Sunday, May 27, 2007

Digital Circuits

Digital systems interface well with computers and are easy to control with software. It is often possible to add new features to a digital system without changing hardware, and to do this remotely, just by uploading new software. Design errors or bugs can be worked-around with a software upgrade, after the product is in customer hands. Information storage can be much easier in digital systems than in analog ones. In particular, the great noise-immunity of digital systems makes it possible to store data and retrieve it later without degradation. In an analog system, aging and wear and tear will degrade the information in storage, but in a digital system, as long as the wear and tear is below a certain level, the information can be recovered completely.

Friday, May 18, 2007


The colloquial or historical meaning of jitter is to shake or shudder one's body or limbs. Also:” Jittery" synonymous with "nervous" or "tense" to the extent of shaking, usually implying fear rather than excitement. May involve a neurological seizure, or withdrawal symptoms from drunkenness - Delirium tremens. Also: jitterbug - a dance that involved shaking the arms and legs.

In electronics and telecommunications, jitter is an unwanted variation of one or more signal characteristics, such as the interval between successive pulses, the amplitude of successive cycles, or the frequency or phase of successive cycles. Jitter is a significant factor in the design of almost all communications links.

Jitter can apply to a number of signal qualities, and can be quantified in the same terms as all time-varying signals. Also like other time-varying signals, jitter can be expressed in terms of spectral density. Jitter frequency is the time between the maximum and minimum effect of a jitter characteristic for a jitter that varies frequently with time. Generally, very low jitter frequency is not of interest in designing systems, and the low-frequency cutoff for jitter is typically specified at 1 Hz.

In the context of digital audio extraction from CDs "jitter" causes extracted audio samples to be doubled-up or skipped entirely. The problem occurs because the Philips CD requirement doesn't require block-accurate addressing. As a result, the extraction process will restart a few samples early or late, resulting in doubled or omitted samples. These glitches often sound like tiny repeating clicks during playback. An approach that has produced good results is to do jitter correction in software. This involves performing overlapping reads, and then sliding the data around to find overlaps at the edges. Most DAE programs will perform jitter correction.

Sunday, May 13, 2007

World Wide Web

The World Wide Web is a system of interlinked, hypertext documents that runs over the Internet. With a Web browser, a user views Web pages that may include text, images, and other multimedia and navigates between them using hyperlinks. The World Wide Web is the combination of four basic ideas: Hypertext, Resource Identifiers, The Client-server model of computing, Markup language. On the World Wide Web, a client program called a client agent retrieves information resources, such as Web pages and other computer files, from Web servers using their URLs. If the user agent is a kind of Web browser, it displays the resources on a user's computer. The user can then follow hyperlinks in each web page to other World Wide Web resources, whose position is embedded in the hyperlinks. Web pages are often set in collections of related material called Web sites. The act of following hyperlinks from one Web site to another is referred to as browsing or sometimes as surfing the Web.

Wednesday, May 09, 2007


A bullet is a solid shell propelled by a weapon or air gun and is normally made from metal. A bullet does not contain explosives, and damages the planned target solely by imparting kinetic energy upon impact. Modern bullets for firearms are usually part of a cartridge, also known as a round. In contrast, bullets for air guns are not part of a cartridge. The word "bullet" is sometimes used to refer to the combination of bullet, case, gunpowder and primer more properly known as a cartridge or round."

1 History
1.1 The first bullets
1.2 Shaped bullets
1.3 The modern bullet
2 Designs
3 Equipments
4 Treaties
5 Bullet acronyms
6 Figurative uses
7 References
8 See also
9 External links


The history of bullets parallels the history of firearms. Advances in one either resulted from or precipitated advances in the additional. Originally, bullets are round metallic or stone balls placed in front of a volatile charge of gunpowder at the end of a closed tube. As firearms became more scientifically advanced, from 1500 to 1800, bullets changed very little. They remained simple round lead balls, called rounds, conflicting only in their diameter. The growth of the hand culverin and matchlock harquebus brought about the use of cast lead balls as projectiles. "Bullet" is derived from the French word "boulette" which approximately means "little ball". The original musket bullet was a globular lead ball two sizes smaller than the bore, wrapped in a loosely-fitted paper patch which served to hold the bullet in the barrel firmly upon the powder. The loading of muskets was, therefore, easy with the old smooth-bore Brown Bess and similar military muskets.

Tuesday, May 01, 2007

Lens construction

The majority type of lenses are spherical lenses, which are fashioned from surfaces that have spherical curvature, that is, the front and back surfaces of the lens can be anticipated to be part of the surface of two spheres of given radii, R1 and R2, which are called the radius of curvature of each surface. The sign of R1 gives the form of the front surface of the lens: if R1 is positive, the surface is convex. If R1 is negative, the front surface is concave. If R1 is infinite, the surface is flat, or has zero curvature, and is said to be plane. The same is true for the back surface of the lens; apart from that the sign conversion is reversed: if R2 is positive, it is concave, and if R2 is negative, the back surface is convex. The line joining the centers of the spheres making up the lens surfaces is called the axis of the lens; in almost all cases the lens axis passes through the physical centre of the lens.

Lenses are divided by the bend of these two surfaces. A lens is biconvex if both surfaces are convex; similarly, a lens with two concave surfaces is biconcave. If one of the surfaces is flat, the lens is termed Plano-convex or Plano-concave depending on the curvature of the other surface. A lens with one convex and one concave side is named convex-concave, and in this case if both curvatures are equal it is a meniscus lens. If the lens is biconvex or Plano-convex, a collimated or parallel beam of light passing along the lens axis and through the lens will be converged to a spot on the axis, at a certain distance behind the lens. In this case, the lens is called a constructive or converging lens.

If the lens is biconcave or Plano-concave, a collimated beam of light passing through the lens is diverged; the lens is thus called a negative or diverging lens. The beam after passing through the lens appears to be emanating from a particular point on the axis in front of the lens; the detachment from this point to the lens is also known as the focal length, although it is negative with respect to the focal length of a converging lens.
If the lens is convex-concave, whether it is converging or diverging depends on the relative curvatures of the two surfaces. If the curvatures are equal, then the beam is neither converged nor diverged.