio):
    It is a streaming technique of music distribution that was originally developed by Microsoft. Recently, it has been used as a sound compression technique.