Digital Audio Midterm Vocabulary
A/D converter Circuit for converting analog waveforms into a series of equally spaced numerical values represented by binary numbers. The analog signal is sampled every few milliseconds and its level is quantized into a digital word. The larger the digital word (i.e. the more 'bits'), the more accurate the digital representation of the analog value.
Aliasing When an analog signal is sampled for conversion into a digital data stream, the sampling frequency must be at least twice that of the highest frequency component of the input signal. If this rule is disobeyed, the sampling process becomes ambiguous as there are insufficient points to define each cycle of the waveform, resulting in enharmonic (false) frequencies being added to the audible signal. Aliasing is also sometimes referred to as fold-over.
AM wide-band tuner Picks up higher frequency range AM signals (from 1610kHz to 1710kHz) to receive newly allocated additional stations.
AMP Ampere; unit of electrical current.
Amplifier A device which increases signal level. Many types of amplifiers are used in audio systems. Amplifiers typically increase voltage, current or both. The resulting signal is a reproduction of the input signal as well as this increase.
Amplitude Another word for level. Can refer to sound levels or electrical signal levels. It is the height of a waveform. The greater a sound wave's amplitude, the louder it sounds.
Analog audio The traditional means of recording and reproducing sound, using fluctuating electronic voltages to replicate audio waveforms. The origin of the term is that the electrical signal can be thought of as being analogous to the original signal.
ADC Analog-to-Digital converter.
Attenuate To make lower in level.
Audio frequency Signals in the human audio range: nominally 20Hz to 20kHz.
Balance The relative levels of the left and right channels of a stereo recording, or the relative levels of the various instruments and voices within a mix.
Balanced wiring Wiring system which uses two out-of-phase conductors and a common screen to reduce the effect of interference. Audio signals require two wires. In an unbalanced line the shield is one of those wires. In a balanced line, there are two wires plus the shield. For the system to be balanced requires balanced electronics and usually employs XLR connectors. Balanced lines are less apt to pick up external noise. This is usually not a factor in home audio, but is a factor in professional audio requiring hundreds or even thousands of feet of cabling. Many higher quality home audio cables terminated with RCA jacks are balanced designs using two conductors and a shield instead of one conductor plus shield. For balancing to be effective, both the sending and receiving device must have balanced output and input stages respectively.
Bandwidth A means of specifying the range of frequencies passed by an electronic circuit such as an amplifier, mixer or filter. The frequency range is usually measured at the points where the level drops by 3dB relative to the maximum. So a system's bandwidth is the total frequency range of the system, usually specified as something like: 20-20,000Hz plus or minus 3 db.
Abbr. b Abbreviation for binary unit or binary digit. 1. The smallest amount of digital information. A bit can store or represent only two states, 0 and 1. [The original term binary unit was coined by John Tukey of Bell Laboratories to represent the basic unit of information as defined by Shannon as a message representing one of two states.] 2. A little bit -- from Old English bita, meaning a piece bitten off.
Usually a multiple of 8 which is then called a byte of audio data. A higher bit depth means that more amplitude accuracy can be represented. 16-Bit capability means (2 to the 16th power) or 65,536 available values for each audio sample. When you have a 24-bit process (2 to the 24th power), there are 16,777,216 available values for each audio sample.
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). [The oft-quoted CD bit rate of 4.3218 MHz is for the raw bit rate which comes from multiplying 7,350 frames per second by 588, the number of channel bits.
Buffering The method for temporarily storing or delaying data samples before processing or conversion.
Burning This term usually refers to the act of writing information (data, music, etc.) on to a compact disc.
Byte A piece of digital data comprising (normally) eight bits. See also Bit.
Capacitance Property of an electrical component able to store electrostatic charge.
Cardioid Meaning heart shaped, describes the polar response of a unidirectional microphone.
CD Compact disc.
CD-R A recordable compact disc format, allowing just one record cycle (it cannot be erased and reused).
CD-R burner A device capable of recording data on to blank CD-R discs.
CD-RW A rewritable CD. Unlike record-once CDs (CD-Rs) you can erase and re-record
Clipping Distortion that occurs when an amplifier is driven to play louder than its power supply will allow. Usually the clipped waveform contains an excess of high-frequency energy. The sound becomes hard and edgy. Hard clipping is the most frequent cause of burned out tweeters. Even a low-powered amplifier or receiver driven into clipping can damage tweeters which would otherwise last virtually forever.
Coaxial Cable A single copper conductor, surrounded with a heavy layer of insulation, covered by a thick surrounding copper shield and jacket. A constant-impedance unbalanced transmission line..
