# Re: Pelli-Zhang Video Attenuator & Attenuator.mex **URL:** \ **Category:** Uncategorized\ **Created:** [April 17, 2002, 6:44pm UTC](https://psychtoolbox.discourse.group/t/re-pelli-zhang-video-attenuator-attenuator-mex/1161 "2002-04-17T18:44:19Z")\ **Posts on this page:** 1\ **Page:** 1
**Author:** ![denispelli](https://avatars.discourse-cdn.com/v4/letter/d/cdc98d/32.png) [@denispelli](https://psychtoolbox.discourse.group/u/denispelli)\ **Post date:** [April 17, 2002, 6:44pm UTC](https://psychtoolbox.discourse.group/t/re-pelli-zhang-video-attenuator-attenuator-mex/1161/1 "2002-04-17T18:44:19Z")
> \>Denis, > \> > \>Thanks for the feedback. I have now purchased and installed an ISR video > \>attenuator on our Mac G4 with a Sony GDM F500 monitor. However, I am unclear > \>on how to determine if I am getting the increased resolution, or perhaps > \>more precisely, if the matlab routines I am using are able to access the > \>full resolution provided by the attenuator. Any advice would be greatly > \>appreciated. > \> > \>1. What exactly is the resolution provided by the attenuator? Is it > \>12-bits? Are there test routines that will confirm the exact resolution? > \> > \>2. I have modified the existing ContrastThresholdDemo routines to present > \>multiple moving gratings at different locations on the screen and to > \>estimate contrast thresholds for the detection of movement at each > \>location. However, I have not tinkered with the code that determines > \>contrast steps, CLUTs, or QUEST parameters. It appears that these routines > \>assume that only 256 luminance levels are available (8 bits). Do I need to > \>alter the statements that apparently refer to 256 levels in various matlab > \>routines to reflect the higher resolution provided by the attenuator in > \>order to get the QUEST routines to modulate stimulus contrast with finer > \>resolution? I've tried doing this with the statements I thought were > \>relevant, but I typically break the programs when I do so. Generally, if one > \>has a contrast threshold estimation program based on the QUEST routines set > \>up to expect 8-bit resolution, how does one alter the program to expect a > \>resolution of 12-bits (or whatever the resolution of the video attenuator > \>actually is)? > \> > \>Thanks for your advice, > \> > \>Vince Samar dear vince 1. you'll get an effective resolution of around 12 bits. no, we don't have any MATLAB test routines to show that. i would be good to have such. 2. even though we use the Pelli-Zhang attenuator in nearly all our work, we're still using the original software that I wrote for it to do that, which has only minimal documentation (usage help) and is needlessly restricted to the Mac. We use a free-standing application called CalibrateLuminance and Attenuator.mex (formerly called "CLUT.mex"). (Both are available, see below; you seem to have them already.) Attenuator.mex reads the luminance calibration file produced by CalibrateLuminance and computes appropriate color lookup table (i.e. clut) values to linearly map a range of pixel values (entry numbers) to luminances. It takes advantage of the video attenuator to achieve very fine steps in that mapping. The normal way to use the video attenuator is to set the video card to use 8-bit pixels. In this case the pixel value (0 to 255) is used to select one of the entries (0 to 255) in the hardward color lookup table (clut). Each of those entries is a triplet of numbers, which drive the three DACs. The video attenuator combines those three voltages, with unequal gains, to produce one voltage, which drives your monitor. Thus, for a fixed clut, you can produce only 256 luminances. If you are simultaneously displaying several gratings you should optimize the clut for the highest contrast grating. Of course, the clut can be changed very quickly between frames, allowing you to have a completely different clut on each frame if you like. in practice, i rarely do that, but we do always compute a new clut for each stimulus presentation (i.e. each new trial at a new contrast). we have thought for some time that it would be very nice to replace the C programs CalibrateLuminance (standalone application) and Attenuator.mex by plain MATLAB code, but we don't have a schedule for that. the Pelli and Zhang article contains all the necessary information. Attenuator.mex will be included in the next release of the PsychToolbox, in a new category called "PsychBeta" of unsupported software that nevertheless seems to be of wide interest. I'm also enclosing it in this message. good luck best denis p.s. announcement for Attenuator.mex [http://psychtoolbox.org/mac.html#changes](http://psychtoolbox.org/mac.html#changes) attenuator web page [http://vision.nyu.edu/Tips/Attenuator.html](http://vision.nyu.edu/Tips/Attenuator.html) CalibrateLuminance (application and sources included in VideoToolbox) [http://vision.nyu.edu/VideoToolbox/Contents.html](http://vision.nyu.edu/VideoToolbox/Contents.html) [http://vision.nyu.edu/VideoToolbox/Download.html](http://vision.nyu.edu/VideoToolbox/Download.html) NOTE TO ALL: [denis.pelli@...](mailto:denis.pelli@...) is my preferred email address. Please use that address. My "verizon.net" address will go away soon, partly because Verizon forces me to use that as my "From:" address. My current email program won't allow me to designate a separate "Reply-to:" address.