Audio quality measurement
Encyclopedia
Audio 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.
measurement, and A-weighted
noise measurement, without any proper consideration of whether these related to subjective effects. The proper approach to measurement, which is largely adopted by broadcasters and other audio professionals, is to first devise measurements that can quantify the various forms of corruption in terms of subjective annoyance to a human listener, ideally the most critical listener based on tests using many suitably rested subjects. Once this is done, measurement has the advantage of not being dependent on a particular listener, or his/her state of hearing on a given day. It also has the advantage of being able to quantify corruption levels that would not be audible to even the most sensitive ear, which is important because a typical audio path from source to listener can involve many items of equipment, and just listening to each is not a guarantee that they will still sound acceptable when cascaded so that all their deficiencies add up.
A measure for testing audio quality for codecs is also given by the Mean Opinion Score
.
FSK
signal for synchronizing purposes, also became CCITT standard O.33 in 1985.
Lindos Electronics
expanded the concept, retaining the FSK concept, and inventing segmented sequence testing, which separated each test into a 'segment' starting with an identifying character transmitted as 110-baud FSK
so that these could be regarded as 'building blocks' for a complete test suited to a particular situation. Regardless of the mix chosen, the FSK provides both identification and synchronization for each segment, so that sequence tests sent over networks and even satellite links are automatically responded to by measuring equipment. Thus TUND represents a sequence made up of four segments which test the alignment level
, frequency response
, noise
and distortion in less than a minute, with many other tests, such as Wow and flutter
, Headroom, and Crosstalk
also available in segments as well as a whole.
The Lindos sequence test system is now a 'de-facto' standard in broadcasting and many other areas of audio testing, with over 25 different segments recognized by Lindos test sets, and the EBU standard is no longer used.
. It relies on the fact that with appropriate choice of frequencies, distortion components and noise can be made to appear between the tones, and measured using digital comb filtering. Even noise and wow and flutter can be extracted from the spectrum in principle.
In practice, though the use of a single brief test is attractive, and might even be used between programmes, this method presents several problems. Digital distortions produce a fine spectrum which can swamp the measurement of true noise in the absence of signal. The composite signal also has a high peak to mean ratio, with peak levels occurring whenever all the tones hit maximum simultaneously. Although the Probability density function
can be controlled to some extent, it is not possible to separate out distortion at high level, from low level distortion. Quite high amounts of the former can be considered acceptable, but low level distortion is more critical.
Fast sequence tests are possible, and there have been attempts to make these appear like jingles for incorporation into broadcast programmes.
The need for measurement
Measurement allows limits to be set and maintained for equipment and signal paths, and different pieces of equipment to be compared. While the issue of measurement is controversial, to the extent that Hi-Fi magazines these days tend to shun measurement in favor of listening tests, it is important to realize that audio quality measurement has in the past got a bad name by failing to produce results that correlated well with listening tests. This was because certain basic measurements were used, such as THDTHD
THD may refer to:* That Handsome Devil, an American alternative rock band* Total harmonic distortion, a measure of the distortion of an audio signal* Total Hi Def , an optical disc format* Th.D. or D.Th...
measurement, and A-weighted
A-weighting
A 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,...
noise measurement, without any proper consideration of whether these related to subjective effects. The proper approach to measurement, which is largely adopted by broadcasters and other audio professionals, is to first devise measurements that can quantify the various forms of corruption in terms of subjective annoyance to a human listener, ideally the most critical listener based on tests using many suitably rested subjects. Once this is done, measurement has the advantage of not being dependent on a particular listener, or his/her state of hearing on a given day. It also has the advantage of being able to quantify corruption levels that would not be audible to even the most sensitive ear, which is important because a typical audio path from source to listener can involve many items of equipment, and just listening to each is not a guarantee that they will still sound acceptable when cascaded so that all their deficiencies add up.
A measure for testing audio quality for codecs is also given by the Mean Opinion Score
Mean Opinion Score
The Mean Opinion Score test has been used for decades in telephony networks to obtain the human user's view of the quality of the network. In multimedia especially when codecs are used to compress the bandwidth requirement , the mean opinion score ...
