Bit Depth can apply to many types of files in the digital domain. Here we are
. Specifically, those created by
Pulse Code Modulation
There are some terms that need to be understood before we start
, or Sample Frequency, (the two terms are used interchangeably):
Sample rate and Bit Depth are the two terms most often used to define the
parameters of PCM audio.
, or: cycles per second. Sample rate is defined in terms of Hertz.
: The Bit Depth defines the dynamic range of a PCM audio file.
, or dB: Dynamic range is defined in Decibels.
The above links are to the Glossaries section of this website. The definitions are
simple and brief but adequate for this discussion. If you want to know more about
the terms used in this article, simply plug any of them into your favorite search
engine and you will discover infinitely more regarding your query.
The first thing that needs to be understood is that the computer you are using to
read this, stores everything in
. Binary code is a mathematical
language that is Base 2. For example: Our standard numbering system (0
through 9) is a Base 10 system. In the mathematical language of Base 2,
everything is defined by a 1 or a 0. Thus, everything from a keystroke to a mouse
click to an audio or picture file is represented in the digital domain by individual
strings of 1's and 0's referred to as digital words. A digital word is comprised of
Bits and Bytes. A “Bit” represents a single state: Either a 1 or a 0. A “Byte”
consists of 8 Bits and is the standard size digital word used for a single character
on your keyboard.
The lower case
for example, is represented by the bit string 01100001 (a
Byte!). Your computer is designed to store data and execute instructions in
groups of Bytes.
As mentioned above, PCM digital audio is defined by Bit Depth and Sample
Rate. For example: the CD you were last listening to, the Bit Depth was 16 bits
with a Sample Rate of 44,100 times per second. Or: 16Bit/44.1kHz.
The Bit Depth divides a given sample by its value. That seems simple enough.
Based on that definition, each sample of a CD file would be divided by 16,
resulting in 16 data points for that particular sample. But, there's more: Each bit
of Bit Depth in a binary system increases the number of data points by the power
of 2 (Base 2). Thus, a 16 Bit file contains 16 X 2 to the power of 16 or 65,536
data points for each sample.
Bit Depth also defines the parameters for
of an audio file. For
every 1 bit increase in Bit Depth, there is a 6dB increase in dynamic range.
Therefore, an audio file with a bit depth of 16 bits, has a dynamic range of 96dB
(16X6). The chart below shows a comparison of Bit Depths with their associated
data points per sample and dynamic range.
If we multiply the number of data points by the sample rate (65,536 X 44,100) for
the CD audio format, we come to the realization that your CD player is sending
almost 3 billion data bits per second to the Digital-to-Analog Converter (DAC) to
be turned into something that sounds like audio in your ear buds.
I've been using the CD file format as a reference in this document because the
Compact Disc, having been around for more than 45 years, is familiar to
everyone. However, by today's standards, the Compact Disc is obsolete and the
sample rate of 44.1kHz is unique to that format. As such, the CD digital file
format is not considered an archival format. The U.S. Library of Congress, the
European Union and, several other countries have adopted a Bit Depth of 24 Bits
at a Sample Rate of 96kHz (24Bit/96K) as the archival standard. Many of today's
recordings and the re-visitation of classic recordings are recorded at 32Bit/192K
(or more) in order to capture the maximum amount of detail.
As mentioned previously, there are volumes of information on this subject
available on the internet. Simply plug any of the terms used in this article into
your favorite search engine and you will find hours worth of reading. Here are a
few places to start:
Plug any of the terms used in this article into the search window and read on!
Easy to understand with some good illustrations.
This is an article named “Digital Dharma of Audio A/D Converters”. An in-depth
read of how digital audio works and, it's available in pdf format if you want to add
it to your reference material.
© Corey Bailey Audio Engineering
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