Video is just like audio. Video is quite different from audio. The truth of this paradox is yours to uncover.

While many musicians have a good technical understanding of audio, they are often confused when it comes to understanding the corresponding aspects of video. This edition of Video Focus, then, is designed for those with some technical knowledge about audio but who have difficulty translating that knowledge over to video. Don't panic - video and audio have quite a lot in common.

Both audio and video are most commonly represented by an analog voltage varying in time. A video signal has a bandwidth of 4.5 MHz or more, compared to audio's 20 kHz. This is a factor of 250, so video could casually be called 250 times faster than audio. A rough comparison is that of a comfortable walk, about 5 mph, with the cruising speed of the Concorde supersonic jetliner, 1,200 mph. Yes, there really is that much difference!

Both audio and video are AC signals with industry-specified levels referenced to ground. Video white level (maximum) is specified at 0.714 Volts peak-to-peak into 75 ohms, familiar line-level terrain for audio people. Both video and audio signals rely on their high frequency content for fidelity. In video, the high frequencies supply the details, and in audio they make up the essential high harmonics.

During playback, both video and audio voltages are first amplified and then transformed into magnetic fields. In audio, a power amplifier sends the amplified audio to a coil which generates the magnetic field that moves the loudspeaker's paper cone. In video, the signal is amplified and sent to several coils which generate magnetic fields which deflect an electron beam, causing it to trace out a picture on the viewing screen. In critical applications requiring high-speed response, electrostatic force is used rather than magnetic force. Examples are 100 MHz oscilloscopes, and electrostatic speakers and headphones.

When it comes to analog video synthesis, the parallels between audio and video are very strong. High-speed VCOs control the vertical and horizontal size of objects; triggers, gates, and multi-level comparators all function as visual controllers and modifiers; subcarrier phase-shifters modulate colors; and feedback systems with a colorizer or frame-buffer in the loop perform the visual equivalents of echo and ambience. Now, let's leave the similarities and look a little closer at what makes up video.

The magic intangible stuff floating through the air that you pick up on your TV is not video. It comprises of RF (radio frequency) electro-magnetic waves, which are detected and downshifted in frequency to form video by the tuner in you TV set. That glow-behind-the-glass we know as video is a result of a beam of electrons striking a phosphor, and causing it to emit light. The two main methods of displaying video are the vector and raster systems.

If the electron beam is used to directly draw the image, this process is known as vector graphics. Vector graphics are found in oscilloscopes and some arcade video games. Vector graphics cannot be recorded or broadcast by conventional means without conversion to the raster system.

In audio synthesis, one way to create a sound is to fill the spectrum with harmonics and then use filters to whittle down that nasty buzz to exactly the sound desired (the familiar concept of subtractive synthesis). Raster graphics, the form of video known as television, is analogous to this. In the CRT (cathode ray tube) the electron beam traces out a series of horizontal lines called a raster, and within the raster selected dots on the horizontal lines are brightened to form an image. The raster is always present independently of the image, and in the American television system a complete picture, called a "frame," is made up of 525 horizontal lines scanned from top to bottom thirty times a second (in other words, each complete scan takes 1/30th of a second). Each frame is made up of two fields of 262.5 lines each: first the odd-numbered lines are scanned and then the even-numbered ones. Each complete field is presented in 1/60th of a second.

This method of generating video is called the interlaced scanning method, and it is used to prevent flickering of the image by effectively doubling the display rate from 30 Hz to 60 Hz. North American video (as well as Japanese video) uses the system of video standardized by the National Television Systems Committee, NTSC (known to broadcast people as "Never Twice the Same Color" for the problems NTSC video has with color fidelity). As pioneers in television technology, the USA was the first to codify its television standards, but unfortunately being first means that the system is less than perfect.

The video signal itself is a composite signal made up of five primary components. Three of the components are important for timing and synchronization. In audio, the timing determines the musical pitch of the music: industry standards, such as a 33-1/3 rpm for records and 1-7/8 ips tape speed for cassettes ensure that the music you hear at home is a more or less accurate reproduction of the original recording. Video timing is also standardized.

VERTICAL SYNC is a 59.94 Hz narrow pulse wave that goes low between each video field. During the brief time between fields the electron beam is turned off (blanked) and repositioned at the top of the screen, ready to scan the next field. During this time, called the vertical interval, there are 21 horizontal lines you never see which contain a variety of test signals. This interval can also contain captions for the hearing impaired and videotext services. HORIZONTAL SYNC is a 15.75 kHz pulse wave that goes low during a portion of the horizontal interval, the time between adjacent horizontal lines. In most CRTs there is a slight mechanical sympathetic vibration at this frequency, causing the high-pitched whine heard when the set is on with the volume all the way down. Don't worry if you don't hear it; my theory is that most Americans are deaf at exactly that frequency--I picture that single hair in the cochlea burnt to the root due to long and assiduous exposure to the ever-present 15.75 kHz whine of horizontal sync.

