The following article was published on the vidi-yo site in 2001.
The aesthetics of underscan: resolution artefacts as art.
This article discusses the problems often experienced by VJs working with scan line converters, what some different scan line converters (in-built & external) offer in features and quality, what to look for and how to use them. It also discusses an approach to appreciation of underscan versus overscan, determining what is quality and what isn't in 'output aesthetics'.
To create and output video sequences, a VJ (visual jammer, video performance artist or act) may be using videocassettes, DVD, video effects and mixing devices, and a computer (probably a laptop for portability). You'd probably think it was simple to connect all of these video-outputting devices together but it can be quite a challenge.
DIFFERENCES BETWEEN PAL AND COMPUTER VIDEO SIGNAL
In Australia, all video devices operate on the PAL standard, which is 768 pixels wide and 576 pixels high. PAL video devices automatically output a video signal that conforms to this standard. Computers and laptops however, use a different standard (VGA and supersets of VGA) to output images and signals to screen. This is because computers output video that is designed to be displayed on a computer monitor, and is quite different to a normal video signal.
The main differences between the computer video signal and PAL video signal are:
Normal video signal is blitted to a screen in a series of zigzag lines, 50 Hz (times per second). Computer video scan happens more rapidly, from 65 Hz up to 100 Hz and higher. The horizontal scan rate (the left to right motion) is also faster in computers than in normal video.
Standard PAL is 768 pixels wide and 576 pixels high. The computer signal may be anything from 640 x 480, 800 x 600 up to 1920 x 1440 or more.
Computer signal is divided into separate red, green, blue, and sync elements, which the computer monitor neatly puts back together in the display. Video signal has all of these elements already combined in composite video signal.
Video signal blits each frame in two passes (odd field and even field) and the 50 passes in PAL makes up the 25 interlaced frames of PAL video. Computers however don't interlace their frames, each frame is a discreet pass of video. (newer video standards such as HD-TV are non-interlaced, but let's not go there!).
Images on a television (ie, normal video signal) go past the edge of the screen, and there are some parts which are part of the scan but you just don't see them because they are outside the edge. This is known as overscan (the scan gies over the edge). Images on a computer screen have a definite border, and is inside the edge of the monitor. This is known as underscan.
To match PAL video signal and computer video output, you need to convert the computer signal to normal video, using a scan line converter.
SCAN LINE CONVERTERS
Scan line converters are a piece of hardware that converts the signal from computer video to normal PAL (or other video standards).
Internal Scan line converters do the same job, but are built-in to the computer, and may be a video card, or a built-in scan line converter so that the computer can output normal PAL video signal. If your laptop has an in-built scan line converter, it will have an RCA composite video jack or S-Video jack that is designed to output standard video.
External Scan line converters are designed to plug in to the computer monitor jack of a computer, and usually also require extra power and cables.
Features vary amongst different brands and models, and some examples
- conversion from computer signal to PAL and/or NTSC (US / japanese format) video signal, or back the other way,
- ability to zoom in on part of the computer signal and convert it to full screen video,
- some special effects, such as dissolve or chromakey, or titles,
- genlocking (syncing two different video signals together so that each frame begins at the same moment).
You may be thinking of connecting your computer directly into a video projector, to obtain the highest quality signal - knowing that most video projectors are capable of displaying computer signal (this is known as data projection). However not all data projectors are capable of all resolutions or colour depths, meaning that work you have designed to be output at 1024 x 768 (for example) cannot be displayed.
Computer signal is carried using VGA cables that are expensive (compared to cables designed to carry normal PAL video signal) and not very long, limiting how far your laptop can be from the projector. In a situation where the visual jammer is working on stage and the projector is front of house, some 6 to 25 metres away and 4 metres in the air, a standard 3 metre VGA cable will not reach. And permanent installations make this situation even more impractical.
Finally, in many cases a VJ will be expected to patch their equipment into a video system such as professional mixers or matrix switcher, which will only accept composite or S-Video signal. Therefore, it is in the VJ's best interest to have an external or in-built scan line converter, to produce a standard PAL video signal which can easily be plugged into video equipment.
