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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
A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector is an adjustable band-pass filter that blocks troublesome out-of-tune frequencies from passing through from the antenna into the radio receiver or preamplifier that otherwise would be directly connected to the antenna.
A preselector improves the performance of nearly any receiver, but is especially helpful[a] to receivers with broadband front-ends that are prone to overload, such as scanners, wideband software-defined radio receivers, ordinary consumer-market shortwave and AM broadcast receivers – particularly with receivers operating on frequencies where static is pervasive – below 10~20 MHz (lower-frequency half of the short waves, and all of medium waves, long waves, and longer wavelengths). Sometimes faint signals that occupy a very narrow frequency span (such as radiotelegraph or 'CW') can be heard more clearly if the receiving bandwidth is made narrower than the narrowest that a general-purpose receiver may be able to tune; likewise, signals which individually use a fairly wide span of frequencies, such as broadcast AM, can be made less noisy by narrowing the bandwidth of the signal, even though making the span of received frequencies narrower than was transmitted will sacrifice some audio fidelity. A good preselector often can reduce a radio's receive bandwidth to a narrower frequency span than many general-purpose radios can manage on their own.
A preselector typically is tuned to have a narrow bandwidth, centered on the receiver's operating frequency. The preselector passes through the signal on its tuned frequency unchanged (or only slightly diminished) but it greatly reduces or removes off-frequency signals, cutting down or eliminating unwanted interference.[b]
Extra filtering can be useful because the first input stage ("front end") of receivers contains at least one RF amplifier, which has power limits ("dynamic range"). Most radios' front ends amplify all radio frequencies delivered to the antenna connection. So off-frequency signals constitute a load on the RF amplifier, wasting part of its dynamic range on unwanted and unused signals. "Limited dynamic range" means that the amplifier circuits have a limit to the total amount of incoming RF signal they can amplify without overloading; symptoms of overload are nonlinearity (tonal "distortion") and ultimately clipping ("buzz").
When the front-end overloads the performance of the receiver is severely reduced, and in extreme cases can damage the receiver.[2] In situations with noisy and crowded bands, or where there is loud interference from nearby, high-power stations, the dynamic range of the receiver can quickly be exceeded. Extra filtering by the preselector limits frequency range and power demands that are applied to all later stages of the receiver, only loading it with the desired signals within the preselector's pass-band.
Spectrum analyzers, heavy-duty network analyzers, and other RF measuring equipment can incorporate switchable banks of preselector circuits individually similar to preselector circuits in conventional radios, that reject out-of-band noise at the frequencies being analyzed.[3] Automatically switched and tuned filter banks can likewise be incorporated into various high quality, general purpose, broadband receivers.
A preselector may be engineered with extra features, so that in addition to attenuating interference from unwanted frequencies it can provide additional services which may be helpful for a receiver:
None of these extra conveniences are necessary for the function of preselection, and in particular, for the typical noisy frequency bands where a preselector is needed, an amplifier in the preselector has no useful function.
On the other hand, when an antenna preamplifier (preamp) is actually needed,[c] it can be made "tunable" by incorporating a front-end preselector circuit to improve its performance. The integrated device is both a preamplifier and a preselector, and either name is correct. This ambiguity sometimes leads to confusion – conflating preselection with amplification.
Ordinary, regular preselectors (that are just preselectors) contain no amplifier: They are entirely passive devices. A standard, ordinary preselector, with no amplifier, sometimes has the word "passive" prefixed – hence "passive preselector" means "standard preselector". Since preselectors are normally "passive", adding the redundant word is pedantic, but even so, emphasizes to those only familiar with tunable preamplifiers that the "passive" preselector has no internal amplifier and does not require any power source.
In the noisy longwave, mediumwave, and shortwave bands where preselectors are typically used, when used with "modern" (post-1940) receivers[d] they function with no noticeable loss of signal strength.
