Digital Sound

PAGE (2 of 5)

Sampling

Sampling involves the rate at which the converted sound is captured. There exists a direct relationship between the sampling rate, sound quality (fidelity), and storage space. The higher the sampling rate the higher the fidelity and the higher the storage requirements. Digital sound sampled at high fidelity rates requires massive storage.

For example, the CD audio format can hold 600 million characters of text, but only 74 minutes of uncompressed music. This requires a tradeoff to be made between the quality of the sound and the storage requirements.

Nyquist Effect

Knowing the type of audio to be sampled and the intended purpose of the audio allows for a reasonable choice of sampling rate. In deciding upon a sampling rate one must be aware of the difference between playback rate and capturing (sampling), rate. These two rates are not the same. In fact the sampling rate must be two times the playback rate. The reason for this discrepancy is due to the Nyquist Effect (or Nyquist Theorem). In a worst-case scenario with only one sample per period, instead of two samples as the graph below depicts, the reproduced sound might be played back as a continuous tone. While in practical applications this maximum loss of fidelity would not likely occur, unacceptable errors due to the Nyquist effect would be exhibited.

Although many sampling rates exist, only the most popular and most common will be discussed here. Human speech can be effectively reproduced at a rate of 5.5 kHz (kilo-Hertz). This requires a sampling rate of only 11kHz. Most natural world sounds and medium fidelity music can be reproduced at 11kHz with acceptable losses of fidelity (approx. FM radio frequencies). To reproduce high fidelity music at CD audio quality the sampling rate must be 44.1 kHz, giving a playback rate of 22 kHz, which is just above the limit of human hearing.