CCIR (ITU) 468 Noise Weighting
   HOME

TheInfoList



Click Here for Items Related To - CCIR (ITU) 468 Noise Weighting
OR:
CCIR (ITU) 468 Noise Weighting on:   [Wikipedia]   [Google]   [Amazon]

ITU-R 468 (originally defined in CCIR recommendation 468-4, therefore formerly also known as CCIR weighting; sometimes referred to as CCIR-1k) is a standard relating to noise measurement, widely used when measuring noise in audio systems. The standard, now referred to as ITU-R BS.468-4, defines a
weighting filter A weighting filter is used to emphasize or suppress some aspects of a phenomenon compared to others, for measurement or other purposes. Audio applications In each field of audio measurement, special units are used to indicate a weighted measure ...
curve, together with a
quasi-peak A quasi-peak detector is a type of electronic detector or rectifier. Quasi-peak detectors for specific purposes have usually been standardized with mathematically precisely defined dynamic characteristics of attack time, integration time, and dec ...
rectifier having special characteristics as defined by specified tone-burst tests. It is currently maintained by the
International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
who took it over from the CCIR. It is used especially in the UK, Europe, and former countries of the
British Empire The British Empire was composed of the dominions, colonies, protectorates, mandates, and other territories ruled or administered by the United Kingdom and its predecessor states. It began with the overseas possessions and trading posts e ...
such as Australia and South Africa. It is less well known in the USA where A-weighting has always been used.
M-weighting ITU-R 468 (originally defined in CCIR recommendation 468-4, therefore formerly also known as CCIR weighting; sometimes referred to as CCIR-1k) is a standard relating to noise measurement, widely used when measuring noise in audio systems. The s ...
is a closely related filter, an offset version of the same curve, without the quasi-peak detector.


Explanation

The A-weighting curve was based on the 40 phon equal-loudness contour derived initially by Fletcher and Munson (1933). Originally incorporated into an ANSI standard for sound level meters, A-weighting was intended for measurement of the audibility of sounds by themselves. It was never specifically intended for the measurement of the more random (near-
white White is the lightest color and is achromatic (having no hue). It is the color of objects such as snow, chalk, and milk, and is the opposite of black. White objects fully reflect and scatter all the visible wavelengths of light. White o ...
or
pink Pink is the color of a namesake flower that is a pale tint of red. It was first used as a color name in the late 17th century. According to surveys in Europe and the United States, pink is the color most often associated with charm, politeness, ...
) noise in electronic equipment, though has been used for this purpose by most microphone manufacturers since the 1970s. The human ear responds quite differently to clicks and bursts of random noise, and it is this difference that gave rise to the CCIR-468 weighting curve (now supported as an
ITU The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
standard), which together with quasi-peak measurement (rather than the rms measurement used with A-weighting) became widely used by broadcasters throughout Britain, Europe, and former
British Commonwealth The Commonwealth of Nations, simply referred to as the Commonwealth, is a political association of 56 member states, the vast majority of which are former territories of the British Empire. The chief institutions of the organisation are the Co ...
countries, where engineers were heavily influenced by
BBC #REDIRECT BBC #REDIRECT BBC Here i going to introduce about the best teacher of my life b BALAJI sir. He is the precious gift that I got befor 2yrs . How has helped and thought all the concept and made my success in the 10th board exam. ...
...
test methods. Telephone companies worldwide have also used methods similar to 468 weighting with quasi-peak measurement to describe objectionable interference induced in one telephone circuit by switching transients in another.


