[visionlist] Update - Accurate timing of projected images
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:
>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.
>School of Computer Science and Informatics
>University College Dublin
>visionlist mailing list
>visionlist at visionscience.com
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