Corey Bailey

Audio Engineering

USEFUL INFORMATION Digital Audio: BIT DEPTH

Bit Depth can apply to many types of files in the digital domain. Here we are talking about audio files. Specifically, those created by Pulse Code Modulation (PCM). There are some terms that need to be understood before we start; (Besides PCM) Sample rate, 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. Hertz, or cycles per second; Sample rate is defined in terms of Hertz. Dynamic Range; The Bit Depth defines the dynamic range of a PCM audio file. Decibel, 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, search the internet, and you will find infinitely more information. The first thing that needs to be understood is that the computer you are using to read this, stores everything in Binary code. 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 audio or a 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 of a digital word used for a single character (on your keyboard, for example). The lower case “a” is represented by the bit string 01100001 (a Byte!). Your computer is designed to execute instructions and store data in groups of Bytes. As mentioned above, PCM digital audio is defined by Bit Depth and Sample Rate. For example; An audio CD has a Bit Depth of 16 bits with a Sample Rate of 44,100 times per second. Or; 16/44.1 Bit Depth divides a given sample by its value. That seems to be simple enough. Based on that definition, each sample of a CD file would be divided by 16, resulting in 16 data points for a 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. 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 the dynamic range of PCM digital audio. 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 the 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 earbuds. 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 (24/96) 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 couple of places to start: https://www.wikipedia.org/ Wikipedia is search engine based so, plug in any of the terms used in this article. https://www.ranecommercial.com/legacy/note137.html This is an article named “Digital Dharma of Audio A/D Converters” An which, is an in-depth read of how digital audio works. It's available in PDF format if you want to add it to your reference material. Return to TOP of page © Corey Bailey Audio Engineering

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