to TV Video
VGA Scan Frequency Conversion
April 15, 1999
In 1997 there were 250,000,000 television sets in the United States alone. 35% of American households own a PC, and US residents are purchasing new PCs at a steady 8.9% growth rate. The question arises, when will the TV and PC clash? The answer is --it has happened already, if you donít mind wading through a few glitches. For the perfectionist, it could be a few years, unless the wallet is exceptionally wide this year.
As the multimedia capabilities compound, the future seems determined to unite TV, movies, cable, telephone, games, email, video, music, Internet, presentation, and word processing all under one roof. For this vision to become a reality, many changes will need to take place --one important one being the merging of television and the home PC. The purpose of this paper is to discuss different attributes of todayís formats that are used to send video signals from a source such as the PC to a television, and shed light on some of the obstacles encountered in integrating them.
Today the most common types of interfaces used for televisionís NTSC or PAL input and output in linking video devices are RF, composite, s-video (Y/C), component video, SCART, and 15 kHz RGB (red-green-blue). The most common interface used for PC, on the other hand, is video transfer by way of separate RGB signals in the form of SVGA, VGA, and XGA.
A RF modulator is a device that many of the older TVís used to allow the signal to be attached to a TV channel via a coaxial cable. This technology is slowly being replaced because other TV and audio signals become intertwined with the PCís signal, causing the poorest picture quality of any format.
The most common type of NTSC interface, composite, uses RCA jacks nearly identical to those used to transfer audio. It is a standard feature that is found on many familiar consumer video devices such as; camcorders, VCRs, video game consoles, and video capture cards. Reaching cable distances over 10 feet while maintaining a clear picture becomes a challenge with composite video.
With higher quality and a longer reach than composite video, S-video is making its way from high-end stations into the living rooms of many avid home cinema enthusiasts. Because it separates the color/chrominance (C), from the brightness/luminance (Y), there is less color bleeding and dot crawl than in both composite and RF interfaces.
Component video, (variations include Y/Pr/Pb, Betacam, and MII), is used almost exclusively in high-end expensive video production.
Like component video, the NTSC RGB 15 kHz output is a very high quality format. It is characteristic in expensive equipment and sometimes output by scan converters. In 1996 Sony released a few consumer TV models that possessed this 15 kHz RGB input, but discontinued shortly because most consumers didnít know what to do with it. Today we find this NTSC RGB format only in the scarce 1996 Sony TV models, very old 13-14" Atari/Amiga type monitors, arcade game consoles, and a few HDTV sets costing in excess of 5,000 dollars.
SCART is an interface that is gaining popularity in Europe. Its 21 pin DIN carries video via RGB and composite format. The remaining pins, carry audio and various control signals. Not all pins are required to be utilized to complete a SCART connection, since it is a conglomeration of many different signals.
The video transfer formats used in monitors are years ahead of the previously discussed television standards. Video cards, digital cameras, and high-end video equipment output signals in high frequency 60 kHz RGB format. The RGB format is utilized by almost all monitor types such as VGA (video graphics adapter), SVGA (Super VGA), and XGA (extended graphics adapter). Almost all devices work internally with video in this fast RGB format, but since around 99% of all todayís televisions cannot accept direct RGB input because of the way their electron guns are setup, the output is converted to NTSC or PAL format before it leaves the machine.
Think of an analog video signal as a wave similar to the pen scribbling produced by a FBI lie detector. It has peaks and valleys denoted by high and low voltages. Within the spectrum, each peak, valley, and stretch in between represents a specific color. The monitor or TV circuitry decodes these signals and the electron beam illuminates the phosphorus particles accordingly. A device will output TTL, Analog, or ECL signals. TTL, or Transistor-Transistor Logic, signals have 4 to 5 volt peak-to-peak levels. Analog signals have .7 volts peak-to-peak levels. ECL, or Emitter-Coupled Logic, signals are high-resolution monochrome signals. Standard televisions interpret analog signals.
