(Installation instructions are at the end of this file.)
The Mac II is great for synthesizing and displaying images for vision experiments. However, the 8-bit resolution of its digital-to-analog converters is inadequate to render threshold-contrast stimuli, typically yielding on-screen contrasts that are only accurate to about 7 bits, because of the monitor's factor-of-two contrast gain. Pelli and Zhang (1991) describe a simple electronic circuit, a video attenuator, that provides accurate contrast control, achieving 12-bit on-screen accuracy. You can build one yourself, from their schematic, or you can order one pre-made from the Institute for Sensory Research, which calls it the "ISR Video Attenuator."
The ISR Video Attenuator is a small box that is inserted between the video cable and the Apple High-Resolution (or 12-inch) Monochrome Display. It combines the three color RGB signals from a color video card to produce a single monochrome signal of higher grayscale resolution. Only programs using special software (provided) will achieve this benefit. Physically, it is a small satin-finish machined-aluminum box, about the size of a matchbox (2"x1.2"x0.7"), with a 15-pin Apple video connector at each end. Inside are precision resistors on a two-sided printed circuit board with controlled-impedance microstrip traces. This passive resistor network combines the three 8-bit RGB signals from your video card to produce a single much-higher-accuracy signal to drive your Apple High-Resolution (or 12-inch) Monochrome Display.
D. G. Pelli and L. Zhang (1991) Accurate control of contrast on microcomputer displays. Vision Research, 31:1337-1360.
PACKING LIST
- ISR Video Attenuator. ISR stands for "Institute for Sensory Research".
- Reprint of Pelli and Zhang (1991), which includes instructions for use and a schematic.
- 3.5" Macintosh disk with free "Video Toolbox" software written in THINK C for generating visual stimuli and making optimal use of the Video Attenuator on a Mac II. The software is also available separately, still free; just send me email or a reprint request, but be sure to include your mailing address and the VideoToolbox is too big for email. (I use this software. I think it's well documented and works well, but I'm not offering to support it. You're on your own. That's why the software is free.) However, there are some restrictions on distribution and commercial use of the VideoToolbox software. Please read the restrictions in the "Read me" file.
YOU WILL ALSO NEED
Symantec: THINK C
Apple High-Resolution Monochrome Display
color video card (or built-in video on Mac IIci, IIsi, or LC)
photometer for luminance calibration
YOU MAY ALSO WANT TO HAVE
Cambridge University Press: Numerical Recipes in C
Data Translation FORERUNNER A/D Card, or equivalent, to digitize photometer output
ORDERING INFORMATION
The ISR Video Attenuator is for research. As such it is licensed, not sold. The license has no time limit and is transferable. The accompanying Video Toolbox software for creating visual stimuli is supplied free, and is available separately to anyone, but may not be sold without permission.
Checks should be made payable to Syracuse University. The price of US$150 includes shipping in US. If you're in New York State then you either need to add sales tax or attach the New York State document excusing your institution from paying sales tax.
SEND ORDERS TO
Electronics Shop
Institute for Sensory Research
Syracuse University
Merrill Lane
Syracuse, NY 13244-5290
(315)-443-4164
Art_Wixson@ISR.SYR.EDU
DISCLAIMER
Denis Pelli has no financial involvement in the ISR Video Attenuator.
INSTALLATION
To install your ISR Video Attenuator, unplug your video cable from your Apple High-Resolution (or 12-inch) Monochrome Display, plug the cable into the attenuator and plug the attenuator into your monitor.
Once the attenuator is installed, you should run the program CalibrateLuminance to calibrate your monitor, attenuator, and video card. You will need a photometer in order to use that program. Once it's finished CalibrateLuminance will produce a new file called LuminanceRecord?.h, where "?" is the screen number of your monitor. (This screen number is similar to but not the same as the monitor number used by the Monitors control panel device.) This file describes the gamma function and RGB gains of your video card and attenuator. It is a C include file. You can cause its contents to be included in any program by writing
#include <LuminanceRecord?.h>
where "?" must be replaced by the screen number of your monitor, as determined by the program CalibrateLuminance or the subroutine GetScreenDevice(). Later, when you write programs (or compile the supplied demonstrations), your LuminanceRecord file will allow them to accurately control the luminance of your display. The relevant subroutines are all in the file called Luminance.c. Documentation is in the file Luminance.h. Note that LuminanceRecord1.h and LuminanceRecord2.h are provided as samples in the VideoToolboxSources folder, so that the demonstration programs can be compiled even before you have calibrated your own display. However, before doing any serious experiment it is essential that you replace the example by a new LuminanceRecord file that describes your own display.
Alternatively, you can read the LuminanceRecord?.h file at runtime, using the new subroutine ReadLuminanceRecord.c in the VideoToolbox.
The ISR video attenuator is intended for use with the Apple High-Resolution (or 12-inch) Monochrome Display. It also works perfectly with the Apple High Resolution RGB Display, but uses only the green input, ignoring the monitor's red and blue inputs.
With a minor modification, the ISR video attenuator can be used with any monitor that uses the Apple standard D-15 connector and accepts a separate synch. Subsequent to manufacture of the ISR video attenuator, Apple introduced new video cards that automatically sense the particular kind of monitor that is attached. To do this they use pins 4, 7, and 10 of their 15-pin video connector as sense lines. These pins were formerly either grounded (pin 4) or unused (pins 7 and 10). All the video cards currently sold by Apple (and the built in video of the Mac IIci and the LC) have auto-sensing. These video cards will be fooled by the ISR attenuator (since it grounds pin 4 and leaves pins 7 and 10 open) into thinking that they are connected to an Apple High-Resolution (or 12-inch) Monochrome or RGB Display (all three have 640x480 pixels at 67 Hz). If you want to use a different display you could either make yourself a custom cable, copying the treatment of pins 4, 7, & 10 from your monitor, or you could modify the ISR Video Attenuator. The modification is to disconnect the ISR attenuator's pin 4 from ground, on both input and output connectors, and to wire the input sensing pins to the output sensing pins, i.e. 4 to 4, 7 to 7, and 10 to 10. This will allow your video card to correctly sense the monitor type.
