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In 1968 Shirley Chisholm became the first Black woman elected to the United States Congress. In 1972 she launched a campaign for the Democratic presidential nomination, breaking another political barrier.
MORE →Reflects the personal views, recollections, and perspective of the author, Mike Davis.
This is a personal recollection on the Move fire on May 13, 1985
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A peak meter is a type of measuring instrument that visually indicates the instantaneous level of an audio signal that is passing through it (a sound level meter). In sound reproduction, the meter, whether peak or not, is usually meant to correspond to the perceived loudness of a particular signal. The term peak is used to denote the meter's ability, regardless of the type of visual display, to indicate the highest output level at any instant.
A peak-reading electrical instrument or meter is one which measures the peak value of a waveform, rather than its mean value or RMS value.
As an example, when making audio recordings it is desirable to use a recording level that is just sufficient to reach the maximum capability of the recorder at the loudest sounds, regardless of the average sound level. A peak-reading meter is typically used to set the recording level.[1]
In modern audio equipment, peak meters are usually made up of a series of LEDs (small lights) that are placed in a vertical or horizontal bar and lit up sequentially as the signal increases. They typically have ranges of green, yellow, and red, to indicate when a signal is starting to overload.
A meter can be implemented with a classic moving needle device such as those on older analog equipment (similar in appearance in some ways to a pressure gauge on a bicycle pump), or by other means. Older equipment used actual moving parts instead of lights to indicate the audio level. Because of the mass of the moving parts and mechanics, the response time of these older meters could have been anywhere from a few milliseconds to a second or more. Thus, the meter might not ever accurately reflect the signal at every instant of time, but the constantly changing level, combined with the slower response time, led to more of an average indication.
By comparison, a peak meter is designed to respond so quickly that the meter display reacts in exact proportion to the voltage of the audio signal. This can be useful in many applications, but the human ear works much more like an average meter than a peak meter. The analog VU meters are actually closer to the human ear's perception of sound level because the response time was intentionally slow - around 300 milliseconds,[2] and thus, many audio engineers and sound professionals prefer to use older analog style metering[citation needed] because it more accurately relates to what a human listener will experience in terms of relative loudness.
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A peak programme meter (PPM) is an instrument used in professional audio that indicates the level of an audio signal. Different kinds of PPM fall into broad categories: True peak programme meter. This shows the peak level of the waveform, no matter how brief its duration. Quasi peak programme meter (QPPM). This only shows the true level of the peak if it exceeds a certain duration, typically a few milliseconds. On peaks of shorter duration, it indicates less than the true peak level. The extent of the shortfall is determined by the 'integration time'. Sample peak programme meter (SPPM). This is a PPM for digital audio. It shows only peak sample values, not true waveform peaks (which may fall between samples and may be higher in amplitude). It may have either a 'true' or a 'quasi' integration characteristic. Over-sampling peak programme meter. This is a sample PPM that first oversamples the signal, typically by a factor of four, to alleviate the problems of a basic sample PPM. In professional use, which requires consistent level measurements across an industry, audio level meters often comply with a formal standard. This ensures that all compliant meters indicate the same level for a given audio signal. The principal standard for PPMs is IEC 60268-10. It describes two different quasi-PPM designs that have roots in meters originally developed in the 1930s for the AM radio broadcasting networks of Germany (Type I) and the United Kingdom (Type II). The term Peak Programme Meter usually refers to these IEC-specified types and similar designs. Though originally designed for monitoring analogue audio signals, these PPMs are now also used with digital audio. PPMs do not provide effective loudness monitoring. Newer types of meters do, and there is now a push within the broadcasting industry to move away from the traditional level meters described in this article to two new types: loudness meters based on EBU Tech. 3341 and oversampling true PPMs. The former would be used to standardise broadcast loudness to −23 LUFS and the latter to prevent digital clipping.
