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LED 25fps - need to capture every frame with camera

vvvv beta question
#1

I’ve got a LED that blinks with 25fps, each frame a different colour. I need a camera which is able to capture each frame colour. Have tried Sony FX1 (50i) which only works for a while. For testing I just have a standard beamer with 56Hz to 97Hz

Do you guys have an idea what the problem could be?. I think that the hardware runs that asynchron, that the camera captures one field of the old colour, and the second one of the next colour. Or do the Hz of the beamer interfere with my fields?

This video shows the old monchrome version.

greets :)

#2

as the LED sendiing 25fps and the camera receiving 25fps are not synced its inevitable that they run out of sync and will sooner or later drop or double one frame.

two basic strategies which are both not trivial:

you cound try oversampling with a 60fps camera and dropping all frames which are identical to the prevous one (the new sony eyetoy camera can capture 120fps (but i presume not in vvvv)). due to the frame based nature of vvvv this will not be possible without some jitter.

alternatively you could try to find a camera which can be triggered externally and use some photosensor/hardware to derive a frame sync signal from the LEDs to trigger the camera.

#3

note that the low tech version works quite well: connect a photo transistor to an rs232 port. mount the photo transistor in some kind of telescope, so you can target individzal pixels. set the rs232 port to 25baud (!) (25 bit/s = 25fps) this is uncommon but should work with many chipsets. as soon as the LED runs on rock solid 25fps (which should be tested beforehand, as this is kind of unlikely) you can receive the data directly with the RS232 UART, if you send them with the usual start+stop bits.

at least i was quite successful receiving 1024x768 pixels at 60baud from a video projector with a photo transistor and an rs232 cable.

#4

…only works for a while.
this indicates that something is drifting, i.e. not at the exact frequency of 25Hz, running out of sync.
therefore, you either need some sync or genlock signal or something to calibrate/lock the signal…one led which is always blinking (1-0-1-0) as a reference for example, but i think you already know that?
you could use a camera which is faster (e.g. sony PS3eye, 120hz@320x240) to generate the lock from the blinking pattern, but your patch also has to run at a higher framerate.

edit: owned by oschatz ;-)

#5

to sync the signal I use a black picture every third frame.

mounting this phototransistor on some kind of telescope sounds quite difficult to me. how would I assure that it just recieves the colour information of one pixel?

the ps3 eye offers a nice framerate for this price. but do you have any control on exposure and aperture?

and what about this one?: Philips SPC1330NC Webcam USB

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