AV News 190 - November 2012
Digital Audio Quality
The quality of a digital audio recording depends heavily on two factors: the
sample rate and the sample format or bit depth. Increasing the sample rate
or the number of bits in each sample increases the quality of the recording....
but also increases the amount of space used by audio files on a computer or
disk.
Sample rates
Sample rates are measured in hertz (Hz), or cycles per second. This value
simply represents the number of samples captured per second in order to
represent the waveform and to then subsequently reconstruct the waveform
when it is converted back to analogue. The more samples per second, the
higher the resolution and thus the more precise the measurement is of a point
on the waveform at a given time.
The normal human ear is sensitive to sound patterns with frequencies
between approximately 20 Hz and 20,000 Hz. Sounds outside that range are
essentially inaudible, although Rupert Neve has subjectively proven the
existence of pysychoacoustic fidelity that can be heard above this supposed
limit of 20,000kHz.
Capturing a sound at a particular frequency requires a sampling rate of at
least twice that frequency (known as the 'Nyquist frequency'). Therefore a
sample rate of 40,000 Hz is the absolute minimum necessary to reproduce
sounds within the range of human hearing, though higher rates (called over
sampling) may increase quality even further by avoiding any aliasing artifacts
around the Nyquist frequency. The sample rate used by audio CDs is 44100
Hz. Human speech is intelligible even if the frequencies above 4,000 Hz are
missing; in fact telephones only transmit frequencies between 200 Hz and
4,000 Hz. (hence that is why when 'your call is really important to them'....and
they play you some 'music' - it sounds awful!) A common sample rate for
audio recordings is 8,000 Hz, which is sometimes called speech quality.
The most common sample rates are 8 kHz, 16 kHz, 22.05 kHz, 22.25 kHz,
44.1 kHz, 48 kHz, 96 kHz, and 192 kHz with the most common sample rate
being 44.1 kHz (44100 Hz).
In the image below, the left half has a low sample rate, and the right half
has a high sample rate (ie. higher resolution). With the low sample rate the
computer software is actually having to guess what the waveform is like
between the various sample points as it does not have enough information.
Waveform with low sample rate and high sample rate
Page 43