Codec Acronym for Compressor-Decompressor; the complicated mathematical algorithms used to create and read compressed digital audio files. There are several MP3 codecs that vary widely in their quality and efficiency.
Compression In audio, compression means to reduce the dynamic range of a signal. Compression may be intentional or one of the effects of a system that is driven to overload.
Compressor A compressor provides a form of automatic level control. It attenuates high levels, thus effectively reducing the dynamic range, making it much easier to control signals and set appropriate fader levels. Reducing the dynamic range also means that recording levels can be set higher, therefore improving the signal-to-noise performance. Limiting is an extreme form of compression, where the output signal is sharply attenuated so that it cannot exceed a particular level.
Cycle One complete vibration of a sound source or its electrical equivalent. One cycle per second is expressed as 1 Hertz (Hz).
DAC Converter Circuitry that converts a digital (binary) signal into an equivalent analog waveform. In an audio system, this is done so the signal from digital storage media, such as MiniDisc, CD, or Digital Audio Tape, can be handled by analog system components, such as the receiver and speakers. Often abbreviated to just D/A or DAC.
dB deciBel; one-tenth of a Bel, a unit of measurement (named after Alexander Graham Bell) that describes the strength of a signal relative to a standard reference. Different kinds of decibels measure a variety of signal types. One type, dBv, compares an electrical signal such as amplifier output with a 1-volt standard. Another type, dBr, can be used to measure the output of a speaker system relative to a reference sound level that represents the lower threshold of human hearing. 0 dB is the threshold of hearing, 130 dB is the threshold of pain
· Whisper: 15-25 dB
· Quiet background: about 35 dB
· Normal home or office background: 40-60 dB
· Normal speaking voice: 65-70 dB
· Orchestral climax: 105 dB
· Live Rock music: 120 dB+
· Jet aircraft: 140-180 dB
Decoding This is the process whereby the information in a compressed digital audio file is read so that it can be played and heard. Software MP3 players like UltraPlayer and Winamp decode and play MP3 files.
Digital Audio Refers to a way of synthesizing sound. In a digital format sound waves are broken down and assigned a numerical value, represented by groups of 1s and 0s. When these groups of numbers are read by a receiver or amplifier, they are converted back into sound. This kind of mathematical precision creates crisp, clean sounds.
Distortion Anything that alters the musical signal (a process often found desirable by guitar players). There are many forms of distortion, some of which are more audible than others. Distortion specs are often given for electronic equipment which are quite meaningless. As in all specifications, unless you have a thorough understanding of the whole situation, you will not be able to make conclusions about the sonic consequences.
Dynamic microphone A type of microphone that works on the electric generator principle, where a diaphragm moves a coil of wire within a magnetic field.
Dynamic Range The difference between the loudest (maximum output level) and quietest (residual noise floor) sounds produced in an audio system without distortion. The dynamic range in a digital system is determined by the data resolution, about 6 dB per digital bit. A 20-bit system has a theoretical dynamic range of 120 dB. In speech, the range rarely exceeds 40 dB; in music, it is greatest in orchestral works, where the range may be as much as 75 dB.
Dynamics the relative loudness or softness of a piece of music.
Encoding This is the name for the process in which larger sound files are compressed into smaller ones. When you convert a file from WAV format into an MP3 file, you are encoding. When you record a song from a CD into MP3 format, you are ripping and encoding it.
Fade in/out A feature of most audio editing software that allows the user to apply a gradual amplitude increase or decrease over some segment of the sound.
Fletcher-Munson curve Our sensitivity to sound depends on its frequency and volume. Human ears are most sensitive to sounds in the midrange. At lower volume levels humans are less sensitive to sounds away from the midrange, bass and treble sounds seem reduced in intensity at lower listening levels
Frequency the rate per second at which an oscillating body vibrates. Usually measured in Hertz (Hz), humans can hear sounds whose frequencies are in the range 20 Hz to 20kHz.
Hertz (Hz) A unit of measurement denoting frequency, originally measured as cycles per second, (CPS): 20 Hz = 20 CPS. Kilohertz (kHz) are Hertz measured in multiples of 1,000.
Impedance Impedance is a measure of electrical resistance specified in ohms. Speakers are commonly listed as 4 or 8 ohms but speakers are reactive devices and a nominal 8 ohm speaker might measure from below 4 ohms to 60 or more ohms over its frequency range. This varying impedance curve is different for each speaker model and makes it impossible to design a really effective generic speaker level high-pass filter. Active devices like amplifiers typically have an input impedance between about 10,000-100,000 ohms and the impedance is the same regardless of frequency.
