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Transcript
From "hearing"sounds to "visualizing"sounds VU (Volume Unit) Meters and Loudness Meters Introduction With the transition of analog TV broadcast to digital TV broadcast, the digitized production of TV programs, the resulting availability of wide dynamic range, and the rapid advancement of audio equipments, the sophisticated sound technology has become widely used. TV program creators and particularly TV commercial creators try to increase sound level to attract the audience. As a result, the sound levels vary greatly between channels, between programs, or between a program and commercial break, requiring the viewers to adjust TV volume at every program or at every channel change. To reduce the annoyance to TV viewers, loudness meter measuring human-perceived sound level has been required instead of conventional VU meter measuring sound level in electrical strength. Once, there were several sound meters to indicate human-perceived sound level such as noise meters. Recently, however, international organizations such as ITU (International Telecommunication Union) and EBU (European Broadcasting Union) developed standards that describe how to measure sound level (ITU BS.1770-2, R-128). Also in Japan, ARIB (Association of Radio Industries and Businesses) established a standard called ARIB TR-B 32. This guide explains briefly about "sound", "volume meter" and “loudness" to help you to better understand. 2 Quick review of sound and types of G uidance 1 Quick Review Let’s quickly go through what is sound. In general, sound is a series of compression waves that move through air (i.e., changes in pressure). Sound is composed of 3 factors. ❶Sound Intensity (Strength) ❷Frequency (High-Low) ❸Tone We perceive sounds by detecting vibration with the hearing organ - the ear. 3 components of sound are outlined below. 1 Sound Intensity (Strength) The sound intensity is the strength of compression waves. 2 Frequency (High-Low) The high and low of sound is the frequency of compression waves. Human ear has a range of audible frequency, i.e., audibility range. 3 Tone T he tone is a feature that different sounding bodies (instruments) have different sound tones even at the same frequency. An instrument produces the instrument-specific sound including complex frequency content so that we are able to identify the peculiar sounds by each instrument. 3 loudness meters 2 Types of Volume Meters Volume meters have 2 major types: ❶Measured by vibration (pressure) ・Noise meter ・S ound pressure meter (SPL Meter: Sound Pressure Level Meter) Circuit inside VU meter sensitivity control resistor Set to 0VU at reference input rectifier circuit germanium diode ❷Measured by electrical signals ・VU Meter (Volume Unit Meter) ・PPM Meter (Peak Program Meter) ・Loudness Meter The below describes the second type of volume moving-coil meter sensitivity of approx. 80μA damping resistor control not to overswing the needle meters. They are generally used for adjusting or monitoring sound level on recording or broadcasting. The rectifier in VU meter is a simple circuit with a diode. VU meter provides wide-range measurement capability from -20VU to -3VU (dB), by the logarithmic i Volume Unit Meter (VU Meter) characteristics of diode at an extremely low input voltage. The indicator uses a high-sensitivity DC meter in a range from 80μA to100μA. VU meter is adjusted to have 300ms response time to a step signal in a range from –20VU to 0VU. Considering its slow response time (around 300ms), the meter tends to ignore short-term sound and to be actually closer to the human ear's perception of sound level. Due to those features, VU meter is still one of the most common volume meters for measuring sound signals. VU meter was jointly developed by Bell Labs, CBS and NBC in 1939 for monitoring and standardizing the levels of telephone lines. The VU meter consists of a high ii PPM (Peak Program Meter) sensitivity DC meter and a rectifier circuit converting Compared to VU Meter, Peak Program Meter, mostly sound signals from AC into DC. used in Europe, has shorter response time of 10ms for monitoring the peak on sounds recorded. 4 The needs for loudness meter and G uidance 3 Why We Need Loudness Meters? Until recently, VU meter and PPM were the only options available for“visualizing”sound level. Meanwhile, with the transition from analog to digital sound, audio technologies are rapidly evolving. As digital sound offers wider dynamic range in full bit depth than analog sound and easy-to-operate sound processing equipments such as limit controller is available, a technique increasing sound level without significant changes in electrical signal level is becoming commonplace. Through these situational changes, the volume indicated by VU meter is not always matched with the loudness level perceived by listeners and the limit of VU meters has become apparent. Also in broadcasting, digitization brings variations in volume levels between programs, commercial breaks, or channels, forcing viewers to adjust volume at every commercial break or at every channel change. To address those issues, loudness meter has been developed to meet demand for accurately measuring human-perceived volume incorporating not only electrical level but also hearing characteristics. 