History


Original research

Developments in the 1960s, in particular the spread of
FM broadcasting FM broadcasting is a method of radio broadcasting using frequency modulation (FM). Invented in 1933 by American engineer Edwin Armstrong, wide-band FM is used worldwide to provide high fidelity sound over broadcast radio. FM broadcasting is cap ...
and the development of the compact audio cassette with
Dolby-B Noise Reduction A Dolby noise-reduction system, or Dolby NR, is one of a series of noise reduction systems developed by Dolby Laboratories for use in analog audio tape recording. The first was '' Dolby A'', a professional broadband noise reduction sy ...
, alerted engineers to the need for a weighting curve that gave subjectively meaningful results on the typical random noise that limited the performance of broadcast circuits, equipment and radio circuits. A-weighting was not giving consistent results, especially on FM radio transmissions and Compact Cassette recording where
preemphasis Typically, prior to some process, such as transmission over cable, or recording to phonograph record or tape, the input frequency range most susceptible to noise is boosted. This is referred to as "pre-emphasis"before the process the signal will u ...
of high frequencies was resulting in increased noise readings that did not correlate with subjective effect. Early efforts to produce a better weighting curve led to a DIN standard that was adopted for European Hi-Fi equipment measurement for a while. Experiments in the
BBC #REDIRECT BBC #REDIRECT BBC Here i going to introduce about the best teacher of my life b BALAJI sir. He is the precious gift that I got befor 2yrs . How has helped and thought all the concept and made my success in the 10th board exam. ...
...
led to
BBC Research Department BBC Research & Development is the technical research department of the BBC. Function It has responsibility for researching and developing advanced and emerging media technologies for the benefit of the corporation, and wider UK and European ...
Report EL-17, ''The Assessment of Noise in Audio Frequency Circuits'', in which experiments on numerous test subjects were reported, using a variety of noises ranging from clicks to tone-bursts to
pink noise Pink noise or noise is a signal or process with a frequency spectrum such that the power spectral density (power per frequency interval) is inversely proportional to the frequency of the signal. In pink noise, each octave interval (halving ...
. Subjects were asked to compare these with a 1 kHz tone, and final scores were then compared with measured noise levels using various combinations of weighting filter and quasi-peak detector then in existence (such as those defined in a now discontinued German DIN standard). This led to the CCIR-468 standard which defined a new weighting curve and quasi-peak rectifier. The origin of the current ITU-R 468 weighting curve can be traced to 1956. The 1968 BBC EL-17 report discusses several weighting curves, including one identified as ''D.P.B.'' which was chosen as superior to the alternatives: A.S.A, C.C.I.F and O.I.R.T. The report's graph of the DPB curve is identical to that of the ITU-R 486 curve, except that the latter extends to slightly lower and higher frequencies. The BBC report states that this curve was given in a "contribution by the D.B.P. (The Telephone Administration of the Federal German Republic) in the Red Book Vol. 1 1957 covering the first plenary assembly of the CCITT (Geneva 1956)". D.B.P. is
Deutsche Bundespost The Deutsche Bundespost (German federal post office) was a German state-run postal service and telecommunications business founded in 1947. It was initially the second largest federal employer during its time. After staff reductions in the 19 ...
, the German post office which provides telephone service in Germany as the GPO does in the UK. The BBC report states "this characteristic is based on subjective tests described by Belger." and cites a 1953 paper by E. Belger. Dolby Laboratories took up the new CCIR-468 weighting for use in measuring noise on their noise reduction systems, both in cinema (Dolby A) and on cassette decks (Dolby B), where other methods of measurement were failing to show up the advantage of such noise reduction. Some Hi-Fi column writers took up 468 weighting enthusiastically, observing that it reflected the roughly 10 dB improvement in noise observed subjectively on cassette recordings when using Dolby B while other methods could indicate an actual worsening in some circumstances, because they did not sufficiently attenuate noise above 10 kHz.


Standards

CCIR Recommendation 468-1 was published soon after this report, and appears to have been based on the BBC work. Later versions up to CCIR 468-4 differed only in minor changes to permitted tolerances. This standard was then incorporated into many other national and international standards (IEC, BSI, JIS, ITU) and adopted widely as the standard method for measuring noise, in broadcasting, professional audio, and ' Hi-Fi' specifications throughout the 1970s. When the CCIR ceased to exist, the standard was officially taken over by the
ITU-R The ITU Radiocommunication Sector (ITU-R) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU) and is responsible for radio communications. Its role is to manage the international radio-frequency sp ...
(
International Telecommunication Union The International Telecommunication Union is a specialized agency of the United Nations responsible for many matters related to information and communication technologies. It was established on 17 May 1865 as the International Telegraph Unio ...
). Current work on this standard occurs primarily in the maintenance of IEC 60268, the international standard for sound systems. The CCIR curve differs greatly from A-weighting in the 5 to 8 kHz region where it peaks to +12.2 dB at 6.3 kHz, the region in which we appear to be extremely sensitive to noise. While it has been said (incorrectly) that the difference is due to a requirement for assessing noise intrusiveness in the presence of programme material, rather than just loudness, the BBC report makes clear the fact that this was not the basis of the experiments. The real reason for the difference probably relates to the way in which our ears analyse sounds in terms of spectral content along the
cochlea The cochlea is the part of the inner ear involved in hearing. It is a spiral-shaped cavity in the bony labyrinth, in humans making 2.75 turns around its axis, the modiolus. A core component of the cochlea is the Organ of Corti, the sensory or ...
. This behaves like a set of closely spaced filters with a roughly constant Q factor, that is, bandwidths proportional to their centre frequencies. High frequency
hair cell Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates, and in the lateral line organ of fishes. Through mechanotransduction, hair cells detect movement in their environment. ...
s would therefore be sensitive to a greater proportion of the total energy in noise than low frequency hair cells. Though hair-cell responses are not exactly constant Q, and matters are further complicated by the way in which the brain integrates adjacent hair-cell outputs, the resultant effect appears roughly as a tilt centred on 1 kHz imposed on the A-weighting. Dependent on spectral content, 468-weighted measurements of noise are generally about 11 dB higher than A-weighted, and this is probably a factor in the recent trend away from 468-weighting in equipment specifications as cassette tape use declines. It is important to realise that the 468 specification covers both weighted and 'unweighted' (using a 22 Hz to 22 kHz 18 dB/octave bandpass filter) measurement and that both use a very special quasi-peak rectifier with carefully devised dynamics (A-weighting uses RMS detection for no particular reason). Rather than having a simple 'integration time' this detector requires implementation with two cascaded 'peak followers' each with different attack time-constants carefully chosen to control the response to both single and repeating tone-bursts of various durations. This ensures that measurements on impulsive noise take proper account of our reduced hearing sensitivity to short bursts. This
quasi-peak A quasi-peak detector is a type of electronic detector or rectifier. Quasi-peak detectors for specific purposes have usually been standardized with mathematically precisely defined dynamic characteristics of attack time, integration time, and dec ...
measurement is also called
psophometric weighting Psophometric weighting refers to any weighting curve used in the measurement of noise. In the field of audio engineering it has a more specific meaning, referring to noise weightings used especially in measuring noise on telecommunications circuits ...
. This was once more important because outside broadcasts were carried over 'music circuits' that used telephone lines, with clicks from Strowger and other electromechanical telephone exchanges. It now finds fresh relevance in the measurement of noise on computer 'Audio Cards' which commonly suffer clicks as drives start and stop.