With all preselectors there is some very small loss at the tuned frequency; usually, most of the loss is in the inductor (the tuning coil). Turning up the inductance gives the preselector a narrower bandwidth (or higher Q, or greater selectivity) and slightly raises the loss, which nonetheless is still very small.
Most preselectors have separate settings for one inductor and one capacitor (at least).[e] So with at least two adjustments available to tune to just one frequency, there are often a variety of possible settings that will tune the preselector to frequencies in its middle-range.
For the narrowest bandwidth (highest Q), the preselector is tuned using the highest inductance and lowest capacitance for the desired frequency, but this produces the greatest loss. It also requires retuning the preselector more often while searching for faint signals, to keep the preselector's pass band overlapping the radio's receiving frequency.
For lowest loss (and widest bandwidth), the preselector is tuned using the lowest viable inductance and highest capacitance (and the lowest Q, or least selectivity) for the desired frequency range. The wider bandwidth allows more interference through from nearby frequencies, but reduces the need to retune the preselector while tuning the receiver, since any one low-inductance setting for the preselector will pass a broader span of nearby frequencies.
Typically, the radio operator alternates a preselector's bandwidth between wide and narrow: Wide-band (low inductance, high capacitance, lowest Q) while searching for signals, and then re-adjusted to narrow-band (high inductance, low capacitance, highest Q) to closely inspect an interesting signal.
Although a preselector is placed between the radio and the antenna, in the same electrical location as a feedline matching unit, it serves a different purpose: A transmatch or "antenna" tuner connects two transmission lines with different impedances and only incidentally blocks out-of-tune frequencies (if it blocks any at all).
A transmatch matches transmitter impedance to feedline impedance and phase, so that signal power from the radio transmitter smoothly transfers into the antenna's feed cable; a properly adjusted transmatch prevents transmitted power from being reflected back into the transmitter ("backlash current"). Some antenna tuner circuits can both impedance match and preselect,[4] for example the Series Parallel Capacitor (SPC) tuner, and many 'tuned-transformer'-type matching circuits used in many balanced line tuners (BLT) can be adjusted to also function as band-pass filters.[f]
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Source: Wikipedia. Article content is retrieved live through the MediaWiki API.
A preselector is a name for an electronic device that connects between a radio antenna and a radio receiver. The preselector is an adjustable band-pass filter that blocks troublesome out-of-tune frequencies from passing through from the antenna into the radio receiver or preamplifier that otherwise would be directly connected to the antenna.
A preselector gearbox is a type of manual transmission mostly used on passenger cars and racing cars in the 1930s, in buses from 1940 to 1960 and in armoured vehicles from the 1930s to the 1970s. The defining characteristic of a preselector gearbox is that the gear shift lever allowed the driver to "pre-select" the next gear, usually with the transmission remaining in the current gear until the driver pressed the "gear change pedal" at the desired time. The design removed the need for the driver to master the timing of using a clutch pedal and shift lever in order to achieve a smooth shift in a non-synchromesh manual transmission. Most pre-selector transmissions avoid a driver-controlled clutch entirely. Some use one solely for starting from a standstill. Preselector gearboxes were most common prior to the widespread adoption of the automatic transmission, so they were considered in comparison to the "crash gearbox" type of manual transmission. Preselector gearboxes were often marketed as "self-changing" gearboxes, however this is an inaccurate description as the driver is required to choose the gear (and often manually actuate the gear change). An automatic transmission is a true "self-changing gearbox" since it is able to change gears without any driver involvement. There are several radically different mechanical designs of preselector gearbox. The best known is the Wilson design. Some gearboxes, such as the Cotal, shift gears immediately as the control is moved, without requiring the separate gear change pedal.