Jitter Timing variations, a tendency towards lack of synchronization caused by electrical changes. Technically the unexpected (and unwanted) phase shift of digital pulses over a transmission medium. A discrepancy between when a digital edge transition is supposed to occur and when it actually does occur. Think of it as nervous digital, or maybe a digital analogy to wow and flutter.
kHz 1000 hertz
kOhm 1000 ohms
MIC level The low level signal generated by a microphone. This must be amplified many times to increase it to line level.
Monophonic The ability to play only one note at a time. A characteristic of some older synthesizers.
MP3 Stands for MPEG-1, Audio Layer 3. MP3 is a digital audio file format. It is currently the most popular format on the web because of its high sound quality and small file size, and because MP3 encoders are easily available on the Internet. The file extension for an MP3 file is usually ".mp3".
Muddy Listening term. A sound that is poorly defined, sloppy or vague. For example, a muddy bass is often boomy with all the notes tending to run together.
Normalization An automatic process available in most audio software whereby the gain of all program material is adjusted so the peak level will just arrive at 0 dBFS. Normalization can be extremely damaging to your digital audio if misused. The first serious peril is that it is a mistake to use normalization to try to correct the apparent loudness of different songs on an album. In many cases, normalization may aggravate an existing condition. The second peril of normalization is that the very process of changing gain can add a veil or truncation distortion if improperly done.
Nyquist sampling theorem Defines the process of sampling audio with a digital system. Among other things, it states that the sampling frequency of a digital audio system must be at least twice that of the highest audio frequency, otherwise aliasing will occur. The Nyquist theorem was developed at Bell Labs by C. Shannon and H. Nyquist.
Ohm Unit of electrical resistance.
Oscillator An electronic device capable of generating a recurring waveform, or a digital process to generate the same.
Out of phase An audio signal that has its waveform 180 degrees out of phase with another.
Overload A condition in which a system is given too high an input level. A common cause of distortion or product failure.
Peak Maximum instantaneous level of a signal.
Phase The relative position of a wave to some reference point; the frequency coherence of a signal; the timing difference between two electrical waveforms expressed in degrees, where 360 degrees corresponds to a delay of exactly one cycle. If two signals are "out of phase", the trough of the first waveform corresponds with the peak of the second, resulting in cancellation.
Pitch A continuous frequency over time.
Preamplifier A device that takes a source signal, such as from a turntable, tape deck or CD player, and passes this signal on to a power-amplifier. The preamplifier may have a number of controls such as source selector switches, balance, volume and possibly tone controls. Often shortened to "preamp".
Ripping Ripping is the process of taking audio data from your CD and making it into a sound file on your computer. It is called ripping because in most cases the audio data is digitally "ripped" directly from the CD. This process can be very fast (a four minute song might only take 30 seconds to record). An analog recording process on the other hand records a song by playing the CD and recording the sound output. The analog process can only happen in realtime (a four minute song takes four minutes to record). The digital extraction process is faster because it copies the data instead of recording the sound output. Software applications that rip from CDs create the new audio file in the WAV or AIFF formats. Software applications that rip and encode usually create the new audio file in the MP3 format.
Sample A digitized sound used as a musical sound source in a sampler or additive synthesizer.
Sample rate A digital representation of an analog signal created by checking, or sampling, the analog voltage a fixed number of times per second. The greater the number of samples, the more accurate the representation of the analog signal. The CD uses a sampling rate of 44.1kHz; it checks the analog signal 44,100 times per second.
Stereo From the Greek meaning solid. The purpose of stereo is not to give you separate right and left channels but to provide the illusion of a three-dimensional, holographic image between the speakers.
Unison To play the same melody using two or more different instruments or voices.
VU meter Meter designed to interpret signal levels in roughly the same way as the human ear, which responds more closely to the average levels of sounds rather than to the peak levels.
WAV A digital audio file format. It is not a compressed format, so WAV files are usually very large. It is often used as an intermediate format when recording from a CD. WAV files can be compressed as MP3 files. The file extension for a WAV file is ".wav".
Wavelength The distance the sound wave travels to complete one cycle. The distance between one peak or crest of a sine wave and the next corresponding peak or crest. The wavelength of any frequency may be found by dividing the speed of sound by the frequency. (Speed of sound at sea level is 331.4 meters per second, that's 1087.42 feet per second).
White noise A random noise that contains an equal amount of energy per frequency band. That is, 100-200, 800-900, and 3000-3100. Pink noise has an equal amount of energy per octave. The bands 100-200, 800-1600, and 3000-6000 all contain the same amount of energy.
XLR Type of connector commonly used to carry balanced audio signals, including the feeds from microphones.