AV News 190 - Novembert 2012
Sample formats
The other measure of audio quality is the sample format (or as it is more
usually known, bit depth), which is usually measured by the number of
computer bits used to represent each sample. The more bits that are used, then
the more precise the representation of each sample. Increasing the number of
bits also increases the maximum dynamic range of the audio recording - the
difference in volume between the loudest and softest possible sounds that can
be represented.
In the image below, the left half has a sample format with few bits, and the
right half has a sample format with more bits. If you think of the sample rate as
the spacing between vertical gridlines, the sample format is the spacing
between horizontal gridlines.
Dynamic range is measured in decibels (dB). The human ear can perceive sounds
with a dynamic range of at least 90 dB. However, whenever possible it is a good idea
to record digital audio with a dynamic range of far more than 90 dB, in part so that
sounds that are too soft can be amplified for maximum fidelity. Note however that
although signals recorded at low levels can be raised (that is, be 'normalized') to take
advantage of the available dynamic range, the recording of low level signals will not
use all of the available bit depth. This loss of resolution cannot be re-captured simply
by normalizing the overall level of the digital waveform.
Common sample formats, and their respective dynamic range include:
8-bit integer : 48 dB, 16-bit integer : 96 dB, 24-bit integer : 145 dB and the maximum
32-bit floating point : near-infinite dB.
What does all this mean when we are recording or mixing multitracks?
Audio CDs and most computer audio file formats use 16-bit integers. Audacity
uses 32-bit floating-point samples internally and, if required, then converts the
sample bit depth when the final mix is exported.
Audacity's default sample format during recording can be configured in the
Quality Preferences or set individually for each track in the Track Drop-Down
Menu. During playback, the audio in any tracks that have a different sample
format from the project will be resampled on the fly using the Real-time
Conversion settings in the Quality Preferences. The High-quality Conversion