Understanding Audio – Bried Glossary
Below are some of the most common terms that it is useful to have a grasp of when evaluating PC audio products. Note that this brief summary borrows heavily from the excellent Pro Audio Reference published by the Rane Corporation, which can be found here. Common audio terms and their definitions follow:
The Acoustic Driver in an audio device is what physically moves the cone(s) and causes sound through controlled (typically magnetic) vibration.
Analog to Digital Converter [ADC]
The Analog to Digital Converter [ADC] is the electronic component which converts the instantaneous value of an analog input signal to a digital word (represented as a binary number) for digital signal processing. The ADC is the first link in the digital chain of signal processing. In other words, it breaks down a continuous signal into a digitized signal of 1’s and 0’s.
An electronic device used to increase an electrical signal. The signal may be voltage, current or both (power). Preamplifier is the name applied to the first amplifier in the audio chain, accepting inputs from microphones, or other transducers, and low output sources (CD players, tape recorders, turntables, etc.). The preamplifier increases the input signals from mic-level, for instance, to line-level. Power amplifier is the name applied to the last amplifier in the audio chain, used to increase the line-level signals to whatever is necessary to drive the loudspeakers to the loudness required.
The Audio Source is the original source of sound where the data is taken from. A vinyl record, mp3 file, CD and DVD are all possible examples of an audio source.
The Bit Rate is the rate or frequency at which bits appear in a bit stream. Applied to digital audio, bit rate (kbits/sec/channel) equals the sampling rate (kHz) times the number of bits per sample. The data bit rate for a CD, for example, is 1.41M bits per second (44.1 kHz x 16 bits per sample x 2 channels). Generally speaking, the higher the bit rate, the more detail you retain and, as a result, higher fidelity can be experienced.
Clipping is a term used to describe the result of an amplifier running into power supply limitation. The maximum output voltage that any amplifier can produce is limited by its power supply. Attempting to output a voltage (or current) level that exceeds the power supply results in a flattoping effect on the signal, making it look cut off or "clipped." A clipped waveform exhibits extreme harmonic distortion, dominated by large amplitude odd-ordered harmonics making it sound harsh or dissonant.
Hard clipping is the term used to describe extreme clipping of a signal, producing highly visible flattoped waveforms as viewed on an oscilloscope; soft clipping refers to moderate clipping that results in waveforms having softly-rounded edges, as opposed to the sharp edges of hard clipping.
A Crossover is an electrical circuit (passive or active) consisting of a combination of high-pass, low-pass and bandpass filters used to divide the audio frequency spectrum (20 Hz - 20 kHz) into segments suitable for individual loudspeaker use. Since audio wavelengths vary from over 50 feet at the low frequency end, to less than one inch at the high frequency end, no single loudspeaker driver can reproduce the entire audio range. Therefore, at least two drivers are required, and more often three or more are used for optimum audio reproduction. Named from the fact that audio reproduction transitions (or crosses over) from one driver to the next as the signal increases in frequency.
For example, consider a two driver loudspeaker crossed over at 800 Hz: Here only one driver (the woofer - "woof, woof" = low frequencies) works to reproduce everything below 800 Hz, while both drivers work reproducing the region immediately around 800 Hz (the crossover region), and finally, only the last driver (the tweeter - "tweet, tweet" = high frequencies) works to reproduce everything above 800 Hz. Crossover circuits are characterized by their type (Butterworth, Bessel and Linkwitz-Riley being the most popular), and by the steepness of their roll-off slopes (the rate of attenuation outside their passbands) as measured in decibels per interval, such as dB/octave, or sometimes dB/decade [useful rule-of-thumb: 6 dB/octave approximately equals 20 dB/decade].
Digital Signal Processing (DSP)
A technology for signal processing that combines algorithms and fast number-crunching digital hardware, and is capable of high-performance and flexibility. This allows for the manipulation of signal
Audio distortion: By its name you know it is a measure of unwanted signals. Distortion is the name given to anything that alters a pure input signal in any way other than changing its size. The most common forms of distortion are unwanted components or artifacts added to the original signal, including random and hum-related noise.
Distortion measures a system's linearity -- or nonlinearity, whichever way you want to look at it. Anything unwanted added to the input signal changes its shape (skews, flattens, spikes, alters symmetry or asymmetry, even if these changes are microscopic, they are there). A spectral analysis of the output shows these unwanted components. If a piece of gear is perfect, it does not add distortion of any sort. The spectrum of the output shows only the original signal -- nothing else -- no added components, no added noise -- nothing but the original signal.
Electro-Magnetic Interference [EMI]
A measure of electromagnetic radiation from equipment that must be taken into account, as this radiation tends to bleed into an unprotected, or unshielded, audio circuit and can create noise.
The frequency range of an audio device represents the lowest to the highest frequencies of sound it is capable of producing. With today’s technology, it is easy to produce below 20Hz and above 20,000Hz, but 20-20,000Hz represents the average human’s audible hearing range, and anything beyond that spectrum usually goes unheard. However, our bodies can still feel the physical rumble of frequencies below 20Hz and the frequencies above 20,000Hz may not be noticed as distinct notes, but they add a sense of atmosphere to the acoustic image. This is why very high-end audio systems will still provide a frequency range well beyond our noticeable hearing capabilities.
Audio electronics: It connotes amplitude-frequency response and quantifies a device's maximum and minimum frequency for full-output response. The electrical passband of an audio device. The measure of any audio device's ability to respond to a sine wave program, and therefore is a complex function measuring gain and phase shift (see phasor). It is used to express variation of gain, loss, amplification, or attenuation as a function of frequency, normally referred to a standard 1 kHz reference point.
General. Sound or a sound that is loud, unpleasant, unexpected, or undesired. Physics. A disturbance, especially a random and persistent disturbance, that obscures or reduces the clarity of a signal. Computer Science. Irrelevant or meaningless data.
Audio signals are complex AC (alternating current) periodic phenomena expressed mathematically as phasors, or vectors. Phase refers to a particular value of t (time) for any periodic function, i.e. it is the relationship between a reference point and the fractional part of the period through which the signal has advanced relative to an arbitrary origin. In short, Phase is responsible for creating acoustic positioning. When you hear two speakers but think that sound is coming from some spot between them, that is a result of phase manipulation.
Power Output (Wattage)
1. Electricity a. The product of applied voltage (potential difference) and current in a direct-current circuit (or the voltage squared divided by the resistance, or the current squared times the resistance). b. The product of the effective values of the voltage and current with the cosine of the phase angle (between current and voltage) in an alternating-current circuit.
2. Physics The rate at which work is done, expressed as the amount of work per unit time, and measured in units such as the watt (1 joule per second, which equals the power dissipated (as heat) by 1 ohm of resistance when 1 ampere of current passes through it) and horsepower (equal to 745.7 watts).
The frequency or rate at which an analog signal is sampled or converted into digital data. Expressed in Hertz (cycles per second). For example, compact disc sampling rate is 44,100 samples per second or 44.1 kHz, however in pro audio other rates exist: common examples being 32 kHz, 48 kHz, and 50 kHz.
With these fundamental concepts now understood, users may purchase an audio device with knowledge and confidence. Speakers must be test heard, just as cars should be test driven. The more knowledge you have, the better you will be able to discern various characteristics between two juxtaposed products. Even further, once a product is decided upon, the user must know what he/she is doing in order to fine tune the device to fit his/her ideal needs. For those who wish to learn more, I highly recommend the full reference that can be found at http://www.rane.com/digi-dic.html.