Lux, James P skrev:
On 5/10/09 11:21 AM, "Magnus Danielson" <mag...@rubidium.dyndns.org> wrote:
Lux, James P skrev:
I'm looking for a way to take GPS time and generate a signal that can
be recorded on the audio track of a video recording to time stamp it.
This is so it can be aligned with other data that's collected with GPS
based time. It needs to be portable/small (i.e. Something you could
attach to a small camcorder, or such).
Traditionally SMPTE 12M LTC is being used for this purpose. For most
cases it is only known as LTC but SMPTE/EBU time code is among other
names fairly common name.
Yes.. But most consumer recorders (e.g. The cheap AIPtek) don't do timecode
(or genlock, or other useful stuff)..
They should be hackable thought.
So what they're really looking for is someway to time align after the fact.
This does not prohibit you from just recording the LTC onto the
soundtrack anyway.
I actually have a GPS with IRIG-B output in about the same size of a
Thunderbolt.
Is that an off the shelf, not hideously expensive, widget? (There are other
applications...)
I bought one of those Brandywine GPS4 devices as announced available on
the list not too long ago. Not hideously expensive IMHO.
Another alternate is if something like a iPhone records accurate time
with the video stream. (another of the data sources is an iPhone
recording something else)
I think the basic requirement is accuracy to some few milliseconds
(e.g. Frame rate of the video)..
SMPTE LTC encodes the frame numbers.
(It's for a high school science project, where they want to record
various things, and line them up.. I think they could deal with
looking at the timestamps over many frames to do interpolation)
SMPTE LTC should be preferred in that case, as it is something that
editing equipment understands.
I don't think "real" editing gear is in the cards. Probably more like
iMovie or something on a PC.
There should be editing gear that chews LTC over audio-interface. We did
frame-grabbing tricks with a cheap video-recorder, LTC on audio track
and an SGI Indy back in the days... and a remote hacked with CMOS
switches steered by a DTMF decoder so the Indy played DTMF tone-pairs on
the port, real-time decoded LTC and frame-grabbed 10 frames at the time,
rewinded, played again etc. Some of the cheap/free programmes would be
able to decode LTC and tag the pictures accordingly.
As a model for what they're trying to do, say you were going to measure the
acceleration due to gravity by videotaping a falling object against a scale
in the background. Except that the motion is more complex.. Maybe imagine
putting an accelerometer in the "payload" of a trebuchet... And you want to
time align the position of the trebuchet components (from the video) with
the forces on the payload. That's not what they're doing, but now that I
describe it, that WOULD be a cool science project. And get away from the
"here, I built a trebuchet and launched a projectile X meters" which is kind
of tired. (get one of those analog devices 3 axis accelerometers, hook it
to a suitable datalogger..)
In that case you want the line-frequency locked or traceable by other
means. You have more use for the frequency aspect than time-notation
actually, which is more handy for a matter logging which event. Still,
LTC should be easier to get locked to the phase attached to the frames,
as the infrastructure is expected to be there for some apps, but the
IRIG-B support is not expected to be there.
If you do not hack the camera to accept a reference signal (hacking the
crystal oscillator may be all you need to do), after the fact frequency
calibration can be done with either IRIG-B, LTC or just a 1 kHz sine.
Anyway, cheap and cheerful consumer gear is what is sought. (so no
suggestions of synthesizing SMPTE from the output of my Z3801 and
feeding it to a RED camera.. We're talking AIPtek and iPhone here..)
Locking the camera with a black burst GENLOCK signal with VITC does need
to be done, but the LTC may very well be practical for the purpose.
BTW. Cameras lacking GENLOCK is highly annoying, especially when they
have el cheapo crystals... el cheapo crystals is fine, as long as they
lock up to a GENLOCK when needed.
That pretty well describes just about everything they'll have available.
Well, the two initial strategies is to either hack the cameras and
replace the XO with a VCXO which locks to a 10 Mhz. Usually it is 27 MHz
and relationship to 10 MHz is fairly trivial. After the fact
synchronisation using a 1 kHz signal (such as given by TADD-2, tvb
PIC-div or something) into the audio signal would also do, if only the
audio sample rate and the video rate is locked in the el cheapo camera,
which one can hope for at least.
Cheers,
Magnus
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