Talk:Digital Audio

Gale 24Feb15: ToDo I still argue the current text on Digital Audio is more repetitive than ideal and that we should consider linking to http://xiph.org/~xiphmont/demo/neil-young.html.

Gale 17Jan15:

Possible further revised text about sample rates.

Sample rates are measured in hertz (Hz), or cycles per second. This value is the number of samples captured per second in order to represent the waveform. Higher sample rates allow higher audio frequencies to be represented. Provided that the sample rate is more than double the highest audio frequency present, the waveform can be reconstructed exactly from the digital samples. Frequencies that are more than half the sample rate cannot be correctly represented in digital samples, so must be filtered out from the analog signal before converting to digital so as to avoid aliasing distortion. This upper frequency limit of "half the sample rate" is called the Nyquist frequency.

The human ear can hear sounds with frequencies between approximately 20 Hz and 20000 Hz (although the upper range of hearing declines markedly with age). Therefore a sample rate of 40000 Hz is the absolute minimum necessary to reproduce sounds within the range of human hearing. This "minimum necessary sample rate" is called the Nyquist rate. However higher rates than the minimum necessary (called oversampling) are often used. Oversampling provides more room between the upper required frequency (20000 Hz in this case) and the Nyquist frequency (half of whatever sample rate is used) so as to avoid potential aliasing artifacts around the Nyquist frequency.

For these reasons the sample rate used by audio CDs and the standard rate on most soundcards is 44100 Hz rather than 40000 Hz. While 44100 Hz gives excellent music reproduction, human speech is still intelligible if frequencies above 4000 Hz are eliminated - in fact telephones only transmit frequencies between 200 Hz and 4000 Hz. Therefore another common sample rate is 8000 Hz, sometimes called speech quality. Note that the digital converter requires a very steep anti-aliasing filter at the Nyquist frequency of 4000 Hz in order to prevent frequencies above this cutoff point being "folded back" into the audible range as aliasing noise.