Electronic Photography

Video starts with a camera, as does all picture taking. In still and motion-picture film photography, there is a mechanical system that controls the amount of light falling on a strip of film. Light is then converted into a pattern of varying chemical densities on the film.
As a physical medium, film can be cut, spliced, edited, and manipulated in other ways as well. In electronic photography, the light from an object goes through a lens, as it does in film photography. On the other side of the video camera lens, however, light is converted to an image by an electronic process as opposed to a mechanical or chemical process. The medium for this conversion has changed over the years. It began with tube cameras and has progressed to completely electronic components. The tube cameras will be discussed first, followed by a discussion of the same process as it occurs in digital cameras.

Tube Cameras
In a video tube camera, the lens focuses the image on the face of a pickup tube inside the camera. The face of the pickup tube is known as the target (Figure 2.1). The target is light-sensitive, like a piece of film. When light shines on the face of the target, it conducts electricity in proportion to the amount of light that is striking its surface. Without light on the face of the target, the target resiststhe flow of electricity.

A stream of electrons, called the beam, comes from the back end of the tube and scans back and forth across the face of the target on the inside of the pickup tube. The electrical current generated is either allowed to pass from the target to the camera output or not, depending on the amount of resistance at the face of the target. The amount of resistance varies depending on how much light is shiningon the target. In every video camera, there are adjustments for the beam intensity and the sensitivity of the face of the target. The target acts as either an insulator, when it’s not exposed to light, or as a conductor when light shines on its face. The electrical signal that flows from the target is, in effect, the electronic recreation of the light coming from the scene at which the camera is aimed.

Scanning the Image
Scanning the image begins with the beam of electrons sweeping back and forth across the inside face of the target. Where the electron beam strikes the face of the target, it illuminates an area the same size as the electron beam. This “dot” of electron illumination is called the aperture (Figure 2.2).


The dot or aperture is the smallest size that an element of picture information can be. The larger the aperture, the less detail in the picture. The smaller the aperture, the more detail in the picture. Dot size, or beam aperture, is comparable to drawing with large, blunt crayons or fine-tipped pens. Crayons can outline shapes or color them in. A fine-tipped pen can add texture and small highlights toa drawing. In a digital video signal, these picture elements are called pixels, short for picture elements (Figure 2.3).
The electron beam must always be kept perpendicular to the face of the target. If it were not perpendicular as it scanned back and forth, then only one line in the center of the television image would be in focus. The lines closest to the top and bottom of the picture would be badly distorted, because at these angles the aperture dot would be shaped like an ellipse.

How Video Works

Introduction
Since the development of broadcast cameras and television sets in the early 1940s, video has slowly become more and more a part of everyday life. In the early 50s, it was a treat simply to have a television set in one’s own home. In the 60s, television brought the world live coverage of an astronaut walking on the moon. With the 70s, the immediacy of television brought the events of the Vietnam War into living rooms. In the 21st century, with additional modes of delivery such as satellite, cable and the Internet, video has developed into the primary source of world communication.

Video Evolution
Just as the use of this medium has changed over the years, so has its physical nature evolved. The video signal started as analog and has developed into digital with different types of digital formats, including some for the digital enthusiast at home. When television was first created, cameras and television sets required a great deal of room to house the original tube technology of the analog world. In today’s digital society, camera size and media continue to get smaller as the quality continues to improve.
Today, a video image is conveyed using digital components and chips rather than tubes. Although the equipment has changed, some of the processes involved in the origination of the video signal have remained the same. This makes the progression of video from analog to digital not only interesting to study, but crucial in providing a foundation of knowledge upon which the current digital video world operates. So much of today’s digital technology is the way it is because it evolved from analog.

Analog and Digital
No matter how digital the equipment is that is used to capture an image, the eyes and ears see the final result as analog. All information from the physical world is analog. A cloud floating by, an ocean wave, and the sounds of a marching band all exist within a spectrum of frequencies that comprise human experience. Thisspectrum of frequencies can be converted to digital data, or zeros and ones. Human beings, however, do not process digital information, and eventually what a human being sees or hears must be converted back from digital data to an analog form. Even with a digital home receiver, the zeros and ones of the digital signal mustbe reproduced as an analog experience (Figure 1.1).
In the early days of television, video was captured, recorded, and reproduced as an analog signal. The primary medium for storage was videotape, which is a magnetic medium. The primary system for reproduction was mechanical, using a videotape machine.
 


Videotape, which was developed based on mechanical concepts, is a linear medium. This means that information can only be recorded or reproduced in the order in which it was created. With the advent of digital, the primary system for signal reproduction has become solid-state electronics, incorporating servers and computers. This change has created a file-based system, rather than the tapedbased system of the analog era. File-based systems allow random, or nonlinear, access to information without respect to the order in which it was produced or its placement within the storage medium.

Video Applications
Facilities such as cable or broadcast stations, as well as production or post-production companies, are constantly transmitting and receiving video signals. They generally have a number of devices that can be used to capture and reproduce a video signal, such as cameras, videotape recorders (VTRs), videocassette recorders (VCRs),
computer hard drives, FireWire drives, and multiple hard drives called RAID arrays, short for Redundant Array of Independent (or Inexpensive) Disks, which are controlled by computer servers. shows different ways in which VTRs or computers might be used to capture, transmit, or reproduce a video signal.

70V audio speaker systems

70 volt systems are generally used in commercial applications where many speakers need to be run from one amplifier. It is also called high impedance speaker system. The advantage is that, being high impedance, long cable runs of relatively small gauge (usually 20-24 gauges) do not significantly affect the output as they would in a common low impedance speaker system.

The speakers themselves are commonly 4 or 8 ohms, but there's a transformer at each speaker that matches that low impedance, to a high impedance, which is on the line side. Typically those transformers have multiple output taps so the sensitivity of the speaker system (output volume) can be adjusted as needed.

Amplifiers designed for 70 volt operation often have an output transformer as well for matching purposes. Typically a 70V line can be driven with normal audio amplifiers if that matching transformer is added. If you have powerful enough amplifier (high output voltage), you might be able to run the line directly (for example some powerful PA amplifiers can be used to drive a 70V line directly in bridged mode).

Where 70 volt system is used?
A 70 volt system is used in restaurants, small bars, department stores etc. You would want to use this type of system if you plan on powering say 10 speakers with one amp. If the amp produces 100 watts then each speaker would get 10 watts. The speakers have a transformer inside to prevent them from blowing up. You couldn't really connect 10 normal speakers to an average amp because the impedance would be too low, and it's not the best idea to parallel and series your speakers.

Advantages
To an extent, speakers can be added or removed from a 70 volt system without regard for impedance matching. Something you simply can't do with a low impedance system.

Disadvantages
There are two primary disadvantages to a 70 volt system.
1. You are limited to a max amplifier power of 250 watts. Beyond that the transformers saturate and your signal goes to hell.
2. Frequency response is limited, unless serious money is spent on transformers (and it usually isn't). I don't have hard numbers off the top of my head, but a range of 250-10k is in the ballpark. This means it works well for voice application (hence where it's usually seen), but poorly for music applications.