[libdc1394-devel] Re: [Coriander-devel] CCD Shutter question

[Arne C. <ar...@in...>]

Caleb J. Howard wrote:
> At 30 FPS, with around 1000 trials, I lost precisely 0 strobes!
> Woohoo!
> I suppose the ratio of my small strobe time (5 milliseconds) to the much 
> larger exposure time (unknown, but < 1/30th  of a second) has made my 
> odds of  hitting the CCD during the  period of light integration very high.
> 

For the curious ( if there are any ;-) ), I measured the time between 
the a RSG pulse and the XSUB pulse of a DFK 21F04 ( a TI based camera ). 
This is the time where no light is integrated for the next image.

It is some 860 usec.

/Arne














> Anyway, thanks to those who helped, and to those who wrote the software 
> (Coriander), I have a hope of meeting a difficult schedule.
> 
> Thanks, thanks, thanks!
> 
> -caleb
> 
> Caleb J. Howard wrote:
> 
>> I apologize for the bandwidth on this  question, and the 
>> crossposting.  I'm stuck on a question  which  seemed  to me at first 
>> to be straightforward.  I  am  guessing now that it may be a bit 
>> esoteric  in the eyes of many.
>>
>> Any light will be gratefully appreciated.  The following is a single, 
>> simple question.  All the words are to be perfectly clear.
>>
>> ------
>>
>> I will go through the IIDC spec which I have, and see what it says, 
>> but perhaps someone here know from experience.  As the CCD is passing 
>> the pixel  information down the line to Coriander, or whatever app, is 
>> there a blank time analogous to the flyback period of analog video?  
>> That is, if I am flashing a strobe, say, 100 times during a frame 
>> interval (1/30 of a frame, for example), will the image for that frame 
>> contain  all 100 flashes, or will it lose some.
>>
>> To go back to the mechanical  shutters of Film, there is a time at 
>> which the shutter opens, and  then a time when the shutter is closed  
>> while the film is advanced to the next frame.  The  interval is the 
>> shutter speed, and it is not uncommon for a shutter to  be open for 
>> only a small fraction of the 1/24th second frame interval.  In this 
>> film example, I might only capture 25% of the strobes, with the other 
>> 75% occuring during the time when the shutter is closed.  This would 
>> suck, for my purposes.
>>
>> I know that CCDs require some time to recharge, and that there are 
>> exposure settings to play with in the IIDC spec, which  would seem to 
>> be analogous to the shutter speed  of a mechanical  camera.  I have 
>> also read about the differences between a rolling shutter and a global 
>> shutter, which also seems to pertain to the question I have.
>>
>> What I really hope to know is this:  If I am triggering a single event 
>> which strobes a flash six times at millisecond intervals (for a total 
>> of 5/1000 - start to end), with no trigger, nor syncing mechanism - 
>> just capturing steadily off the 1394 bus while the event is being 
>> waited for - then what is the liklihood that some or all of that 
>> 5/1000 interval will occur during a time when the CCD is not 
>> capturing?  How long may I keep the camera capturing,  out of every 
>> frame interval.  It's possibly/probably camera dependent, but is it 
>> reasonable to hope that a cheap camera (IIDC compliant) will  capture 
>> all six flashes - either on a single frame, or accross two.
>>
>> I set my cameras to 3.75 FPS, and I set the exposure to 511 in 
>> Coriander, and I seem  to miss capturing the flashes much more often 
>> then when I'm running at 30 FPS.  This is not what I    would have 
>> guessed.
>>
>> Whatever anyone can tell me about this, I'll be grateful.  Please feel 
>> free to  be much less wordy in the answer than I was in the question.  
>> :-)
>>
>> Thanks!
>>
>> -caleb
>>
>> =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
>> Caleb J. Howard - ca...@ad... (home) - ca...@rh... (work)
>>      http://howards.vigilante.net/Caleb/Personal/CalebMain.html
>>      'It was for this you were called, created, formed and made.'
>> -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
>>
>>
>>
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> 
>