Sound quality
Encyclopedia
Sound quality is the quality of the audio
output from various electronic devices. Sound quality can be defined as the degree of accuracy with which a device records or emits the original sound waves. Sound quality is also the physical pleasure or fatigue experienced by a listener.
In a live setting the skill of the musicians, the tonal quality of their instruments, and the physical traits of the venue determine sound quality.
In a playback setting, sound quality is characterized by the same traits as in a live setting but is also affected by the recording techniques and equipment used, from the microphones at the session, through the pressing at the record or compact disc factory, to the quality of electronics and speakers used to recreate the sound in a listener's home.
s such as MP3
and Ogg Vorbis, sound quality is a quantifiable factor that determines how much sound data the encoder is allowed to discard in order to reduce file size
. MP3-encoded sound is generally CBR, so its quality is defined by its bitrate, in kilobits per second (kbit/s). Quality of Ogg Vorbis-encoded files, which are most commonly VBR
, is a decimal value ranging from –1 to 10, with –1 being suitable only for low-quality voice.
The frequency range of sound (in hertz
) which the equipment is capable of sampling affects sound quality. Humans can hear frequencies ranging from about 20 Hz
to approximately 20 kHz, so sampling that doesn't extend this far will have a detrimental effect on the resultant quality.
The sound wave being recorded is an analogue
wave, that is, it is continuous and has some real value at every instant. The digital
quantization
of the analogue
sound wave means that much of the continuous sound wave is not recorded. The rate at which the sound is sampled refers to the amount of information the detection equipment records for each second of the sound. The higher the sampling frequency is, the more accurate the resultant samples will be.
The conversion of sample range and sample rate between different pieces of equipment in a sound recording and reproduction system usually lower the quality of the sound.
methods, where the quality is preserved also in smaller sound file sizes.
The sound quality is determined by the bit rate, which is the number of samples per second times the number of bits per sample. It also depends on number of audio channels (mono, stereo
, multichannel
) and compression method.
The most common are lossy compression formats as MP3, OGG Vorbis, AAC
and many others. For example, MP3 files commonly have a bit rate of 128 kbit/s, because it typically offers adequate audio quality in a relatively small space. MP3 has a maximum bit rate of 320 kbit/s.
By contrast, uncompressed audio as stored on an Audio CD
has a bit rate of 1,411.2 kbit/s, or four times the best MP3 can do, and twelve times that of common MP3 files.
The WAV
, AIFF
or AU
audio file formats can store audio using various bit rates, in accordance with the number of channels and compression. They often store uncompressed PCM audio with a bit rate of 1,411.2 kbit/s.
There are also lossless compression formats, such as FLAC
, ALAC
, Monkey's Audio
and many others, which preserve quality of original uncompressed audio, but can reduce the needed storage space. Advantageously, these lossless formats can also store metadata
(eg. images, title, artist, album, label, etc.) like lossy formats can, which cannot be done with pure uncompressed formats like WAV, AIFF, AU.
quality over a transmission medium. This metric is often used to quantify the quality of audio heard over a radio
system. DAQ levels are defined by the following scale.
Sound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
output from various electronic devices. Sound quality can be defined as the degree of accuracy with which a device records or emits the original sound waves. Sound quality is also the physical pleasure or fatigue experienced by a listener.
In a live setting the skill of the musicians, the tonal quality of their instruments, and the physical traits of the venue determine sound quality.
In a playback setting, sound quality is characterized by the same traits as in a live setting but is also affected by the recording techniques and equipment used, from the microphones at the session, through the pressing at the record or compact disc factory, to the quality of electronics and speakers used to recreate the sound in a listener's home.
Recording
For digital recording/digital playback, sound quality depends on the range of sound which is sampled, the rate at which it is sampled, and the various conversions that occur in any sound reproduction system. With lossy codecLossy data compression
In information technology, "lossy" compression is a data encoding method that compresses data by discarding some of it. The procedure aims to minimize the amount of data that need to be held, handled, and/or transmitted by a computer...
s such as MP3
MP3
MPEG-1 or MPEG-2 Audio Layer III, more commonly referred to as MP3, is a patented digital audio encoding format using a form of lossy data compression...
and Ogg Vorbis, sound quality is a quantifiable factor that determines how much sound data the encoder is allowed to discard in order to reduce file size
File size
File size measures the size of a computer file. Typically it is measured in bytes with a prefix. The actual amount of disk space consumed by the file depends on the file system....
