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Movements, leaders, victories and the continuing fight for equality.
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Explore the people, places, events, achievements, struggles and stories that shaped our journey.
Movements, leaders, victories and the continuing fight for equality.
Innovation, patents, science, technology and world-changing contributions.
Pioneers, champions, Negro Leagues, records, activism and excellence.
Meet the people whose lives, choices and achievements shaped the journey.
Black towns, communities, institutions and places where history happened.
Moments that changed communities, movements, institutions and the nation.
In August 1908, a white mob attacked Springfield, Illinois’s Black community, destroying homes and businesses and lynching two Black men. National outrage over the violence helped spur the movement that created the NAACP the following year.
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
The Fourier transform of a function of time, s ( t ) {\displaystyle s(t)} , is a complex-valued function of frequency, S ( f ) {\displaystyle S(f)} , often referred to as a frequency spectrum. Any linear time-invariant operation on s ( t ) {\displaystyle s(t)} produces a new spectrum of the form H ( f ) ∗ S ( f ) {\displaystyle H(f)*S(f)} , which changes the relative magnitudes and/or angles (phase) of the non-zero values of S ( f ) {\displaystyle S(f)} . Any other type of operation creates new frequency components that may be referred to as spectral leakage in the broadest sense. Sampling, for instance, produces leakage, which we call aliases of the original spectral component. For Fourier transform purposes, sampling is modeled as a product between s ( t ) {\displaystyle s(t)} and a Dirac comb function. The spectrum of a product is the convolution between S ( f ) {\displaystyle S(f)} and another function, which inevitably creates the new frequency components. But the term 'leakage' usually refers to the effect of windowing, which is the product of s ( t ) {\displaystyle s(t)} with a different kind of function, the window function. Window functions happen to have finite duration, but that is not necessary to create leakage. Multiplication by a time-variant function is sufficient.
The phase velocity of a wave is the speed of any wavefront, a surface of constant phase. This is the velocity at which the phase of any constant-frequency component of the wave travels. For such a spectral component, any given phase of the wave (for example, the crest) will appear to travel at the phase velocity. The phase velocity of light waves is not a physically meaningful quantity and is not related to information transfer.
In electronics, rapid single flux quantum (RSFQ) is a digital electronic device that uses superconducting devices, namely Josephson junctions, to process digital signals. In RSFQ logic, information is stored in the form of magnetic flux quanta and transferred in the form of single flux quantum (SFQ) voltage pulses. RSFQ is one family of superconducting or SFQ logic. Others include reciprocal quantum logic (RQL), ERSFQ – energy-efficient RSFQ version that does not use bias resistors, etc. Josephson junctions are the active elements for RSFQ electronics, just as transistors are the active elements for semiconductor electronics. RSFQ is a classical digital, not quantum computing, technology. RSFQ is very different from the CMOS transistor technology used in conventional computers: Superconducting devices require cryogenic temperatures. picosecond-duration SFQ voltage pulses produced by Josephson junctions are used to encode, process, and transport digital information instead of the voltage levels produced by transistors in semiconductor electronics. SFQ voltage pulses travel on superconducting transmission lines which have very small, and usually negligible, dispersion if no spectral component of the pulse is above the frequency of the energy gap of the superconductor. In the case of SFQ pulses of 1 ps, it is possible to clock the circuits at frequencies of the order of 100 GHz (one pulse every 10 picoseconds). An SFQ pulse is produced when magnetic flux through a superconducting loop containing a Josephson junction changes by one flux quantum, Φ0 as a result of the junction switching. SFQ pulses have a quantized area ʃV(t)dt = Φ0 ≈ 2.07×10−15 Wb = 2.07 mV⋅ps = 2.07 mA⋅pH due to magnetic flux quantization, a fundamental property of superconductors. Depending on the parameters of the Josephson junctions, the pulses can be as narrow as 1 ps with an amplitude of about 2 mV, or broader (e.g., 5–10 ps) with correspondingly lower amplitude. The typical value of the pulse amplitude is approximately 2IcRn, where IcRn is the product of the junction critical current, Ic, and the junction damping resistor, Rn. For Nb-based junction technology IcRn is on the order of 1 mV.
In psychoacoustics and signal processing, a pure tone is a sound or other signal with a sinusoidal waveform; that is, a sine wave of constant frequency, phase-shift, and amplitude. A pure tone has the property – unique among real-valued wave shapes – that its wave shape is unchanged by linear time-invariant systems; that is, only the phase and amplitude change between such a system's pure-tone input and its output. Sine and cosine waves can be used as basic building blocks of more complex waves. As additional sine waves having different frequencies are combined, the waveform transforms from a sinusoidal shape into a more complex shape. When considered as part of a whole spectrum, a pure tone may also be called a spectral component. In clinical audiology, pure tones are used for pure-tone audiometry to characterize hearing thresholds at different frequencies. Sound localization is often more difficult with pure tones than with other sounds.
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.