Corey Bailey

Audio Engineering

GLOSSARIES DIGITAL AUDIO GLOSSARY

Originally, this glossary was limited to digital audio terminology. However, since digital audio involves the use of a computer, some computer terminology has snuck in. Aliasing Aliasing refers to an effect that causes different audio signals to become indistinguishable when sampled (aliases of one another). It also refers to the distortion or artifacts that result when the signal that is reconstructed from samples, is different from the original analog signal. In the world of digital audio, the highest frequency information often suffers from aliasing in the form of poorly or incorrectly reconstructed waveforms. Piano, cymbals and some stringed instruments can be noticeably affected. Analog-to-digital converter Abbreviated A/D, ADC or A to D. An A/D converter is an electronic device used to convert analog electrical signals to digital values whose numbers (combinations of ones and zeros) represent the level (volume) and frequency information contained in the original analog signal. Bandwidth Defined simply as a given range of frequencies (audible or not). We humans, for example, have an audible hearing bandwidth ranging from approximately 20 Hz (cycles per second) to about 20 kHz. (20 kilohertz or 20,000 cycles per second) BIOS An acronym that means Basic Input Output System. In the computer world, the BIOS is the first thing that is read by the CPU when power is applied and subsequently determines the configuration of the computer. The BIOS is a chip that contains ROM and is made using LSI technology. The BIOS chip is generally specific to the motherboard. Bit Depth Within the realm of sampling an analog waveform (using Pulse Code Modulation or PCM), bit depth divides a given sample by its value (16 bit, 24 bit, etc.). Bit depth is binary code that uses a mathematical language that is Base 2. It defines the parameters of the Dynamic Range of an audio signal. By increasing the bit depth, smaller fluctuations of the audio signal can be resolved. The noticeable result for the listener is usually increased clarity and tonality. Listening tests have shown that an increase in bit depth is more readily detected than an increase in Sample Frequency. The rule-of-thumb for bit depth resolution is; For every 1-Bit increase in Bit Depth, the dynamic range will increase by 6dB. A bit depth of 16 Bits yields a dynamic range of 96 dB (16X6). A bit depth of 24 Bits yields a (theoretical) dynamic range of 144 dB. Notice that an 8 Bit increase in Bit Depth yields a difference of 48 dB in dynamic range. I say theoretical because the best analog equipment we can use to playback that decoded 24 Bit audio has a realistic dynamic range of about 120 dB. I have written an article on this subject: Bit Depth Bit Rate The bit rate is typically the amount of data bits per second that can be transmitted over a specified interface. Interface is this context could be USB, Firewire, HDMI, Thunderbolt or any number of specified digital protocols. The transfer or bit rate for any given protocol is different. USB 2.0 for example, has a maximum bit rate of 480 Megabits per second (MBit/s) or 480 million data bits per second. The terms Bit Rate and Data Rate are used interchangeably. Common bit rate (or data rate) terms: Kilobit per second (kbit/s): one thousand bits per second. Megabit per second (Mbit/s): one million bits per second. Gigabit per second (Gbit/s): one billion bits per second. So, now when you see or hear the term Gigabit Ethernet, you will know that it is a network interface that can convey or transfer one billion bits per second. CD Quality A term (an oxymoron, in my opinion) that refers to the 16 bit-44.1kHz digital audio file specification used for the Compact Disk. CPU In computers, it stands for Central Processing Unit. The CPU is the chip that controls everything. It determines the speed at which the computer operates. The smartphone that you carry has a CPU in it. In fact, Apollo 11 (that landed on the Moon) had much less computational power than your smartphone. Digital-to-Analog Converter Abbreviated D/A, DAC, it performs the inverse of the A/D converter by converting those digital ones and zeros to an analog waveform that is hopefully pleasing to listen to. Dither Dither is used to randomize noise at discrete frequencies in a digital audio recording. In computer audio editing, dither is an intentionally applied form of noise. Dither can be applied during recording or after the fact. It is often one of the last stages of audio production used for the Compact Disc. When digital audio with lower