ITU-R 468 noise weighting

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The ITU-R 468-weighting curve (originally defined in CCIR recommendation 468) is widely used when measuring noise in audio systems, especially in the UK, Europe, and former countries of the British Empire such as Australia and South Africa. It is less well known in the USA where A-weighting has always been used.

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[edit] Explanation

While most audio engineers are familiar with the A-weighting curve, which was based on the 40 phon equal-loudness contour derived initially by Fletcher and Munson (1933) the later CCIR-468 weighting curve, now supported as an ITU standard is less well known outside of the UK and Europe. Originally incorporated into an ANSI standard for sound level meters, A-weighting was never specifically intended for the measurement of the more random (near-white or pink) noise in electronic equipment, though it came to be used for this purpose. It is now known than the human ear responds quite differently to random noise, and it is this difference that gave rise to the 468-weighting, which became widely used by broadcasters throughout Britain, Europe, and former British Commmonwealth countries, where engineers were heavily influenced by BBC test methods.

[edit] History

[edit] Original research

Developments in the 1960s, in particular the spread of FM broadcasting and the development of the compact audio cassette with Dolby-B Noise Reduction, alerted engineers to the need for a weighting curve that gave subjectively meaningful result 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 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 led to BBC Research Department 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. 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 stnadard which defined a new weighting curve and quasi-pk rectifier.

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 deckss (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 10dB improvement in noise obseved subjectively on cassette recordings when using Dolby B while other methods could indicate an actual worsening in some circumstances, owing to the fact that they did not sufficiently attenuate noise above 10kHz.

[edit] 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 CCIR468-4 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 1970's. When the CCIR ceased to exist, the standard was officially taken over by the ITU-R (International Telecommunication Union). 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. 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 cells 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 measurement is also called psophometric weighting.

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.

[edit] Uptake

Engineers in the USA never 'caught on' to 468-weighting, probably because for many decades they were part of a strong independent manufacturing economy that tended to import little from abroad. For the same reason they never adopted the PPM (Peak programme meter), which also came out of BBC Research. Nevertheless, 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.

[edit] 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 is known to be far more audible than normal THD(Total harmonic distortion) measurements would suggest.

Measurements of microphone noise are easier using 468-weighting because it emphasises the audible noise more in comparison to low-frequency noise. A-weighted microphone measurements require extremely quiet conditions to avoid the effects of slow pressure variations caused by wind and air conditioning.

[edit] References

  • 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

[edit] See also

[edit] External links