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 decay time or fall-back time. Quasi-peak detectors play an important role in electromagnetic compatibility (EMC) testing of electronic equipment, where allowed levels of electromagnetic interference (EMI), also called radio frequency interference (RFI), are given with reference to measurement by a specified quasi-peak detector. This was originally done because the quasi-peak detector was believed to better indicate the subjective annoyance level experienced by a listener hearing impulsive interference to an AM radio station. Over time standards incorporating quasi-peak detectors as the measurement device were extended to frequencies up to 1 GHz, although there may not be any justification beyond previous practice for using the quasi-peak detector to measure interference to signals other than AM radio. The quasi-peak detector parameters to be used for EMC testing vary with frequency. Both CISPR and the U.S. Federal Communications Commission (FCC) limit EMI at frequencies above 1 GHz with reference to an average-power detector, rather than quasi-peak detector. Conceptually, a quasi-peak detector for EMC testing works like a peak detector followed by a lossy integrator. A voltage impulse entering a narrow-band receiver produces a short-duration burst oscillating at the receiver centre frequency. The peak detector is a rectifier followed by a low-pass filter to extract a baseband signal consisting of the slowly (relative to the receiver centre frequency) time-varying amplitude of the impulsive oscillation. The following lossy integrator has a rapid rise time and longer fall time, so the measured output for a sequence of impulses is higher when the pulse repetition rate is higher. The quasi-peak detector is calibrated to produce the same output level as a peak-power detector when the input is a continuous wave tone. The CISPR quasi-peak detector is used in EMC testing and is defined in Publication 16 of the International Special Committee on Radio Interference (CISPR) of the International Electrotechnical Commission (IEC). The CISPR quasi-peak detector applied to most conducted emissions measurements (0.15–30 MHz) is a detector with an attack time of 1 ms, a decay time of 160 ms and an IF filter setting of 9 kHz. The quasi-peak detector applied to most radiated emissions measurements (30–1000 MHz) has an attack time of 1 ms, a decay time 550 ms and an IF filter bandwidth of 120 kHz. In audio quality measurement, quasi-peak rectifiers are specified in several standards. For example ITU-R 468 noise weighting uses a special rectifier incorporating two cascaded charging time constants. The PPM or peak programme meter used to measure programme levels is actually a quasi-peak reading meter, again with precisely defined dynamics. Flutter measurement also involves a standardised quasi-peak reading meter. In every case the dynamics are chosen to reflect the sensitivity of human hearing to brief sounds, ignoring those so brief that we do not perceive them, and weighting those of intermediate duration according to audibility.
Loudness monitoring of programme levels is needed in radio and television broadcasting, as well as in audio post production. Traditional methods of measuring signal levels, such as the peak programme meter and VU meter, do not give a subjectively valid measure of loudness that many would argue is needed to optimise the listening experience when changing channels or swapping disks. The need for proper loudness monitoring is apparent in the loudness war that is now found everywhere in the audio field, and the extreme compression that is now applied to programme levels.
A peak meter is a type of measuring instrument that visually indicates the instantaneous level of an audio signal that is passing through it (a sound level meter). In sound reproduction, the meter, whether peak or not, is usually meant to correspond to the perceived loudness of a particular signal. The term peak is used to denote the meter's ability, regardless of the type of visual display, to indicate the highest output level at any instant. A peak-reading electrical instrument or meter is one which measures the peak value of a waveform, rather than its mean value or RMS value. As an example, when making audio recordings it is desirable to use a recording level that is just sufficient to reach the maximum capability of the recorder at the loudest sounds, regardless of the average sound level. A peak-reading meter is typically used to set the recording level.
Before the 1921 destruction of Tulsa’s Greenwood District, Black residents had created a remarkable center of business and community life. The district included stores, professional offices, entertainment venues and homes owned by Black citizens. Understanding Greenwood means learning what was built—not only what was burned.
MORE →Mae Jemison, aboard Space Shuttle Endeavour in 1992.
