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Recording - What Bit Depth & Sampling Rates to Use |
Modern audio interfaces and softwares can work at a variety
of bit depths and sampling rates and it can be a bit confusing when choosing what to
use when recording. There is a lot of information around on this subject and much of it is
hypertechnical, while the rest is based on hearing perception. I'm going to attempt to cut
through a lot of the formulae and arm you with enough knowledge to you to improve
your recordings. Then you can get back to recording - which is the point of the whole
thing after all.
Digital Audio
The main device used in digital recording is an ADC (Analog-to-Digital Converter).
The ADC captures a snapshot of the electric voltage on an audio line and represents it as
a digital number that can be stored by a computer. By capturing the voltage thousands of
times per second, you can get a very good representation of the original audio signal.
Each dot in the figure below represents one audio sample.
Bit Depth
Essentially this is the number of digits in the digital representation of
each sample. Higher bit depths allow for more dynamic range - enabling louder peaks,
softer quiet sounds and a lower noise floor (less noise). WIth a higher bit depth the
amplitude decay of the waveform is also more natural.
Sample Rate
The sampling rate controls how many samples are taken per second and is
measured in samples per second or Hertz (Hz). The higher the sampling rate, the higher
frequencies of sound that can be captured. The sampling rate should be at least twice the
highest frequency you want to represent. Humans can't hear frequencies above about 20,000
Hz, so 44,100 Hz was the rate chosen for audio CDs. However, EQ and dynamics processing at
higher sample rates is far more accurate and analogue-like at higher sample rates.
Metering is much more accurate at higher sample rates too.
Think of the sample rate as the horizontal precision of the
digital waveform, and the sample format as the vertical precision.
| The Catch |
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From this it would seem that the best thing for
audio recording is to use the highest sampling rates and bit depths available to you,
there are some important drawbacks to keep in mind:
- Projects stored at 24-bit depth will require 50% more disk
space than those stored at 16-bits.
- Recording, playing, and processing 24-bit data utilizes more
CPU resources than working with 16-bit data. This means that you will be able to
record/play/process less simultaneous tracks in a 24-bit project than you would be able to
in a 16-bit project. This is also true when working at higher sampling rates.
- Software processes must be used to convert the digital audio
data down to it's final format after recording, and the quality can suffer during
conversion process.
Is It Worth It?
It’s also worth noting that not all audio projects benefit equally from the
use of higher bit-depth/sampling rates. 24-bit, higher-sampling-rate audio is most
beneficial when working with music that focuses on natural acoustic instruments or vocals
recorded with very high-quality microphones. The benefits are less audible when working
with highly synthetic or highly compressed program material such as pop or rock. So
depending on the size of your project and your computer capabilities, you may want to
reconsider whether working at the higher bit-depth and/or sampling rate is justified... or
even possible. |
System
Requirements |
Format |
per
mono track |
| 16-bit / 44.1 KHz |
5.2 MB per minute |
| 24-bit / 44.1 KHz |
7.5 MB per minute |
| 24-bit / 96 KHz |
17 MB per minute |
| 24-bit / 192 KHz |
34MB per minute |
Example: A
24 track 24-bit / 196 KHz recording must move information at 816 MB per minute. A single 4
minute recording will result in a 3.3 GB file for your system to deal with! |
|
In Use
If used carefully, it is possible to make great sounding recordings at 16bit /
44.1KHz. For over 15 years, this has been the format used for CDs, and there are many
really good sounding discs around to prove the point.
However, if you want higher quality, the biggest gains for
the least system stress will be gained using a bit depth of 24-bit or better, which gives
a lot more headroom when recording, the noise floor is much lower and stresses the digital
system less than higher sample rates.
If you want to record with higher sampling rates too, then
use a multiple of your destination format. This avoids many of the errors that can happen
when downsampling from a higher rate. So, if your final format is CD, then sample at
24-bit / 88.2KHz. Do all your post-production at that rate, then sample rate convert to
44.1, and finally dither down to 16 bits for the CD.
If your target is film, TV, DVD-V, DVD-A or SACD, then work
at 96kHz as final material will need to be sample rate converted to 48kHz. |