. MP3-encoded sound is generally CBR, so its quality is defined by its bitrate, in kilobits per second (kbit/s). Quality of Ogg Vorbis-encoded files, which are most commonly VBR
Variable bitrate
Variable bitrate is a term used in telecommunications and computing that relates to the bitrate used in sound or video encoding. As opposed to constant bitrate , VBR files vary the amount of output data per time segment...
, is a decimal value ranging from –1 to 10, with –1 being suitable only for low-quality voice.
The frequency range of sound (in hertz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
) which the equipment is capable of sampling affects sound quality. Humans can hear frequencies ranging from about 20 Hz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
to approximately 20 kHz, so sampling that doesn't extend this far will have a detrimental effect on the resultant quality.
The sound wave being recorded is an analogue
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
wave, that is, it is continuous and has some real value at every instant. The digital
Digital
A digital system is a data technology that uses discrete values. By contrast, non-digital systems use a continuous range of values to represent information...
quantization
Quantization (sound processing)
In signal processing and digital audio, quantization is the process of approximating a continuous range of values by a relatively small set of discrete symbols or integer values...
of the analogue
Analog signal
An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...
sound wave means that much of the continuous sound wave is not recorded. The rate at which the sound is sampled refers to the amount of information the detection equipment records for each second of the sound. The higher the sampling frequency is, the more accurate the resultant samples will be.
The conversion of sample range and sample rate between different pieces of equipment in a sound recording and reproduction system usually lower the quality of the sound.
Bit Rate
Recorded sound is stored in many formats. The need to save space on the storage device is opposed to sound quality. The smaller the sound file for a given recording the poorer the sound quality. This is not true for lossless compressionLossless data compression
Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. The term lossless is in contrast to lossy data compression, which only allows an approximation of the original data to be reconstructed, in exchange...
methods, where the quality is preserved also in smaller sound file sizes.
The sound quality is determined by the bit rate, which is the number of samples per second times the number of bits per sample. It also depends on number of audio channels (mono, stereo
Stereophonic sound
The term Stereophonic, commonly called stereo, sound refers to any method of sound reproduction in which an attempt is made to create an illusion of directionality and audible perspective...
, multichannel
Surround sound
Surround sound encompasses a range of techniques such as for enriching the sound reproduction quality of an audio source with audio channels reproduced via additional, discrete speakers. Surround sound is characterized by a listener location or sweet spot where the audio effects work best, and...
) and compression method.
The most common are lossy compression formats as MP3, OGG Vorbis, AAC
Advanced Audio Coding
Advanced Audio Coding is a standardized, lossy compression and encoding scheme for digital audio. Designed to be the successor of the MP3 format, AAC generally achieves better sound quality than MP3 at similar bit rates....
and many others. For example, MP3 files commonly have a bit rate of 128 kbit/s, because it typically offers adequate audio quality in a relatively small space. MP3 has a maximum bit rate of 320 kbit/s.
By contrast, uncompressed audio as stored on an Audio CD
Red Book (audio CD standard)
Red Book is the standard for audio CDs . It is named after one of the Rainbow Books, a series of books that contain the technical specifications for all CD and CD-ROM formats.The first edition of the Red Book was released in 1980 by Philips and Sony; it was adopted by the Digital Audio Disc...
has a bit rate of 1,411.2 kbit/s, or four times the best MP3 can do, and twelve times that of common MP3 files.
The WAV
WAV
Waveform Audio File Format , is a Microsoft and IBM audio file format standard for storing an audio bitstream on PCs...
, AIFF
AIFF
Audio Interchange File Format is an audio file format standard used for storing sound data for personal computers and other electronic audio devices...
or AU
Au file format
The Au file format is a simple audio file format introduced by Sun Microsystems. The format was common on NeXT systems and on early Web pages. Originally it was headerless, being simply 8-bit µ-law-encoded data at an 8000 Hz sample rate. Hardware from other vendors often used sample rates as...
audio file formats can store audio using various bit rates, in accordance with the number of channels and compression. They often store uncompressed PCM audio with a bit rate of 1,411.2 kbit/s.