5 the outline of loudness 4 What is Loudness? Loudness is sound volume based on human aural Figure 2 Equal-loudness contour ISO226-1996/2003 characteristics. ❷Masking characteristic ❸Time-based characteristic (Figure 1) Human aural characteristics are determined by 3 attributes: Revised Previous Sound Pressure Level[dB] ❶Hearing frequency characteristic Minimum Level 1 Hearing frequency characteristic is illustrated by an equal-loudness contour described in ISO2262003 (Figure 2) . Frequency[Hz] The equal-loudness contour is a curve for which a listener perceives a constant loudness over the frequency spectrum, with using a given loudness at 2 Masking Characteristic 1kHz (e.g., 60phon) as reference for comparison. The The sound occurred right after a loud sound is difficult curve indicates that low frequency sounds appear to to be heard by human ear. This phenomenon is called be less loud than higher frequency and high frequency as masking effect. sounds are less stable. The curves are steep in lower volume, while the curves become flat in higher volume. Figure 1 3 Time-based characteristic Short duration sounds are perceived quieter than longer Sound signal sounds of exactly the same level. For instance, even when a speech gives listeners an impression that the sound level stays the same all the times, the needle Hearing frequency characteristic on VU meter can be swinging. It shows that humanperceived sound level varies by the sound duration. Loudness meter is a device that is designed to measure Masking characteristic the human-perceived loudness by taking account of the hearing characteristics. Time-based characteristic Loudness level 6 Types and Standards of Loudness Meter G uidance 5 Types of Loudness Meters Mainly, there are 4 standards for measuring loudness level, and loudness meter products complied with those standards have been developed. ❶ITU-R BS.1770-2/1771 ❷EBU Tech 3341/R-128 ❸ARIB TR-32 ❹ISO532B ❺Proprietary While ITU-R BS.1770-2 and EBU Tech 3341/R-128 are international standards,“Operational Guidelines for Loudness of Digital Television Programs”(ARIB TR-B32) is Japanese standard established in March 2011 that was based on ITU-R BS.1770-2. ITU is the acronym for the International Telecommunications Union, which is a specialized agency of the United Nations. EBU is the acronym for the European Broadcasting Union, which is an alliance of public service media organizations, with members of Europe and North Africa broadcasters. Yamaki LLM-D1012 is a major product complied with ISO532B. Those standards are outlined below. 7 6 Standards of Loudness Meters 1 ITU -R BS.1770 -2 the loudness value is averaged on all loudness values T h i s s t a n d a r d i s f oc u s e d o n h e a r i n g f r e q u e n c y captured in a whole content. Then, in order to reduce characteristics. the effects caused by intermittent sounds, the values The concept of loudness calculation under this standard equal or less than -10LU (Relative-Gate) are excluded is illustrated below. from the average just calculated. And the resulted value ITU-R BS.1770-2 also includes the recommendations is again averaged to derive the final averaged loudness how to measure audio level for 5.1ch surround-sound value (Program Loudness). (LFE channel not included). Block diagram of multichannel loudness method Pre-filter RLB RMS Pre-filter RLB RMS Pre-filter RLB RMS Pre-filter RLB RMS Pre-filter RLB RMS 2 EBU Tech 3341/R-128 The standard is established by EBU Ploud group for loudness normalization. Loudness Essentially complied with ITU-R BS.1770-2, it defines its own standard concerning EBU Mode. 【What is EBU Mode?】 EBU meter has 3 measurement modes as below: K-filter ● Momentary loudness loudness ● Integrated loudness Relative Lebel[dB] ● Short-term The standard also recommends how to measure Loudness Range. Momentary loudness: Integrated over 400ms ● ● Short-term loudness: Integrated over 3s ● Integrated loudness: Same as defined in ITU-R The standard requires including the functions to Frequency[Hz] start/pause/continue/reset measuring the integrated loudness. Hearing frequency characteristic is approximated by K-filter being composed of Pre-filter *1 and RLB*2 , as shown in above. 3 ARIB TR-B32 “Operational Guidelines for Loudness of Digital Television It also tends to reflect time-based characteristic with Programs”(ARIB TR-B32) was defined in March 2011. The the calculation of loudness level by measuring volume general requirements are conformed to ITU-R BS.1770-2. with every 400ms intervals. These intervals overlap by 75 percent (75%: 100:300ms) for the leveling of Please refer to each written standards for more details. measured values and a new value is obtained every 100ms. After excluding values equal to or less than -70 LKFS (Safety-Gate) to mitigate effects by silent duration, *1 Pre-filter: filter accounting for the acoustic effects of the head. (by +4dB from 1k to 4kHz) *2 RLB: Revised Low-frequency B-weighting 8