.
Automated sequence testing
Sequence testing uses a specific sequence of test signals, for frequency response, noise, distortion etc, generated and measured automatically to carry out a complete quality check on a piece of equipment or signal path. A single 32-second sequence was standardized by the EBU in 1985, incorporating 13 tones (40 Hz–15 kHz at −12 dB) for frequency response measurement, two tones for distortion (1024 Hz/60 Hz at +9 dB) plus crosstalk and compander tests. This sequence, which began with a 110-baudBaud
In telecommunications and electronics, baud is synonymous to symbols per second or pulses per second. It is the unit of symbol rate, also known as baud rate or modulation rate; the number of distinct symbol changes made to the transmission medium per second in a digitally modulated signal or a...
FSK
FSK
FSK can have alternative meanings:* Federal Counterintelligence Service, , Federal Counterintelligence Service of Russia)...
signal for synchronizing purposes, also became CCITT standard O.33 in 1985.
Lindos Electronics
Lindos Electronics
Lindos Electronics is a British manufacturer of test equipment for audio quality measurement.Founded in 1979, by Peter J Skirrow, the business was named after the home where development and manufacture began, as is so often the case, in garage and spare rooms...
expanded the concept, retaining the FSK concept, and inventing segmented sequence testing, which separated each test into a 'segment' starting with an identifying character transmitted as 110-baud FSK
FSK
FSK can have alternative meanings:* Federal Counterintelligence Service, , Federal Counterintelligence Service of Russia)...
so that these could be regarded as 'building blocks' for a complete test suited to a particular situation. Regardless of the mix chosen, the FSK provides both identification and synchronization for each segment, so that sequence tests sent over networks and even satellite links are automatically responded to by measuring equipment. Thus TUND represents a sequence made up of four segments which test the alignment level
Alignment level
The alignment level in an audio signal chain or on an audio recording is a defined anchor point that represents a reasonable or typical level...
, frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
, noise
Noise measurement
Noise measurement is carried out in various fields.In acoustics, it can be for the purpose of measuring environmental noise, or part of a test procedure using white noise, or some other specialised form of test signal....
and distortion in less than a minute, with many other tests, such as Wow and flutter
Wow and Flutter
Wow and Flutter is an EP by the post-rock band Stereolab, released in 1994. A limited edition of 3000 7" copies was released with hand-painted covers. Two of the tracks appear on Mars Audiac Quintet, which was released the same year.-Track listing:...
, Headroom, and Crosstalk
Crosstalk measurement
Crosstalk measurement is made on audio systems to determine the amount of signal leaking across from one channel to another.Interchannel crosstalk applies between the two channels of a stereo system, and is usually not very important on modern systems, though it was hard to keep below the desired...
also available in segments as well as a whole.
The Lindos sequence test system is now a 'de-facto' standard in broadcasting and many other areas of audio testing, with over 25 different segments recognized by Lindos test sets, and the EBU standard is no longer used.
Multitone testing
Another approach to automated testing uses a special multi-tone signal to assess all parameters simultaneously, by analyzing the spectrum of the output from the device under testDevice under test
Device under test , also known as unit under test , is a term commonly used to refer to a manufactured product undergoing testing.-In semiconductor testing:...
. It relies on the fact that with appropriate choice of frequencies, distortion components and noise can be made to appear between the tones, and measured using digital comb filtering. Even noise and wow and flutter can be extracted from the spectrum in principle.
In practice, though the use of a single brief test is attractive, and might even be used between programmes, this method presents several problems. Digital distortions produce a fine spectrum which can swamp the measurement of true noise in the absence of signal. The composite signal also has a high peak to mean ratio, with peak levels occurring whenever all the tones hit maximum simultaneously. Although the Probability density function
Probability density function
In probability theory, a probability density function , or density of a continuous random variable is a function that describes the relative likelihood for this random variable to occur at a given point. The probability for the random variable to fall within a particular region is given by the...
can be controlled to some extent, it is not possible to separate out distortion at high level, from low level distortion. Quite high amounts of the former can be considered acceptable, but low level distortion is more critical.
