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Glossary

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This page gives very brief explanations of technical terms related to digital audio, with some links to

Wikipedia for much more comprehensive explanations.

General Terms

Term Description

ADC: Analog to digital converter. The part of a sound card which records an analog, real

world sound like a voice or guitar and converts it to a numerical representation of

the audio that a computer can manipulate.

Algorithm: A set of steps or a procedure that will produce a desired result.

ALSA: A Linux kernel component for providing device drivers for sound cards. Known as

an audio host in Audacity.

Amplitude: The level or magnitude of a signal. Audio signals with a higher amplitude will sound

louder.

Audacity Project

Format (.aup):

The format in which Audacity stores its projects. This consists of a reference file

with the extension .aup and a large number of small audio files with extension .AU.

This structure makes it quicker for Audacity to move audio around - ideal for cutting

and pasting audio in a project.

Audio CDs: CDs containing PCM audio data in accordance with the Red Book standard. They

can be played on any standalone CD player as well as on computers.

Batch

Processing:

Automation of a series of repetitive tasks on a computer so that the tasks run without

manual intervention. In the early days of computers this was done by processing

stacks of punch cards. In Audacity, repetitive tasks are handled by creating a Chain

of actions. The Chain can apply a predetermined sequence of effects to the current

project, or can be run unattended to apply effects and/or format conversions to a

batch of external audio files.

Bit: A measure of quantity of data. A bit is one binary digit, a 0 or a 1.

Bit Rate: The number of computer bits conveyed or processed per unit of time. Normally

expressed in kilobits per second (kbps). For an uncompressed, PCM file, kbps bit

rate is sample rate multiplied by sample format mutiplied by number of channels,

divided by 1000, giving 1411 kbps for Red Book WAV or AIFF. Rates are much

lower for compressed or lossy formats and are independent of sample rate for MP3.

CBR: Constant Bit Rate - In this format, the rate at which audio uses its data does not vary.

Silence uses as much 'space' as audible sound.

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Cepstrum: The cepstrum of an audio signal is related to the spectrum, but presents the rate of

change in the different spectrum bands. It's particularly useful for properties of vocal

tracks and is used, for example, in software to identify speakers by their voice

characteristics.

Clipping: Distortion to sound, usually due to the audio being too loud. Unless the original

audio is 32-bit sample format, waveforms louder than 0 dB will have their tops

lopped off (flattened) at 0 dB, rather than showing smooth curves. Clipping can also

be an intentional distortion effect that lops off part of the waveform, reducing its

amplitude and changing its frequency content.

Codec: A computer program capable of encoding and/or decoding a digital data stream. The

term is a portmanteau (a blending of two or more words) of coder and decoder.

Companding: Refers to the process of compressing the dynamic range of an audio signal before

storage or transmission, then expanding the signal on retrieval or reception. The term

is a portmanteau (a blending of two or more words) of compressing and expanding.

Compressed

Audio Format:

Any format that will reduce the space required in storing or representing an audio

signal. Space savings can be made for example by discarding certain frequency

components which may be inaudible. MP3 takes this approach. Other formats such

as FLAC compress without audio loss, but achieve lower compression rates.

Compression: A process that tends to even out the overall volume level by increasing the level of

softer passages and decreasing the level of louder passages. See also Compressed

Audio Format.

Cycle: An audio tone consists of an oscillating sound pressure on the ear. One cycle is one

full transition of positive pressure through to negative pressure, back to positive

pressure again.

DAC: Digital to analog converter. The part of a sound card which plays back a numerical

representation of audio as an analog, real world sound like a voice or guitar.

Data CDs: Data CDs contain data intended to be read directly by a computer. The data may

include audio and any other types of file such as images and documents. Most

standalone CD players will not play data CDs, but some DVD players will.

Including compressed audio files on a data CD can greatly increase the playing time

compared to audio CDs.

dB: Decibels. A logarithmic unit (typically of sound pressure) describing the ratio of

that unit to a reference level.

DC Offset: An offsetting of a signal from zero. A signal with DC Offset would appear in the

Audacity Default Waveform view to be not centered on the 0.0 horizontal line. DC

Offset results in reduced headroom and can cause clicks at the start and end or

distortion after running effects. It can be corrected in Audacity by running

Normalize.

