Wednesday, July 25, 2007

traffic light

A traffic light or traffic signal is a signaling device positioned at a road junction or pedestrian crossing to indicate when it is safe to drive, ride or walk, using a universal color code. Traffic lights for usual vehicles or pedestrians always have two main lights, a red one that means stop and a green one that means go. Generally, the red light contains some orange in its hue, and the green light contains some blue, to provide some support for people with red-green color blindness. In most countries there is also a yellow (or amber) light, which when on and not flashing means stop if able to do so securely. In some systems, flashing amber means that a motorist may go in advance with care if the road is clear, giving way to pedestrians and to other road vehicles that may have precedence. A flashing red effectively means the same as a regular stop sign. There may be additional lights (usually a green arrow or "filter") to allow turns (called a lead light in the U.S., because it is usually leading the main green light).

Sunday, July 15, 2007

Abstract art

Abstract art is now usually understood to mean art that does not depict objects in the natural world, but instead uses color and form in a non-representational way. In the very early 20th century, the term was more often used to describe art, such as Cubist and Futurist art, which depicts real forms in a simplified or rather reduced way—keeping only an allusion of the original natural subject. Such paintings were often claimed to capture amazing of the depicted objects' immutable intrinsic qualities rather than its external appearance. The more precise terms, "non-figurative art," "non-objective art," and "non-representational art" avoid any possible ambiguity.

Monday, July 09, 2007

Allan Variance

The Allan variance, named after David W. Allan, is a quantity of stability in clocks and oscillators. It is also identified as the two-sample variance. It is defined as one half of the time average of the squares of the differences between succeed readings of the frequency deviation sampled over the sampling period. The Allan variance depends on the time period used between samples: therefore it is a function of the sample period, as well as the distribution being measured, and is displayed as a graph rather than a single number. A low Allan variance is a characteristic of a clock with good stability over the measured period.

Monday, July 02, 2007


In a computer CPU, an accumulator is a register in which intermediary arithmetic and logic results are stored. Without a register like an accumulator, it would be necessary to write the result of each calculation (addition, multiplication, shift, etc.) to main memory, possibly only to be read right back again for use in the next operation. Access to main memory is slower than access to a register like the accumulator because the technology used for the huge main memory is slower (but cheaper) than that used for a register.

The canonical example for accumulator use is adding a list of numbers. The accumulator is initially set to zero, then each number in spin is added to the value in the accumulator. Only when all numbers have been added is the result seized in the accumulator written to main memory or to another, non-accumulator, CPU register.