The Daimler Company Limited ( DAYM-lər), before 1910 known as the Daimler Motor Company Limited, was an independent British motor vehicle manufacturer founded in London by H. J. Lawson in 1896, which set up its manufacturing base in Coventry. The company acquired the right to use the Daimler name simultaneously from Gottlieb Daimler and Daimler-Motoren-Gesellschaft of Cannstatt, Germany. After early financial difficulty and a reorganisation of the company in 1904, the Daimler Motor Company was purchased by Birmingham Small Arms Company (BSA) in 1910, which also made cars under its own name before the Second World War. In 1933, BSA bought the Lanchester Motor Company and made it a subsidiary of the Daimler Company. Daimler was awarded a Royal Warrant to provide cars to the British monarch in 1902; it lost this privilege in the 1950s after being supplanted by Rolls-Royce. Daimler occasionally used alternative technology: the Daimler-Knight engine which it further developed in the early twentieth century and used from 1909 to 1935, the worm gear final drive fitted from 1909 until after the Second World War, and their patented fluid flywheel used in conjunction with a Wilson preselector gearbox from 1930 to the mid-1950s. Daimler tried to widen its appeal in the 1950s with a line of smaller cars at one end and opulent show cars at the other, stopped making Lanchesters, had a highly publicised removal of their chairman from the board, and developed and sold a sports car, a high-performance luxury saloon, and a limousine. BSA sold Daimler to Jaguar Cars in 1960, and Jaguar briefly continued Daimler's line, adding a Daimler variant of its Mark II sports saloon. Jaguar was then merged into the British Motor Corporation in 1966 and British Leyland in 1968. Under these companies, Daimler became an upscale trim level for Jaguar cars except for the 1968–1992 Daimler DS420 limousine, which had no Jaguar equivalent despite being fully Jaguar-based. When Jaguar Cars was split off from British Leyland in 1984, it retained the Daimler company and brand. Ford bought Jaguar Cars in 1990, and under Ford, it stopped using the Daimler marque in 2009 when the last X358 Daimler models were discontinued. The X351 Jaguar XJ took its place, and there was no Daimler variant. Jaguar Cars remained in its ownership and, from 2000, was accompanied by Land Rover, until they sold both Jaguar and Land Rover to Tata Motors in 2008, who formed Jaguar Land Rover as a subsidiary holding company for them. In 2013, Jaguar Cars was merged with Land Rover to form Jaguar Land Rover Limited, and the rights to the Daimler car brand were transferred to the newly formed British multinational car manufacturer Jaguar Land Rover. On 23 December 2025, the Daimler Company was dissolved after having been dormant for several years.
A semi-automatic transmission is a multiple-speed transmission where part of its operation is automated (typically the actuation of the clutch), but the driver's input is still required to accelerate the vehicle from a standstill and to manually change gears. Semi-automatic transmissions were almost exclusively used in motorcycles and are based on conventional manual transmissions or sequential manual transmissions, but use an automatic clutch system. But some semi-automatic transmissions have also been based on standard hydraulic automatic transmissions with torque converters and planetary gearsets. Names for specific types of semi-automatic transmissions include clutchless manual, auto-manual, auto-clutch manual, and paddle-shift transmissions. Colloquially, these types of transmissions are often called "flappy-paddle gearbox", a term coined by Jeremy Clarkson during his time at Top Gear. These systems facilitate gear shifts for the driver by operating the clutch system automatically, usually via switches that trigger an actuator or servo, while still requiring the driver to manually shift gears. This contrasts with a preselector gearbox, in which the driver selects the next gear ratio and operates the pedal, but the gear change within the transmission is performed automatically. The first usage of semi-automatic transmissions was in automobiles, increasing in popularity in the mid-1930s when they were offered by several American car manufacturers. Less common than traditional hydraulic automatic transmissions, semi-automatic transmissions have nonetheless been made available on various car and motorcycle models and have remained in production throughout the 21st century. Semi-automatic transmissions with paddle shift operation have been used in various racing cars, and were first introduced to control the electro-hydraulic gear shift mechanism of the Ferrari 640 Formula One car in 1989. These systems are currently used on a variety of top-tier racing car classes; including Formula One, IndyCar, and touring car racing. Other applications include motorcycles, trucks, buses, and railway vehicles.
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 →The Greenwood District.