Dynamic Range: The difference between the loudest and softest part in an audio recording, the

maximum possible being determined by its sample format. For a device, the

difference between its maximum possible undistorted signal and its Noise Floor.

Exponential: A non-linear relationship where a change in value is proportional to the current

level. If you double the value in a time period, it doubles again in the next period; if

you halve the level in a time period, it halves again in the next period. For an

exponential fade in, the curve becomes "steeper" with time; an exponential fade out


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becomes "flatter" with time. See also Logarithmic.

FFT: Fast Fourier Transform. A method for performing Fourier transforms quickly.

File name

extension:

A suffix of three or four characters added to a file name which defines the format of

its contents. The suffix is separated from the file name by a dot (period), as in

"song.mp3". The extension of common formats is often hidden on Windows, but can

be turned on in the system's Folder Options.

Filter: A sound effect that lets some frequencies through and suppresses others.

Fourier

Transform:

A method for converting a waveform to a spectrum, and back.

Frequency: Audio frequency determines the pitch of a sound. Measured in Hz, higher

frequencies have higher pitch. See this Wikipedia article.

Gain: A measure of how much a signal is amplified. Usually expressed in dB, positive gain

increases the amplitude of a signal, while negative gain reduces it.

Harmonics: Most sounds are made up of a mix of different frequencies. In musical sounds, the

component frequencies are simple multiples of each other, for example 100 Hz, 200

Hz, 300 Hz. These are called harmonics of the lowest frequency sound.

Headroom: The difference between the peak level of an audio track and the maximum level that

can be achieved without clipping. Recording at -6 dB below maximum level is a

good compromise between getting far enough above the noise floor while having

sufficient headroom to make edits that increase loudness.

High Pass Filter: A filter that lets high frequencies through

Hz: Hertz. Measures a frequency event in number of cycles per second. See Frequency

and Sample Rate, both of which are measured in Hz.

Interpolation: Completing waveform data by estimating missing values. The values are estimated

as being between other known values. To convert a waveform recorded at 22000 Hz

or samples per second to one at a higher rate such as 44000 samples per second

requires interpolation.

kHz: One kilohertz (kHz) is 1000 Hz. For example, the common audio sample rate of

44100 Hz can also be expressed as 44.1 kHz.

LAME: A software library that converts audio to MP3 format.

Latency: A short delay between an audio signal being sent and received. In computer audio

this is due to analog-to-digital and digital-to-analog conversion. Most commonly

refers to the delay between recording a sound and a) hearing its playthrough or b)

laying it down on disk.

Linear: A simple, directly proportional, one-to-one, "straight-line" relationship. This term is

used to contrast with exponential, logarithmic, or other complex relationships.

Logarithmic: A non-linear relationship where one item is proportional to the logarithm of the

other item. So for a logarithmic fade in, the curve becomes "flatter" with time; a

logarithmic fade-out becomes "steeper" with time. Some measures, such as dB, are

logarithmic by definition. See also Exponential.

Lossless: A format that does not lose any information. It may be either a size-compressing

format like FLAC where the quality is exactly as good as before compression, or an

uncompressed format like WAV.


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Lossy: A format for size-compressing audio that may sacrifice a small amount of quality in

order to reduce the file size more than lossless compression. Examples are MP3 and

OGG.

Low Pass Filter: A filter that lets low (bass) frequencies through.

MME: Multimedia Extensions to Windows 3 appeared in Autumn 1991 as the first

standardized Windows interface to support sound cards. It is one of the "audio hosts"

selectable in Device Toolbar. MME was superseded in 1995 by Windows

DirectSound.

MP3 CDs: A specific type of data CD containing only MP3 audio files. All computers can play

them as can some DVD and portable MP3 players.

Noise Floor: A level or amplitude representing the amount of near-continuous background noise

present in the signal. A background hiss would raise the noise floor, and could

prevent a faint signal (one below the noise floor) being heard at all. Unwanted

sporadic noise such as a member of the audience coughing is noise, but it doesn't

contribute to the noise floor.

Pan: Panning is the spread of a sound signal (either monaural or stereophonic pairs) into a

new stereo or multi-channel sound field.

PCM: Pulse code modulation. A method of converting audio into binary numbers to

represent it digitally, then back to audio. The waveform is measured at evenly

spaced intervals and the amplitude of the waveform noted for each measurement.

