Unlike analog signals, which have an infinite range of volume levels, digital audio
samples use binary numbers (bits) to represent the strength of each audio sample. The
accuracy of each sample is determined by its bit depth. Higher bit depths mean your
audio signal is more accurately represented when it is sampled. Most digital audio
systems use a minimum of 16 bits per sample, which can represent 65,536 possible
levels (24-bit samples can represent over 16 million possible levels).
To better understand bit depth, think of each digital audio sample as a ladder with
equally spaced rungs that climb from silence to full volume. Each rung on the ladder is
a possible volume that a sample can represent, while the spaces between rungs are
in-between volumes that a sample cannot represent.
Audio sample rates
8 kHz–22.225 kHz
These lower sample rates are used strictly for multimedia files.
32 kHz is generally used with 12-bit audio on DV.
This sample rate is used for music CDs and some DAT recorders.
Almost all digital video formats use this sample rate.
A multiple of 44.1 kHz. This is useful for high-resolution audio that
needs to be compatible with 44.1 kHz. For example, if you eventually
plan to burn an audio CD, this sample rate is a good choice.
A multiple of 48 kHz. This is becoming the professional standard for
audio post-production and music recording.
A multiple of 48 and 96 kHz, this is a very high-resolution sample
rate used mostly for professional music recording and mastering.
When a sample is made, the audio level of the analog signal often falls in the spaces
between rungs. In this case, the sample must be rounded to the nearest rung. The bit
depth of a digital audio sample determines how closely the rungs are spaced. The more
rungs available (or, the less space between rungs), the more precisely the original
signal can be represented.
Quantization errors occur when a digital audio sample does not exactly match the
analog signal strength it is supposed to represent (in other words, the digital audio
sample is slightly higher or lower than the analog signal). Quantization errors are also
called rounding errors because imprecise numbers represent the original analog audio.
For example, suppose an audio signal is exactly 1.15 volts, but the analog-to-digital
converter rounds this to 1 volt because this is the closest bit value available. This
rounding error causes noise in your digital audio signal. While quantization noise may
be imperceptible, it can potentially be exacerbated by further digital processing.
Always try to use the highest bit depth possible to avoid quantization errors.
The diagram on the far right shows the highest bit depth, and therefore the audio
samples more accurately reflect the shape of the original analog audio signal.
For example, a 1-bit system (a ladder with only two rungs) can represent either silence
or full volume, and nothing in between. Any audio sample that falls between these
rungs must be rounded to full volume or silence. Such a system would have absolutely
no subtlety, rounding smooth analog signals to a square-shaped waveform.
When the number of bits per sample is increased, each sample can more accurately
represent the audio signal.
To avoid rounding errors, you should always use the highest bit depth your equipment
supports. Most digital video devices use 16- or 20-bit audio, so you may be limited to
one of these bit depths. However, professional audio recording devices usually support
24-bit audio, which has become the industry standard.
32-bit floating point
Internal resolution of the Final Cut Pro Audio Mixer. This allows
audio calculations, such as fader levels and effects processing, to be
performed at very high resolution with a minimum of error, which
preserves the quality of your digital audio.
This has become the audio industry standard for most audio
recording formats. Most professional audio interfaces and
computer audio editing systems can record with 24-bit precision.
Used in some video formats such as Digital Betacam and audio
formats such as ADAT Type II.
DAT recorders, Tascam DA-88 and ADAT Type I multitracks, and
audio CDs all use16-bit samples. Many digital video formats, such
as DV, use 16-bit audio.
In the past, 8-bit audio was often used for CD-ROM and web video.
Today, 16-bit audio is usually preferred, but available bandwidth
and compatibility with your target user’s system are your chief
considerations when outputting audio for multimedia use.
Many consumer DV camcorders allow you to record four audio channels using 12-bit mode, but this is
not recommended for professional work.