Present usage of 468-weighting

468-weighting is also used in weighted distortion measurement at 1 kHz. Weighting the distortion residue after removal of the fundamental emphasises high-order harmonics, but only up to 10 kHz or so where the ears response falls off. This results in a single measurement (sometimes called distortion residue measurement) which has been claimed to correspond well with subjective effect even for power amplifiers where
crossover distortion Crossover distortion is a type of distortion which is caused by switching between devices driving a load. It is most commonly seen in complementary, or "push-pull", Class-B amplifier stages, although it is occasionally seen in other types of circu ...
is known to be far more audible than normal THD (
Total harmonic distortion The total harmonic distortion (THD or THDi) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion fact ...
) measurements would suggest. 468-weighting is still demanded by the BBC and many other broadcasters, with increasing awareness of its existence and the fact that it is more valid on random noise where pure tones do not exist. Often both A-weighted and 468-weighted figures are quoted for noise, especially in microphone specifications. While not intended for this application, the 468 curve has also been used (offset to place the 0 dB point at 2 kHz rather than 1 kHz) as "M weighting" in standards such as ISO 21727 intended to gauge loudness or annoyance of cinema soundtracks. This application of the weighting curve does not include the quasi-peak detector specified in the ITU standard. The M-weighting is available in Avee Player version 1.3.0 or later.


Summary of specification

Note: this is not the full definitive standard.


Weighting curve specification (weighted measurement)

The weighting curve is specified by both a circuit diagram of a weighting network and a table of amplitude responses. Above is the ITU-R 468 Weighting Filter Circuit Diagram. The source and sink impedances are both 600 ohms (resistive), as shown in the diagram. The values are taken directly from the ITU-R 468 specification. Note that since this circuit is purely passive, it cannot create the additional 12 dB gain required; any results must be corrected by a factor of 8.1333, or +18.2 dB. Table of amplitude responses: The values of the amplitude response table slightly differ from those resulting from the circuit diagram, e.g. because of the finite resolution of the numerical values. In the standard it is said that the 33.06 nF capacitor may be adjusted or an active filter may be used. Modeling at hand the circuit above and some calculus give this formula to get the amplitude response in dB for any given frequency value : :R_\mathrm(f) = \frac :\mathrm(f) = 18.2+20\log_\left(R_\mathrm(f)\right) where :h_1(f)=-4.737338981378384\cdot10^ \, f^6 + 2.043828333606125\cdot10^ \, f^4 - 1.363894795463638\cdot10^ \, f^2 + 1 :h_2(f)=1.306612257412824\cdot10^ \, f^5 - 2.118150887518656\cdot10^ \, f^3 + 5.559488023498642\cdot10^ \, f


Tone-burst response requirements

5 kHz single bursts:


Repetitive tone-burst response

5 ms, 5 kHz bursts at repetition rate:


Unweighted measurement

Uses 22 Hz HPF and 22 kHz LPF 18 dB/decade or greater (Tables to be added)


See also

*
Noise weighting A 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. Usually, noise means audible noise, in audio ...
*
Audio system measurements Audio system measurements are a means of quantifying system performance. These measurements are made for several purposes. Designers take measurements so that they can specify the performance of a piece of equipment. Maintenance engineers mak ...


References


Further reading

* ''Audio Engineer's Reference Book'', 2nd Ed 1999, edited Michael Talbot Smith, Focal Press * ''An Introduction to the Psychology of Hearing'' 5th ed, Brian C. J. Moore, Elsevier Press


External links


AES pro audio reference definition of ITU-R 468-weighting


Circuit diagrams {{DEFAULTSORT:Itu-R Bs.468 Audio engineering Noise Sound
ITU-R 468 ITU-R 468 (originally defined in CCIR recommendation 468-4, therefore formerly also known as CCIR weighting; sometimes referred to as CCIR-1k) is a standard relating to noise measurement, widely used when measuring noise in audio systems. The s ...