Betreff: Re: [visionlist] Update - Accurate timing of projected images Von: vincent ferrera Datum: Wed, 28 Jun 2006 13:32:10 -0400 An: Stuart Jackson CC: visionlist@visionscience.com 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. vf 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. > Yours > > Stuart Jackson > School of Computer Science and Informatics > University College Dublin > Dublin > Ireland > _______________________________________________ > visionlist mailing list > visionlist@visionscience.com > http://visionscience.com/mailman/listinfo/visionlist > > _______________________________________________ visionlist mailing list visionlist@visionscience.com http://visionscience.com/mailman/listinfo/visionlist