The third timing signal, SUBCARRIER (or COLOR BURST), is also found in the interval in between horizontal lines. This is a short burst of a 3.58 MHz sine wave whose phase is compared with two encoded color signals to determine the colors to be displayed on the next horizontal scan line. In the mid-1950s when color television was under development, we had a monochrome TV standard that was already firmly entrenched; the NTSC color system is an inspired kluge that the engineers at RCA devised to encode color into the existing standard. The LUMINANCE (brightness) signal, which determines the monochrome (black-and- white) image is an amplitude modulated signal that travels between 0. 0535 volts (black level) and 0.714 volts (maximum white level).

The CHROMINANCE (color) signal is the sum of two color-difference signals corresponding to hue and saturation. Both amplitude modulate 3.58 MHz carriers that are offset from each other in phase. The only way the receiver can distinguish between the components of the chrominance signal is by locking onto the phase of the color subcarrier burst found in the horizontal interval before each scan line. Pity the poor receiver that encounters multipath reception or, in this day and age, funky cables, splitter boxes, and other signal-distribution flotsam courtesy of your local cable company. Anything that degrades the phase integrity of the video signal will alter the colors displayed on your set. In reality, color in the NTSC system is left to the discretion of the viewers who adjust the hue and saturation controls on their receivers to suit their taste. In summary, NTSC video is made up of horizontal sync, vertical sync, 3.58 MHz subcarrier, luminance signal, and chrominance signal. The luminance and chrominance signal together determine the color video image, and the other signals convey essential timing information.

If you put five people in the front seat of a compact car no one is going to be very comfortable, and driving could be quite hazardous. The present video system, a composite of five different signals, is difficult enough to deal with under the best of circumstances - but it really pulls into the fast lane as soon as it enters your VCR. Just as you might be influenced by your seatmates in a tightly-packed car, luminance and chrominance crossmodulate each other, and vertical sync occasionally jumps out the window.

All commercial 1/2 inch (VHS and BETA) and 3/4 inch recorders use the "color-under" or "low-band" video recording method which compresses the bandwidth of the recorded video signal to facilitate recording. Only the expensive 1 inch digital video recorders record the video signal "high-band," uncompressed. An alternative method is to record chrominance and luminance separately. Both signals can then have a wider bandwidth, which gives the picture improved resolution and S/N ratio. Sony's Betacam, Panasonic's Recam, and other systems have demonstrated the clear technical superiority of component recording and processing (not unlike the advantages of bi- or tri-amplification in the world of audio).

Now that you know something about the nature of composite video you can appreciate what happens when you attempt editing on your home VCR. When you record new material next to old material, the machine is presented with discontinuities in all the essential timing signals. What you have is a rolling frame or two, perhaps a stray, impetuous burst of noise, and a moment of monochrome video while the VCR locks onto the phase of the new color subcarrier. For about $1600 you can buy an industrial type 1/2 inch video tape machine designed for insert and assembly edits, or you can use the method fondly called "fade to glitch." If you fade your video to black just before the desired edit point, the break in horizontal sync and subcarrier are of no consequence since there is no luminance signal being displayed. The break in vertical sync will cause a slight flash, and if by luck the timing error is small it will hardly be noticeable. You can make your fade optically or electronically using the video level control found on some popular VCR accessory units. Have fun editing on your VCR, and remember how lucky you are to be cutting your audio tape with a razor blade, and creating music with the simple, inexpensive and accessible technology of audio.

For a deeper technical understanding of video, pick up Hewlett Packard's primer and product note opt.005-1, "TV/Video Sync", available from Hewlett Packard. For a fine discussion of color as it relates to video, computer graphics, and perception, see the article "Color Considerations" by Lee Baldwin, Byte Magazine (Sept. 1984). And for analog video synthesis, look for upcoming discussions in this column. Next issue, we'll find out how to augment the lowly Apple computer so that it can produce high-speed graphics of great beauty. See you then!

1994 Commentary:

Not that much has changed over the last nine years. With the popularity of the 8mm video format with its built-in flying erase heads, perfect edits are commonly done in consumer camcorders and the "fade-to-glitch" edits are a thing of the past. Time-Base Correctors are now available as PC accessory cards, and complete video editing by PC ("desktop video") is quickly maturing. Analog video synthesis has completly disappeared and is absolutely unknown to most people in video, as if it had never existed.

As we sit on the edge of new video technologies, the interlaced scanning system might be supplanted by progressive scan in which the raster is scanned in one pass from the first scan line to the last. This is the way VGA video is scanned. With today's higher video bandwidths and refresh rates it makes sense.

As of this writing a low-cost digital consumer VCR is promised "any month now" and "prosumer" equipment brings all the standard video production tools to consumers.

One interesting development has been the application of vector graphic techniques to current computer graphics. Today's primary computer display, the VGA standard, is an analog raster format. The DISPLAY technology is still raster, but the way many graphic objects are described inside the computer is through vectors. In the old days (10 years ago) graphic objects such as fonts were described as bitmaps, a map of the pixels used to display the font. Today fonts are described by vectors, the path you would have to follow to trace out the letter. When you describe a graphic object in the vector method you gain two terrific advantages: the objects are completely scalable without resolution degradation, and the objects are resolution independent. The postscript language is vector based. Windows' font technology is vector, and popular graphic packages such PowerPoint and Corel Draw are based on vector techniques.