THE MAJOR SCAN CONVERSION ISSUES
No matter whether dealing with an in-built or an external scan line converter, there are several issues which are typical for the VJ.
In general, the quality of a computer video signal will be much higher than standard video, and converting a computer video signal will mean a loss of crispness. For example, text that has been produced on a computer video monitor set to a high resolution will appear fuzzy and dull when converted to standard video. For this reason, it is wise to test and check images and designs created on a computer monitor during the production process, to alter them if necessary so that the conversion process will not reduce the legilibity or colour so much. Don't assume that what you see on your computer monitor will look the same once converted to standard video.
Underscan and Overscan. Because of the difference in scan size between video and computer signal, the processing of underscan and overscan is a critical issue. When computer signal is converted to standard video, in underscan mode, a black border will appear around the edge of the image. This is less an issue when this image is the only source, but when it is mixed or juxtaposed with other video signal, the black border will appear as a definite edge on many projections. Many Scan line converters have overscan or scaling functions which should overcome this problem, but many scan line converters are unable to properly scale to full screen overscan (even if they advertise overscan) – meaning that the black border can’t be hidden or removed.
Resolution and Window Size. Many VJs work with applications that
don't display full screen video, but instead present a 320x240 sized
in one corner
with control panels around it. Converting the full screen display
would be a problem as you only want the active window to be shown.
needs to be able to zoom up on specific display window and display
that as a full overscan image (768 x 675 pixels). Many scan line
capable of zooming, or don't offer precise enough control to scan
just the active window. Others which are capable of zoom, require
to be set
up afresh each time, which is a nuisance in a live context. Ideally
scan line converter should allow the setting to be saved and easily
Accessories. Some scan line converters are programmed through on screen menus or remote controls. Onscreen menus require a monitor that is separate to the mix (so that setting up won't have to be done on the main screen). And remotes have a tendency to get lost!
Flicker is a feature of normal video signal - created by subtle differences between interlaced odd and even fields of frames. A scan line converter takes a non-interlaced video signal and creates an interlaced signal. Low end scan line converters do this by duplicating every second frame, more expensive scan line converters apply other processes which give a higher quality motion but may soften other aspects of the image. This is a consideration for computer video signal designed to be interlaced (such as some 3D packages).
The scan line converter may be supplied and set up for the VJ at some major events, but this does not necessarily mean it has been set up according to your needs. Overscan SHOULD have been set up as the default (but it is wise to check as this may have been forgotten). If you require part-screen zooming, you should speak with the events AV tech director about creating presets before the event begins. Considering that the scan line converter may have been hired in for the day, the AV tech team may not be well versed in its use, so allow plenty of time for set up.
THE AESTHETICS OF UNDERSCAN
When viewing an image onscreen produced by a VJ, it's often possible to tell the method of production, left by tell tale signs.
For example, you can easily spot a VJ using a mixer to dissolve back and forth from overscanned video to underscanned computer signal, because the rectangular frame of the image will appear to grow and shrink slightly. With some video source material this may appear to be an attractive aspect, but in general it seems to reinforce the rectangularity of the projection.
In circumstances where the VJ is not mixing back and forth between two sources, but instead has an underscan and overscan image dissolved together or keyed together, the image will appear to be of two images, one being in a rectangle slightly smaller than the other. This can be quite displeasing to the eye if the images are organic, soft images, as the hard rectangular outline slightly inside the frame tends to override the softer qualities.
The above problems appear worse if the underscan is off centre. If you are bothered by the mismatch of normal video signal, and your scan line converters inefficient overscan, you can hide the overscan border in the projection. To do this, set the projector image to be slightly larger than the screen, so that the border spills over the edge. This works well when the screen is suspended, or against a dark wall.
A VJ who is using an iterative content production technique (that
is, mixing work, recording it, and then using that recording as
within a mix)
and displaying the underscan / overscan issue will present three
or more "inner
rectangles" which may all be slightly off centre from each
other. By counting the number of different rectangles, you can
gauge how many
been through to create that particular mix.
There is a point in an iterative content production technique in which the underscan rectangle actually becomes a feature of the work rather than a detriment to it. The gap between underscan and overscan rectangles takes on a life, and history of its own, separate to the aesthetic of the rest of the frame.
