Technology Beat

June 2001 IBEW Journal

Digital Video: Part I

Remember when home movies consisted of the 16 millimeter camera your parents used to shoot footage of some family event like a picnic or a wedding? Over time, the black and white (or color) footage became grainy and cloudy to the point where the quality was compromised and the film became unusable. Well, todays video technology tells a different, more positive story.

The exploding technology of digital video has ushered in an era of moviemaking that benefits full-time professional and amateur videographers alike. Digital video offers home users the ability to edit their home movies and store them to be viewed time and again. Thanks to innovations in digital video technology, anyone with a keen interest in making movies can produce an impressive video.

The video world is currently undergoing a major shift from analog to digital. This transition is occurring at every level of the broadcast/recording industry. Standards have been set as broadcast stations move towards digital television. While analog video signals are still being transmitted, the federal government has mandated the conversion to digital broadcast signals by 2006.

Homes can currently receive digital cable or digital satellite signals. Video editing has evolved from analog tape-to-tape editing and into the world of digital non-linear editing. Home viewers watch high-quality video on digital versatile disk (DVD) players or from DVD drives in computers. Consumer electronics offer digital video cameras at affordable prices.

Analog video signals are composed of continuously varying waveforms

Digital video signals are precise points selected at intervals on the curve.

Analog Versus Digital Video

Analog and digital video are characterized by different signals. Analog signals consist of fluctuating wave forms. At any given time, the value of the signal can range somewhere between the minimum and maximum allowed. Digital signals, by contrast, are digital versions of an analog signal. They are transmitted as precise points selected at intervals on the curve. Digital signals are binary, with signals consisting of a minimum value of zero and a maximum value of one. In a language of ones and zeroes, a signal can be clearly transmitted. This gives digital signals an edge over analog signals in terms of high quality video and audio.

Analog Technology

Many professional video devices and millions of consumer video cameras and tape machines are still using analog. There are currently three types of analog video connections:

• Composite uses cable with a single wire to transmit the video signal. The luminance and color signal are composited together and transmitted simultaneously. This is the lowest quality connection because of the merging of the two signals.
   

• S-Video uses a cable that separates the luminance signal (also called the Y1 signal) onto one wire and the combined color signals (called the C1 and C2 signals) onto another wire. The separate wires are encased in a single cable.
   

• Component, reportedly the best type of analog connection, has cable assigned to each of the YCC signals.

These analog formats bleed color, have low clarity and suffer high generational loss, i.e., a copy of a copy is never as crisp and sharp as the original. Analog video signals do not allow the receiving end to distinguish between the original signal and any noise, or interference, that might be introduced during transmission. Subsequent transmissions mean more noise accumulated, resulting in the poor fidelity that translates into poor quality.

Digital Video Factors

With digital video, we should keep in mind four major factors: frame rate, color resolution, spatial resolution and image quality.

Frame Rate

The standard for displaying any sort of non-film video is 30 frames per second. (Film is 24 frames per second.) This simply means that the video is made up of 30 pictures or frames for every second of video. Additionally, these frames are split in half (odd lines and even lines), to form fields.

When a television set displays its analog video signal, it displays the odd lines (the odd field) first and then the even lines (the even field). The odd field and the even field together form a frame. Sixty of these fields are displayed every second (or 30 frames every second). This is called interlaced video. However, digital video displays use progressive scan to update the screen. With this method, the screen is not broken into fields. Instead, each line is displayed in sequence, from top to bottom. The entire frame is displayed 30 times every second. This is also called non-interlaced video.

Digital video can use a variety of frame rates and resolution. Resolution refers to the amount of information contained in each frame. It is expressed as the number of horizontal pixels times the number of vertical pixels. (For example, 640 x 480 or 720 x 480). This means that a higher resolution yields a higher quality image.

Together the frame rate and resolution of digital video determine the amount of data that must be manipulated, the space required for storage, and the bandwidth needed for transmission.