Re: [photo-3d] Re: Testing synch of twin cameras

[Brian Reynolds <reynolds@xxxxxxxxx>]

Alan Lewis wrote:
> Following up on the thread about how to verify the actual synch of
> twin camera shutters:
> 
> I've had this idea in my head for a couple of years but never got
> around to trying it.  This idea is not complete in the details, and
> may be flawed in concept, but I think you other guys can expound on
> it anyway and make it a workable thing.
>
> If someone else has already stated this idea then as Emily said
> "Never Mind..."
> 
> I've used the single photo transistor with a battery hooked up to
> either an analog or digital scope to check shutter speed.  If both
> shutters were checked with this setup and the shutter speeds
> recorded, then using two photo transitors in series and tripping
> both shutters should superimpose the two signals, thus your output
> signal should be the difference between the two shutters.
> 
> In other words the shorter shutter should have its' signal
> completely hidden within the longest shutter's signal, and this
> would be the best synch possible with this method.  If the the synch
> was off then it should show up in the signal being different than
> either individual shutter speed.
> 

I don't think this would do what you want.  The following is bad ASCII
art diagram.  It's been a long time since my EE school days ("Damn it
Jim, I'm a sys admin not an EE!") so think of this as a black box
diagram not a circuit.

    o
    |
    \|
     | T1 = __|||||__
    /|
    |
    |
    \|
     | T2 = ___|||||_
    /|
    |
    o

    T1&T2 = ___||||__

This is a pair of photo transistors in series.  It is the equivalent
of a logical AND.  You will only get a signal when light strikes both
T1 AND T2.  This will tell you the amount of overlap between the
shutter firings, but not whether the shutters are in sync.  Looking
carefully at the diagrams above you'll notice that both transistors
fire for the same amount of time, but offset by one time tick.  The
output shows only the overlap of the two signals.

> If the scope was able to take two signals then the transistors could
> be in parallel and the two shutters could be superimposed over each
> other.
> 

This should work, but I'm not quite sure that I understand what you
mean by "If the scope was able to take two signals".  You can wire the
photo transistors in parallel and use a scope with only one input.
This is the equivalent of a logical OR.  More bad ASCII art follows.

    o_________
      |     |
      \|    \|        T1          T2          ||||
       | T1  | T2  __|||||__ + ___|||||_ = __||||||_
      /|    /|
      |     |
    o---------

Here the output wave doubles in height when both transistors are on.
By looking at the leading and trailing edges you should be able to
figure out what (if any) offset there is between the firing of the
shutters.  This will also tell you if the shutters fire at the same
speed.  You would want to you a very high sampling rate on the scope
to be sure you weren't losing the offset due to aliasing.

> My guess is someone has already done this.
> 

Like a lot of obvious ideas I think everyone figures that someone else
has done it.

-- 
Brian Reynolds                  | "Dee Dee!  Don't touch that button!"
reynolds@xxxxxxxxx              | "Oooh!"
http://www.panix.com/~reynolds  |    -- Dexter and Dee Dee
NAR# 54438                      |       "Dexter's Laboratory"

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