Fast sequence tests are possible, and there have been attempts to make these appear like jingles for incorporation into broadcast programmes.
Measurements needed
- Frequency responseFrequency responseFrequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
- Signal to Noise Ratio
- Damping factorDamping factorIn audio system terminology, the damping factor gives the ratio of the rated impedance of the loudspeaker to the source impedance. Only the resistive part of the loudspeaker impedance is used. The amplifier output impedance is also assumed to be totally resistive...
- Intermodulation distortion
- Common-mode rejection ratioCommon-mode rejection ratioThe common-mode rejection ratio of a differential amplifier is the tendency of the devices to reject the input signals common to both input leads...
- Phase between channels or drivers
- PhasePhase (waves)Phase in waves is the fraction of a wave cycle which has elapsed relative to an arbitrary point.-Formula:The phase of an oscillation or wave refers to a sinusoidal function such as the following:...
input to output - Total harmonic distortionTotal harmonic distortionThe total harmonic distortion, or THD, of a signal is a measurement of the harmonic distortion present and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency...
- THD+N
- Crosstalk
- Slew RateSlew rateIn electronics, the slew rate represents the maximum rate of change of a signal at any point in a circuit.Limitations in slew rate capability can give rise to non linear effects in electronic amplifiers...
- FFT
- SPLSound pressureSound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure caused by a sound wave. Sound pressure can be measured using a microphone in air and a hydrophone in water...
- Gain or dbG
- Flutter measurement (on analog tape systems)
- Rumble measurement
- JitterJitterJitter is the undesired deviation from true periodicity of an assumed periodic signal in electronics and telecommunications, often in relation to a reference clock source. Jitter may be observed in characteristics such as the frequency of successive pulses, the signal amplitude, or phase of...
(on digital systems) - Impulse responseImpulse responseIn signal processing, the impulse response, or impulse response function , of a dynamic system is its output when presented with a brief input signal, called an impulse. More generally, an impulse response refers to the reaction of any dynamic system in response to some external change...
(speakers) (Waterfall plots, MLSSA) (coloration) - LatencyLatency (audio)Latency refers to a short period of delay between when an audio signal enters and when it emerges from a system...
(satellite links and codecs) (sound for live video) - Headroom
See also
- Sound qualitySound qualitySound 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...
- Audio noise measurementAudio noise measurementAudio noise measurement is carried out when assessing the quality of audio equipment, such as is used in recording studios, broadcast studios, and in the home ....
- Distortion measurement
- 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...
- Flutter measurement
- Rumble measurement
- Loudspeaker measurementLoudspeaker measurementLoudspeaker measurement is one of the most difficult aspects of audio quality measurement, and also probably the most relevant, since loudspeakers, because they are transducers, have higher distortion than other audio system components....
- Alignment levelAlignment levelThe alignment level in an audio signal chain or on an audio recording is a defined anchor point that represents a reasonable or typical level...
- Programme levels
- Headroom
- 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...
- Fletcher-Munson curves
- Sound level meterSound level meterSound level meters measure sound pressure level and are commonly used in noise pollution studies for the quantification of almost any noise, but especially for industrial, environmental and aircraft noise. However, the reading given by a sound level meter does not correlate well to...
- NoiseNoiseIn common use, the word noise means any unwanted sound. In both analog and digital electronics, noise is random unwanted perturbation to a wanted signal; it is called noise as a generalisation of the acoustic noise heard when listening to a weak radio transmission with significant electrical noise...
- Lindos ElectronicsLindos ElectronicsLindos Electronics is a British manufacturer of test equipment for audio quality measurement.Founded in 1979, by Peter J Skirrow, the business was named after the home where development and manufacture began, as is so often the case, in garage and spare rooms...
- PEAQPEAQPEAQ is a standardized algorithm for objectively measuring perceived audio quality, developed in 1994-1998 by a joint venture of experts within Task Group 6Q of the International Telecommunication Union . It was originally released as ITU-R Recommendation BS.1387 in 1998 and last updated in 2001...