Pitch: Generally synonymous with the fundamental frequency of a note, but in music,

often also taken to imply a perceived measurement that can be affected by overtones

above the fundamental.

Red Book: The most widely used standard for representing audio on CD, requiring stereo,

16-bit, 44100 Hz.

Resampling: Converting a sampled signal from one sample rate to another without changing the

length of the audio (hence without changing the playback speed or pitch). This

necessarily changes the number of samples that the audio contains. Resampling can

also mean converting from one sample format to another which changes the

precision of each sample but not the number of samples.

RMS: Root-mean-square. A method of calculating a numerical value for the average sound

level of a waveform. The RMS level (colored lighter blue in Audacity) equates very

approximately to how loud the audio sounds.

Sample: A discrete value at a point in a waveform representing the audio at that point. Also

the act of taking a sequence of such values. All digital audio must be sampled at

discrete points. By contrast, analog audio (such as the sound from a loudspeaker) is

always a continuous signal.

Sample Rate: Measured in Hz like frequency, this represents the number of digital samples

captured per second in order to represent the waveform.

Sample Format: Also known as Bit Depth or Word Size. The number of computer bits present in each

audio sample. Determines the dynamic range of the audio.

Spectrum: Presentation of a sound in terms of its component frequencies.

Uncompressed

Audio Format:

An audio format in which every sample of sound is represented by a binary number.

Examples are WAV or AIFF.


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VBR: Variable Bit Rate. A method for compressing audio which does not always use the

same number of bits to record the same duration of sound.

Waveform: A visual representation of an audio signal.

Windows

DirectSound:

A Windows interface between applications (such as Audacity) and the sound card

driver. It is one of the "audio hosts" selectable in Device Toolbar. DirectSound was

released in 1995 as a replacement for the older MME and has an option to bypass

the kernel mixer and so reduce latency.

Windows

WASAPI:

The most recent Windows interface between applications (such as Audacity) and the

sound card driver. It is one of the "audio hosts" selectable in Device Toolbar.

WASAPI was first officially released in 2007 in Windows Vista.

Zero Crossing: The point where a line joining the audio samples crosses the zero horizontal line.

Audio File Formats

There are numerous audio file formats for storing audio on a computer.

WAV format is widely used on Windows and is needed for creating an audio CD.

AIFF is widely used on Apple's operating systems.

Compressed formats are used on portable music players.

Term Description

AAC: A lossy, size-compressed audio format. AAC files usually have M4A extension,

with variants such as M4P (protected) and M4R (ringtones). Usually gives better

quality for the same bit rate than the older MP3 format. Is default audio format for

iTunes®, iPod® and iPhone®, and Sony PlayStation 3.

AIFF: A container format, almost always used for lossless, uncompressed, PCM audio

with similar file size to WAV. Although the classic AIFF format is in Apple's earlier

Big-endian byte order, Mac OS X has always written "AIFF-C/sowt" files. These

have the same AIFF extension as classic AIFF and are identical to it except for being

Little-endian like WAV format. Rarely, files with AIFC extension can contain

compressed formats.

AU: A container format, used by Audacity for storage of lossless, uncompressed, PCM

audio data. Not be confused with Sun/NeXT AU files, which are usually U-Law

encoded PCM files but may be headerless.

FLAC: An Open Source lossless, size-compressed audio format

MIDI: MIDI is a small-sized file format which stores how to play notes, widely used for

keyboard instruments. It is not an audio file format like WAV that uses thousands of

samples to record the full sound of the notes actually being played.

MP2: A lossy, size-compressed audio format mainly used by the broadcast media

MP3: A lossy, size-compressed audio format which is the main format for transmitting

audio over the internet

Ogg Vorbis: An Open Source lossy, size-compressed audio format

WAV: A container format, almost always used for lossless, uncompressed, PCM audio.

The format is in Microsoft's Little-Endian byte order.


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WMA: A container format. Windows Media Audio is a lossy, size-compressed audio format

developed by Microsoft. It is a proprietary technology that forms part of the

Windows Media framework. WMA consists of four distinct codecs. The original

WMA codec, known simply as WMA, was conceived as a competitor to the popular

MP3 and RealAudio codecs.

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