[visionlist] Update - Accurate timing of projected images

vincent ferrera vpf3 at columbia.edu
Wed Jun 28 17:32:10 GMT 2006

For LCDs it is possible to get reliable timing.  The key is to send a 
video signal at the native framerate and resolution of the monitor.  If 
you send a signal that does not match the display's native 
characteristics, then the DSP circuitry will resample the input and this 
could introduce variable delays.

I've used a CRS VSG system with an NEC2010X flat panel and an EPSON 
EMP9100 projector.  Both have native resolution of 1280 x 1024 @ 60 Hz.  
The VSG allows you to fiddle with the timing parameters of the video 
signal.  I've found that I can tweak the timing so that I get perfectly 
smooth motion, i.e. no dropped frames.  There is still a delay between 
the VSYNC signal and the actual appearance of the stimulus.  This can be 
measured using a photometer.  I've found it to be 34 ms on both 
displays, and it is consistent from trial-to-trial.


Stuart Jackson wrote:

>Dear all,
>Recently I sought information on the the kinds of projectors used for 
>displaying images where accurate timing is an issue. Here's a summary 
>of the information i've gathered on the topic, from responses i 
>received and elsewhere. Thanks again, hope the info. is helpful: 
>1) LCD projectors can have slow response times, as the colour wheel is 
>not synched to the input. Also they have lower contrast than other 
>types of system, and at some magnifications grid lines can be seen 
>between pixels. These mightn't be appropriate where accurate stimulus 
>timing is required, particularly when explicit control over the 
>vertical refresh rate is desired. However, some have used 2 LCD 
>projectors in combination to synchronise two separate presentations. 
>2) Some DLP systems can contain motion artifacts due to a dithering 
>process used to increase resolution. Single panel systems may also 
>contain color-breakup artifacts due to having to sequence between r, g, 
>and b. A newer projection technology, LCoS, has very high levels of 
>contrast and can operate at high refresh rates. However, like some 
>LCD/DLP systems, I'm not sure if any currently available systems offer 
>the user explicit control over the vertical refresh rate. The 
>specifications for many systems claim that vertical refresh rates 
>between, for example, 50-85Hz (LCD/DLP) and 50-120Hz (LCoS), can be 
>achieved. However, some sources suggest that in reality this isn't the 
>case and that the projectors typically display at the lower end (e.g. 
>50/60Hz), even if the input signal is at a higher rate.  
>3) CRT projectors may be the most appropriate where stimulus timing is 
>important, though cost can be prohibitive. Some systems can apparently 
>project at frequencies between 30-200Hz+. When the display rate needs 
>to be the same as the input signal rate, I think the important thing to 
>look for is a reference to something like an 'auto-lock' feature, 
>whereby the projector takes on whatever rate is fed into it.      
>Stuart Jackson
>School of Computer Science and Informatics
>University College Dublin
>visionlist mailing list
>visionlist at visionscience.com

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