bit depths is at the threshold of signal level, it can sound grainy. Noise is often added to mask this anomaly. One of the trade-offs of applying Dither is that it will slightly raise the noise floor of a given audio recording. Dynamic Range In the world of audio, Dynamic Range is defined simply as the range of volume from the loudest to the softest of audible sounds. Dynamic Range is expressed in decibels (dB) which, is a mathematical Log-10 ratio. We Audio Engineers often refer to Dynamic Range in terms of the difference between the loudest undistorted signal that can be recorded down to the noise level (floor) of a given medium. Analog tape has a dynamic range approaching 70dB, when used at 30 Inches Per Second. An audio CD has a theoretical dynamic range of 96dB. The average human can hear a dynamic range of approximately 140dB. HDD From Wikipedia; A Hard Disk Drive (HDD), is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage and one or more rigid rapidly rotating platters coated with magnetic material. The platters are paired with magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. HDD’s are a type of non-volatile storage, retaining stored data even when powered off. Modern HDD’s are typically in the form of a small rectangular box that is usually 3-1/2inches or 2-1/2 inches wide. Introduced by IBM in 1956, HDD’s were the dominant storage device for general- purpose computers beginning in the early 1960s. There are also Solid State hard Drives (SSD’s) that use a different technology. There is a separate description for a SSD in this glossary. LSI In the digital world, LSI refers to Large Scale Integration. LSI technology is used in the manufacture of integrated circuits of all kinds. The technology has been around since the 1920’s, but it wasn’t practical until the late 1950’s. A lot has changed since then and the acronym is now ULSI meaning; Ultra Large Scale Integration. MOTHERBOARD Also known as the “Mainboard” and sometimes referred to as “Mobo.” The Motherboard in your computer is the circuit board that holds everything. Some things are plugged in to it and others are soldered on. MP3 Known officially as MPEG-2 Audio Layer III. It is a patented digital audio encoding scheme using a form of lossy data compression. It’s a common audio exchange format for consumer digital audio players and streaming. The MP3 file format is designed to greatly reduce the amount of data required to represent an audio recording and still sound like the original uncompressed audio for (most) listeners. An MP3 file that is created using the setting of 128 Kilobits per second (kbits/s) will result in a file that is about 11 times smaller than a Compact Disk file created from the original analog audio source. An MP3 file can also be constructed at higher or lower bit rates with the higher bit rates resulting in higher audio quality. It's worth noting that listening tests have shown that the average listener can tell the difference in fidelity between an MP3 file and a CD quality file. Nyquist Frequency Named after the Swedish-American engineer Harry Nyquist, the Nyquist frequency is half the sampling frequency (sample rate) of a discrete signal. If the sample rate is 48 kHz, the Nyquist frequency would be 24 kHz. According to the Nyquist theorem, at least 2 samples of a peak-to-peak audio waveform are required for it to accurately be reproduced. Acquiring less than 2 samples of a peak-to-peak waveform can produce a form of distortion known as Aliasing. All of the frequencies above the Nyquist frequency are attenuated by filtering. OS In the computer world, OS stands for Operating System and there are many. The most popular for personal computers (PC’s) are Microsoft Windows, Apple and LINUX. In that order, I think. Oversampling In digital signal processing, oversampling is the process of sampling a signal with a sampling frequency significantly higher than twice the bandwidth (or the highest frequency) of the signal being sampled. Pulse Code Modulation Pulse Code Modulation (PCM), is the most widely used method of converting analog audio to a digital format. Basically, it’s defined as the uniform sampling of an audio waveform at regular intervals. This applies to both frequency (Sample Rate) and volume or the magnitude of the signal (Bit Depth). RAM An acronym that means Random Access Memory. RAM is what your OS and all of the software that is run on your personal computer use. Think of RAM as a scratch pad. The RAM gets wiped clean every time you boot or re-start your computer. RED BOOK: The CD standard The original set of books containing the specifications