In the 1950s, The National Televisions Systems Committee created the NTSC format which set the horizontal scan frequencies at 15750 Hz and the number of horizontal lines of resolution to 525, (425 of which actually make it to the screen). Because it often exhibits problems displaying the correct hue, it has been given the nickname "Never The Same Color." Europe and many other areas of the world use the Phase Alternate Line, (PAL) format, which has slightly more visible horizontal lines, but a slightly slower scan frequency that causes a bit more flicker. The French government, for political reasons, created SECAM, which has similarities to both PAL and NTSC. All three formats approximately translate to a meager 640x480 resolution or 800x600 with cutoff edges. A television frame is drawn on the screen by an electron beam going from left to right at 15 kHz and top to bottom at about 30 times per second. The human eye will get twisted up in a flicker frenzy when viewing video at the low 30 frames per second, so an interlaced format was created which scanned all the odd lines in one scan followed by even lines in the next scan. This created a smooth transition between frames as well as effectively doubled the scan frequency to 30hz, which corresponds to a refresh rate of 60 frames per second, (60 kHz).
Unlike a television, the electron beam in a monitor scans horizontally between 24 Ė 65 kHz, (usually 60 kHz), and vertically at a user specified rate which can vary between 60 and 180 Hz, (usually set between 60 and 85 Hz). The monitor does not interlace its horizontal lines, (progressive scanning), yet still allows for VGA to crank out between 320x200 and 720x400 and for SVGA to pump up to a steady 1600x1280. The electron gun has the capability to project separated RGB video for increased accuracy and speed.
It is not an easy task converting the SVGA, VGA, or XGA signals, used to drive a PCís monitor to the NTSC or PAL formats that feed a television picture tube. Doing so implies much more than simply connecting a cable from PC to TV. A similar analogy to would be like sliding a Dodge Viper engine under the hood of an old Studabaker. The trick is to downgrade the new technologiesí performance to fit the old, meanwhile keeping the new from blasting the whole effort up in smoke. Fortunately, there are enough brilliant computer engineers in our society to take on the project and present a near perfect jury rigged solution until High Definition Television, (HDTV), can get its foot through the door. Until then, expecting a link from PC to TV to be as crisp as the link from PC to monitor is a false hope. The main tasks that are required to make the VGA converterís connection possible without excessive compromises are as follows: (1) Match VGA and NTSC horizontal synchronization frequencies. (2) Lower the VGA 75 Hz frame rate to match the NTSC 60 Hz rate. (3) Morph the VGAís non-interlaced format into an interlaced format. (4) Scale-down the high VGA resolutions to those acceptable by NTSC. (5) Convert VGA picture signals to a specified output format such as s-video or composite.
Accomplishing the transition requires an army of filtering/encoding IC chips and software drivers that cooperate to perform the necessary conversion. Appendix B depicts a typical conversion circuit. To clear the picture as much as possible, engineers use tricks such as buffering the odd lines and scaling images. Flicker filters use algorithms to interpolate between scan lines in an attempt to reduce flicker. In professional models costing over $25,000, every VGA output frame is stored in a large buffer before being processed through motion estimation algorithms and output to NTSC. For most situations video will be clear enough to be presentable, nevertheless, it is still common practice to increase font sizes and use high contrast colors to increase legibility and prevent color bleeding.
In the pursuit of a clearer picture, the first quarter of 1999 introduced a new type of digital LCD display for portable PCs that represents each pixel and color with a binary number and still displays the necessary 24 bit color. Along that pursuit, 100 Hz NTSC televisions with VGA inputs are trickling their way from the professional scene to consumer homes. Once digital HDTV makes itís way into the mainstream, most of the conversion hurdles are expected to vanish. Departed will be the days of NTSC Ėreplaced by digital signals, 720 non-interlaced or 1080 interlaced scanning lines, 1080x1920 interlaced or 720x1280 non-interlaced resolutions, 16: 9 ratios, and 60 frames per second. It no longer takes a revolutionary to imagine links without compromises via RGB cables or even digitized optical signals.
As PCs and TVs are tied closer to the point of fusion, the line between the two is growing fainter by the minute. Thanks to NTSC/PAL scan converters and capable interfaces such as s-video, RGB, and SCART, we are not left in the dark until high definition sets roll in. The many approaches such as buffering, scaling, filtering, and motion estimation are changing visual presentations that would normally cause eye watering into a sight that is, as they say, "easy on the eyes."
April 15, 1999