BUYING A SCAN LINE CONVERTER
You do get what you pay for in scan line converters...
Cheap to medium quality: $100 - $800
high end: $800 - $3000 plus
Broadcast quality: $4000 plus
Bottom end of the range tend to have poorer conversion methods, meaning that the converted image will be less crisp and readable, They tend to support less resolutions, and the resolutions supported may be clipped (see the aesthetics of underscan). Low end scan line converters may also be operating in fewer colours, sampling image colours in only hundreds or thousands of colours (much less than normal video). Better quality scan line converters will tend to have better colour handling, offer higher resolution processing, and will also have better "anti-flicker filtering" for creating interlaced video signal.
Before you purchase a scan line converter, check:
- what us the resolution and refresh rate you use on your computer; Make sure that the scan converter supports it.
- does the scan line converter support the video standard you need? It is very important to check the quality of the video signal. Lower end scan line converters may output a substandard or low quality signal which may not be apparent when displayed on a TV, but which may be a problem for older or professional quality equipment. For example, the Panasonic WJ-MX50 and MX12 mixers are renown for "allergic reactions" to substandard video signal output by low end scan converters.
- Does the scan line converter have the appropriate outputs? For example, does it supply PAL, composite, or PAL S-Video. S-Video carries separate colour and brightness signals, and is used on most pro-sumer equipment because of its superior quality. High end scan converters may also offer a component video option (signal is split into red green and blue) which is used in some high end broadcast applications.
- How many colours does the converter support? Normal video signal displays millions of colours, whereas a computer video signal may be output at anywhere from 8 colours, through thousands to millions of colours. Not all converters support a good quality colour conversion process. Some claim to be 24 bit (thousands of colours) but actually downsample the image before converting to video. Check that the unit offers "bit sampling and processing" for 24 bit colour.
- samples per line. The scan line converter samples pixels across each horizontal scan line of the computer signal. A low sample rate means a fuzzier signal, higher will be crisper. Bottom end scan converters should offer at least 640 samples per line.
- Anti flicker options. Low-end scan converters reduce flicker by duplicating frames to create odd and even fields. High-end converters create unique fields but use other methods to reduce flicker. Flicker may be an issue for you if your computer images include motion or stripes.
- Underscan and Overscan. A computer signal is normally displayed underscan, but when converted to normal video signal, menu bars and items at the edge of screen may disappear outside the edge of the screen. This is because normal video is overscan. A scan line converter should offer both overscan and underscan modes. But be careful - some offer underscan as the default, and the overscan version is lesser in quality, or worse yet, may not go right to the edge. Or if overscan is the default, the underscan may be poorer in quality and not centred on screen. You need your computer signal to be able to be converted to full overscan, 768 x 576 pixels to the edge, to ensure that it perfectly matches the scan size of normal video.
- Zooming, Magnification, Position options. Some scan converters allow you to zoom in on part of the computer image. Low end scan converters offer a simple 1X, 2X, magnification with rudimentary positioning (eg top left or bottom right). But for many software applications the part of screen you may want to zoom up on is in a particular part of frame and needs more magnification control. Middle range scan line converters allow you to specify the exact dimensions, and save it as a preset, so you can easily jump backwards and forwards from full screen mode. High end scan converters may also offer a continuous linear zoom magnification that maintains the aspect ratio throughout the zoom.
- Genlock. A genlock allows you to take two sources and synchronise the signals to each other, which may be useful if you are working with other technologies (such as mixers) which require all signals to be synced. When looking at Genlock features, check which signal is the "sync source", and whether that is an option you can set. Ideally your genlocking scan converter should also offer options to adjust both horizontal and subcarrier signal timing.
- Other features. Scan converters offer a variety of features depending on manufacturer, including:
remote control or rs-232 control
other formats (eg computer to PAL, NTSC, VGA, XVGA, component)
special effects (eg still or image freeze mode, dissolve, chroma key)
Other accessories (manuals, cables, rackmount options)
- The manufacturer. Is the company a flybynighter or a reliable company with reasonable history, and what is the manufacturer offering in terms of Warranty and Support for service and repairs.
Written by Cindi Drennan, 2001