for all forms of optical Compact Disk were individually color bound. The book containing the specifications for Audio CD’s was bound in red, hence the name. (I’m not kidding!) The Compact Disk standard was developed jointly by Phillips & Sony, and the technical specifications were released in 1980. ROM Another acronym that means Read Only Memory. ROM is often used in LSI chips. Your personal computer uses it in the BIOS, for example. ROM chips can retain their memory because they are connected to a battery or some kind of constant voltage source. SACD SACD is an acronym for Super Audio Compact Disk. SACD’s are high fidelity, read-only optical disks (often, a DVD) for audio playback. Developed jointly by Sony and Philips Electronics, SACD recordings have a wider frequency response and dynamic range than conventional audio CD’s. The SACD format uses a digital audio scheme called Direct Stream Digital (DSD), which has a very high sampling frequency of 2.8224 MHz (Millions of cycles per second). A SACD can stream data at four times the rate of an audio CD. Sample Rate or Sampling Frequency Sample Rate defines the number of samples per second taken of an analog waveform within a given bandwidth. The terms Sample Rate and Sample Frequency are used interchangeably to describe the same thing. I go more into this subject here. Sample Rate Conversion This is where digital audio that was originally digitized at a given sample rate is re-sampled to another sample rate and often to another bit depth. For example, a digital file created at 24 bit/48 KHz has to be sample-rate converted to 16 bit/44.1 kHz so it can be used for Audio CD’s. This process is often accomplished by software using a mathematical formula called an “algorithm” or in real time by a hardware converter. The real time process is most often used in a sound mixing environment where several sources were delivered that are not all the same sample rate or bit depth. This often happens in the making of Radio and TV Commercials, Feature Films and Music Production. Either process can result in the creation of unwanted audible artifacts, which is why everyone tries to maintain the same specifications for a given project. SSD The term is also an acronym that stands for Solid State Drive. The technology comes in several forms; USB drives, SD cards, etc. When compared to a traditional hard drive (HDD), a SSD is faster, will withstand shock better, (no moving parts) and will run quieter. However, because of the technology involved, some forms (USB, SD cards, etc.) can lose or corrupt data when not connected to power. Word Clock (also known as Sample Clock) Digital audio is created by taking a sample of an analog signal on a periodic basis, say 48000 times per second (the “Sample Rate”). A dedicated clock, the “sample clock” ticks at that rate, and, every time it does, a new sample is measured. Sample clocks are built into most devices that handle digital audio and video. Your CD player and your DVD player have sample clocks built into them in order to stream the data accurately enough to convert the signal to analog audio and/or video. Whenever you connect two digital audio or video devices together in order to move data from one to the other, you must ensure they share the same sample clock. Why is this necessary? The oscillating crystals used for sample clocks are generally very stable, but there are always minute differences in the frequency of any two or more sample clocks (A man with two watches, never knows the exact time). When used individually, this is not a problem, but connect two digital devices together and those minute differences will accumulate over time. Eventually, one of the devices will be trying to read a sample in the middle of the other device's tick, and the result is a small click or pop in the audio stream or noticeable jump in the picture. In the consumer realm, when you connect your CD/DVD player to your home theater processor, via a digital interconnect cable, the theater processor will adjust its clock to the incoming data stream, and all works well. In the professional world, many digital devices are often connected together or to a single source (a mixer, for example), and in order to avoid clocking errors over time, a central clock source is used and fed to all of the equipment. This central clock is known as a Word Clock generator. References Rudolph F. Graf, “Dictionary of Electronics” Howard W. Sams, 1974 Glenn D. White, “The Audio Dictionary” University of Washington Press, 1987 Wikipedia, http://www.wikipedia.org/ Return to TOP of page © Corey Bailey Audio Engineering