There are also lossless compression formats, such as FLAC
FLAC
FLAC is a codec which allows digital audio to be losslessly compressed such that file size is reduced without any information being lost...
, ALAC
Apple Lossless
Apple Lossless Apple Lossless Apple Lossless (also known as ALAC (Apple Lossless Audio Codec), or ALE (Apple Lossless Encoder) is an audio codec developed by Apple Inc. for lossless data compression of digital music. After initially being proprietary for many years, in late 2011 Apple open sourced...
, Monkey's Audio
Monkey's Audio
Monkey's Audio is a file format for audio data compression. Being a lossless format, Monkey's Audio does not discard data during the process of encoding, unlike lossy compression methods such as AAC, MP3, Vorbis and Musepack....
and many others, which preserve quality of original uncompressed audio, but can reduce the needed storage space. Advantageously, these lossless formats can also store metadata
Metadata
The term metadata is an ambiguous term which is used for two fundamentally different concepts . Although the expression "data about data" is often used, it does not apply to both in the same way. Structural metadata, the design and specification of data structures, cannot be about data, because at...
(eg. images, title, artist, album, label, etc.) like lossy formats can, which cannot be done with pure uncompressed formats like WAV, AIFF, AU.
Delivered Audio Quality
Delivered Audio Quality, abbreviated as DAQ, is a measure of audioSound
Sound is a mechanical wave that is an oscillation of pressure transmitted through a solid, liquid, or gas, composed of frequencies within the range of hearing and of a level sufficiently strong to be heard, or the sensation stimulated in organs of hearing by such vibrations.-Propagation of...
quality over a transmission medium. This metric is often used to quantify the quality of audio heard over a radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
system. DAQ levels are defined by the following scale.
- DAQ 1: Unusable. Speech present but not understandable.
- DAQ 2: Speech understandable with considerable effort. Requires frequent repetition due to noise or distortion.
- DAQ 3: Speech understandable with slight effort. Requires occasional repetition due to noise or distortion.
- DAQ 3.4: Speech understandable without repetition. Some noise or distortion present.
- DAQ 4: Speech easily understandable. Little noise or distortion.
- DAQ 5: Perfect. No distortion or noise discernible.
Sources
- Carlos Herrero. Subjective and objective assessment of sound quality: solutions and applications
- TIA {Telecommunications Industry Association} TSB-88-A
- http://www.its.bldrdoc.gov/pub/ntia-rpt/99-358/99-358.pdf
See also
- Audio quality measurementAudio quality measurementAudio quality measurement seeks to quantify the various forms of corruption present in an audio system or device. The results of such measurement are used to maintain standards in broadcasting, to compile specifications, and to compare pieces of equipment....
- Weighting filterWeighting filterA weighting filter is used to emphasise or suppress some aspects of a phenomenon compared to others, for measurement or other purposes.- Audio applications :...
- Equal-loudness contourEqual-loudness contourAn equal-loudness contour is a measure of sound pressure , over the frequency spectrum, for which a listener perceives a constant loudness when presented with pure steady tones. The unit of measurement for loudness levels is the phon, and is arrived at by reference to equal-loudness contours...
- Noise weightingNoise weightingA noise weighting is a specific amplitude-vs.-frequency characteristic that is designed to allow subjectively valid measurement of noise. It emphasises the parts of the spectrum that are most important....
- A-weightingA-weightingA Weighting curve is a graph of a set of factors, that are used to 'weight' measured values of a variable according to their importance in relation to some outcome. The most commonly known example is frequency weighting in sound level measurement where a specific set of weighting curves known as A,...
- ITU-R 468 noise weightingITU-R 468 noise weightingITU-R 468 is a standard relating to noise measurement, widely used when measuring noise in audio systems. The standard defines a weighting filter curve, together with a quasi-peak rectifier having special characteristics as defined by specified tone-burst tests...
- Tape biasTape biasTape bias is the term for two phenomena, DC bias and AC bias, that improve the fidelity of analogue magnetic tape sound recordings. DC bias is the addition of a direct current to the audio signal that is being recorded. AC bias is the addition of an inaudible high-frequency signal to the audio...