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Re: Eclair ACL 25fps speed problem
My device counts the openings and closings of the shutter every second on
and on. When I put it in front of a projector that runs exactly 24 fps (not
that common actually, but I have one of those, designed mechanically with a
chain instead of a belt) it gives a static 24 that's regenerated every
second. If I put it in front of a projector running 23.5 fps it will go 23
for one second then 24 for one second then 23 then 24 etc. It is precise :
putting it in front of a PAL TV screen gives a perfect 25. When I put it
behind the shutter of the ACL with the 25 fps crystal, I get sometimes 12
sometimes 13 (because it divides by two) but I can't tell the ratio
precisely, while with the 24 fps crystal it doesn't budge from 12 seeming to
prove that it's running at the right speed. If it was going, say, 12.1 fps,
the way my tool is programmed it would show 13 for one second every ten
seconds. True if it was going 12.001 fps I would have to wait a thousand
seconds to see it go 13, but the speed the bar is moving when shooting a
screen with the 25 crystal indicates a much higher imprecision. However, I
easily recognize my method isn't as reliable as using a stroboscopic speed
checker (I wish I had one !) and maybe I'll pass by the camera repairman to
figure this out for sure. Or I'll wait for the new version of my little
device that my friend is currently working on, this time including a mode
where it counts the light pulsation over 10 seconds and gives the speed in
tenths of fps right away. I also thought of watching the "bloop" signal on
an oscilloscope, but my battery cable (you know, the one with Jaeger
connectors) doesn't output it (it only has pins 1 and 4). Maybe I'll pick it
up behind the female connector of the camera - I notice the cap there was
easy to open by unscrewing the three little screws from the bottom of the
base. But again it would take a frequency meter rather that a oscilloscope
to make a precise reading.
Anyways, I agree that it sounds strange that the crystal would be "slightly"
wrong. I don't think these things age. But I think I'll pay the 3 $ to M.
Tobin to be sure.
At L'Abominable -
Aging Nicolas Rey
le 22/11/01 19:07, Robert Latimer à firstname.lastname@example.org a écrit :
It's unlikely to be your crystal and I still suspect your motor. The
procedure you are using for measuring 24fps isn't nearly as precise as the
procedure you are using to measure 25fps so you can't compare your results.
How accurate is your measuring device for your projector? If it can only
measure to one tenth of a frame accuracy per second, that's not going to be
close enough to test for true precision.
The best option would be to have the frame rate for both 24 and 25 tested
with a crystal locked speed checker. All you need is a crystal locked bulb
that will flash at an exact 24 or 25fps. Mark a rotating part of your
suspect motor with a chinagraph pencil and illuminate it with the speed
checker. If it's precise you'll see a static fan tailed effect. If it's
out of sync the fan tail will seem to move back and forth.
That's what I was afraid of. But you know what, it's strange because I know
the 24 fps speed (using the corresponding crystal) to be right. (I have a
little electronic device conceived for measuring the speed of projectors
that counts the shutter openings in one second and shows that number divided
by two - because projectors usually have double shutters - and that gives me
a perfect 24/2=12. But I can't use that trick for 25 because it doesn't show
tenths of images). Could it be my 25 fps crystal that doesn't output the
right pulsation ? Then all I would need would be a new 25 fps crystal...
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